Aspects of the biology and population dynamics of the geelbek Atractoscion aequidens (Cuvier) (Pisces: Sciaenidae) off the South African coast
- Authors: Griffiths, Marc H
- Date: 1988
- Subjects: Sciaenidae -- South Africa Fish populations -- South Africa Atractoscion Atractoscion -- South Africa -- Ecology Atractoscion -- South Africa -- Physiology
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5194 , http://hdl.handle.net/10962/d1001970
- Description: The geelbek Atractoscion aeguidens is an important commercial and recreational linefish species along the South African eastern seaboard . Aspects of its biology and population dynamics were investigated using data collected from various sampling sites along the South African east coast and from catch statistics obtained from both the Sea Fisheries Research Institute (Cape Town), and dealer records. The biological aspects studied included age and growth, reproduction and feeding. It was established that A. aeguidens is a fast growing species with a relatively late age at sexual maturity and a 1:1 sex ratio. Feeding studies revealed that it has become specialized to feed on small pelagic fish. The South African geelbek stock consists of three distinct age related sub-populations, each representing a particular phase in the life cycle. The sub-adults (1-4 yrs) occur in the South West Cape and feed predominantly on anchovy, Engraulis capensis, which they follow, moving inshore in summer and offshore in winter. The adults (5-9 yrs) undergo a seasonal inshore migration to Natal where they spawn in spring. The dynamics of this migration are strongly influenced by the annual movement of their principal prey viz. , Sardinops ocellatus, to that region. As a result the geelbek first arrive in Natal during mid winter. Evidence is also presented to suggest that a significant number of "adolescent " fish (4-5 yrs) undergo limited gonad maturation and a partial migration to at least as far as the South East Cape. Spawning appears to occur offshore. It is postulated that eggs and larvae are transported southwards by the peripheral waters of the Agulhas Current. After spawning the adults probably remain offshore and use the current to aid their return to the South and South West Cape, where they spend summer feeding on the Agulhas Bank. Juveniles (<1 yr) first appear in the South East Cape between January and March. They apparently use this region as a nursery area for almost a year before moving southwards to join the sub-adults in the South West Cape. In the South East Cape, the juveniles feed initially on mysids whereafter they switch to anchovies and pilchards. A preliminary stock assessment using yield per recruit and spawner biomass per recruit models, indicated that despite the current legislation the South African geelbek population is already overexploited. This is largely attributed to high rates of fishing mortality of the migratory adult population. Further management recommendations are therefore proposed. These include a size limit (660 mm FL), derived from an investigation of the interrelationship between natural mortality and somatic growth in a theoretical unexploited cohort, as well as a uniform bag limit for all user groups (5 fish per person per day)
- Full Text:
- Date Issued: 1988
- Authors: Griffiths, Marc H
- Date: 1988
- Subjects: Sciaenidae -- South Africa Fish populations -- South Africa Atractoscion Atractoscion -- South Africa -- Ecology Atractoscion -- South Africa -- Physiology
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5194 , http://hdl.handle.net/10962/d1001970
- Description: The geelbek Atractoscion aeguidens is an important commercial and recreational linefish species along the South African eastern seaboard . Aspects of its biology and population dynamics were investigated using data collected from various sampling sites along the South African east coast and from catch statistics obtained from both the Sea Fisheries Research Institute (Cape Town), and dealer records. The biological aspects studied included age and growth, reproduction and feeding. It was established that A. aeguidens is a fast growing species with a relatively late age at sexual maturity and a 1:1 sex ratio. Feeding studies revealed that it has become specialized to feed on small pelagic fish. The South African geelbek stock consists of three distinct age related sub-populations, each representing a particular phase in the life cycle. The sub-adults (1-4 yrs) occur in the South West Cape and feed predominantly on anchovy, Engraulis capensis, which they follow, moving inshore in summer and offshore in winter. The adults (5-9 yrs) undergo a seasonal inshore migration to Natal where they spawn in spring. The dynamics of this migration are strongly influenced by the annual movement of their principal prey viz. , Sardinops ocellatus, to that region. As a result the geelbek first arrive in Natal during mid winter. Evidence is also presented to suggest that a significant number of "adolescent " fish (4-5 yrs) undergo limited gonad maturation and a partial migration to at least as far as the South East Cape. Spawning appears to occur offshore. It is postulated that eggs and larvae are transported southwards by the peripheral waters of the Agulhas Current. After spawning the adults probably remain offshore and use the current to aid their return to the South and South West Cape, where they spend summer feeding on the Agulhas Bank. Juveniles (<1 yr) first appear in the South East Cape between January and March. They apparently use this region as a nursery area for almost a year before moving southwards to join the sub-adults in the South West Cape. In the South East Cape, the juveniles feed initially on mysids whereafter they switch to anchovies and pilchards. A preliminary stock assessment using yield per recruit and spawner biomass per recruit models, indicated that despite the current legislation the South African geelbek population is already overexploited. This is largely attributed to high rates of fishing mortality of the migratory adult population. Further management recommendations are therefore proposed. These include a size limit (660 mm FL), derived from an investigation of the interrelationship between natural mortality and somatic growth in a theoretical unexploited cohort, as well as a uniform bag limit for all user groups (5 fish per person per day)
- Full Text:
- Date Issued: 1988
A contribution towards an understanding of the intensive tank culture of an ornamental Cichlid, Aulonocara Baenschi, from Chipoka, Lake Malawi
- Authors: Impson, N D (Neville Dean)
- Date: 1988
- Subjects: Cichlids -- Nysas, Lake , Cichlids -- Nyasa, Lake -- Physiology
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5208 , http://hdl.handle.net/10962/d1004596 , Cichlids -- Nysas, Lake , Cichlids -- Nyasa, Lake -- Physiology
- Description: The intensive tank culture of ornamental mouthbrooding cichlids poses several problems which limit their aquaculture potential. This project addressed some of these problems for Aulonocara baenschi. The production of juveniles in 2501 aquaria was accelerated when: a) aquaria were equipped with refuges, b) females of less than 70mm in standard length were used as broodstock, c) mouthbrooding females were replaced with gravid females at seven day intervals, and d) embryos were removed from the mouths of females at replacement times for artificial incubation. Two sex ratios also accelerated juvenile production. The sex ratio (male:females) 1:30 yielded the highest spawning returns per tank, and therefore represented the most effective utilization of aquarium space (a critical consideration for the small-scale culturist). Contrastingly, the sex ratio 1:12 yielded the highest clutch sizes and a high percentage female spawning return, and therefore represented the most effective utilization of broodstock (an important consideration for culturists inhibited by financial constraints or having an abundance of culture vessels). The reproductive behaviour of A. baenschi was described. Emphasis was given to aspects of reproduction of relevance to culture, for example; spawning times and seasons, clutch size and its relationship with female size, age and size of sexes at first spawning, embryo development rate and size of first swimming juveniles. The slow growth rate of juveniles, combined with a late attainment of marketable size (± seven months) was a major limitation affecting the cuIture potential of A. baenschi. Two factors favouring the cuIture of this species was the high survival rate recorded for both adults and juveniles, and the comparatively high prices fetched by fish on domestic wholesale markets (R4,00 per fish). It is recommended that A. baenschi should not be cultured exclusively for the relatively small South African ornamental fish market. A more profitable strategy for domestic culturists should involve a major production effort with A. baenschi and other desirable species of Aulonocara (e.g . A. ethelwynnae; A. hansbaenschi; A. stuartgranti & A. maylandi) for foreign markets (in particular, the U.S.A.; Western Europe & Japan). Not only are these markets massive, but prevailing exchange rates of the Rand with these currencies favour such a strategy.
- Full Text:
- Date Issued: 1988
- Authors: Impson, N D (Neville Dean)
- Date: 1988
- Subjects: Cichlids -- Nysas, Lake , Cichlids -- Nyasa, Lake -- Physiology
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5208 , http://hdl.handle.net/10962/d1004596 , Cichlids -- Nysas, Lake , Cichlids -- Nyasa, Lake -- Physiology
- Description: The intensive tank culture of ornamental mouthbrooding cichlids poses several problems which limit their aquaculture potential. This project addressed some of these problems for Aulonocara baenschi. The production of juveniles in 2501 aquaria was accelerated when: a) aquaria were equipped with refuges, b) females of less than 70mm in standard length were used as broodstock, c) mouthbrooding females were replaced with gravid females at seven day intervals, and d) embryos were removed from the mouths of females at replacement times for artificial incubation. Two sex ratios also accelerated juvenile production. The sex ratio (male:females) 1:30 yielded the highest spawning returns per tank, and therefore represented the most effective utilization of aquarium space (a critical consideration for the small-scale culturist). Contrastingly, the sex ratio 1:12 yielded the highest clutch sizes and a high percentage female spawning return, and therefore represented the most effective utilization of broodstock (an important consideration for culturists inhibited by financial constraints or having an abundance of culture vessels). The reproductive behaviour of A. baenschi was described. Emphasis was given to aspects of reproduction of relevance to culture, for example; spawning times and seasons, clutch size and its relationship with female size, age and size of sexes at first spawning, embryo development rate and size of first swimming juveniles. The slow growth rate of juveniles, combined with a late attainment of marketable size (± seven months) was a major limitation affecting the cuIture potential of A. baenschi. Two factors favouring the cuIture of this species was the high survival rate recorded for both adults and juveniles, and the comparatively high prices fetched by fish on domestic wholesale markets (R4,00 per fish). It is recommended that A. baenschi should not be cultured exclusively for the relatively small South African ornamental fish market. A more profitable strategy for domestic culturists should involve a major production effort with A. baenschi and other desirable species of Aulonocara (e.g . A. ethelwynnae; A. hansbaenschi; A. stuartgranti & A. maylandi) for foreign markets (in particular, the U.S.A.; Western Europe & Japan). Not only are these markets massive, but prevailing exchange rates of the Rand with these currencies favour such a strategy.
- Full Text:
- Date Issued: 1988
Life history changes of two reef fish species in exploited and unexploited marine environments in South Africa
- Authors: Buxton, Colin David
- Date: 1988
- Subjects: Reef fishes -- Africa, Southern Chrysoblephus laticeps Chrysoblephus cristiceps
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5189 , http://hdl.handle.net/10962/d1001965
- Description: A detailed investigation into the life histories of Chrysoblephus laticeps and c. cristiceps in exploited and unexploited marine environments on the south-east coast of southern Africa is presented. The study provided information necessary for the evaluation of marine reserves as a management option for these and similar reef species. Visual underwater assessments showed a clear spatial separation between adults and juveniles of both species, suggesting a recruitment of juveniles into shallow water followed by a gradual outward migration into deeper water with age. Denslty estlmates of C. laticeps in the Noordhoek (0.0064 fish/m²) and Tsitslkamma (0.0254 fish/m²) areas were significantly different and were attributed to exploitation. Within area differences in density were also significantly correlated with temperature) depth and substratum relief. Similar comparisons for C. cristiceps were not possible because these fish shied away from divers. Habitat preferences were mirrored in the diets of both species which fed on a wide variety of reef associated benthic animals. Size of prey appeared to be gape limited, only larger fish being able to manipulate prey. Quantitative differences between the diets of fish sampled in different areas were a reflection of differences in available prey and not related to the relative density of predators. The dietary characteristics recorded in these fish were typical of search hunters. An age and growth study based on the examination of sectioned otoliths showed that both species were slow growing and long lived. Maximum recorded ages were 18 and 22 years for C. laticeps and C. cristiceps respectively. Growth rates in exploited and unexploited areas were not significantly different although there was an indication that growth was slower in the exploited areas for C. cristiceps. Several inconsistencies in otolith interpretation and terminology were evaluated. Contrary to other South African studies the results suggested that somatic growth was fastest just after spawning. This coincided with the winter months and was reflected by the deposition of a hyaline zone in the otolith. Two independent estimates, the Pauli derivation and visually assessed length frequency analysis, produced the same value for total mortality of C. laticeps in the Tsitsikamma area (0.2). This result, together with a significant difference in the rate of fishing mortality at different areas along the coast suggested that the populations sampled were discrete and supported the postulate that both species were sedentary. Further support was obtained from a mark-recapture study. Although sample numbers were low the results showed no evidence of large scale migratory patterns in these fish. An investigation of the reproductive biology of both fish showed that they were protogynous hermaphrodites, all males being derived from females with no evidence for polyandry. Males were shown to be capable of mating with a number of females and this, together with monandry, monochromatism and a small testis size, suggested a polygynous mating system in both species. Observations of the courtship behaviour in captive c. laticeps also supported polygyny. A significant difference in the mean size at sex reversal between unexploited and exploited populations supported the postulate that sex reversal was independent of the size or age of the animal. The size at which sex reversal occurred appeared to be dependent on sex ratio. A detailed yield per recruit analysis of the response of the population to different levels of fishing mortality and size at recruitment (t [subscript]r), showed that sex reversing species were particularly vulnerable to over fishing. This was due to a dramatic decline in the number of surviving individuals past t[subscript]r which resulted in a skewing of the sex ratio towards females. Evidence is presented to show that current legislation is inadequately protecting the spawner stock of both species. In conclusion it is shown that longevity, sex reversal, restricted movements and the occupation of a demersal habitat make these species particularly vulnerable to over exploitation in a size- selective fishery. The sustained yield in the linefishery is suggested to be a result of distant recruitment from relatively unexploited populations rather than the adequacy of current conservation measures. The need to protect the spawner stock is emphasised and the use of marine reserves as an additional protection for sedentary reef-dwelling species is therefore advocated.
- Full Text:
- Date Issued: 1988
- Authors: Buxton, Colin David
- Date: 1988
- Subjects: Reef fishes -- Africa, Southern Chrysoblephus laticeps Chrysoblephus cristiceps
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5189 , http://hdl.handle.net/10962/d1001965
- Description: A detailed investigation into the life histories of Chrysoblephus laticeps and c. cristiceps in exploited and unexploited marine environments on the south-east coast of southern Africa is presented. The study provided information necessary for the evaluation of marine reserves as a management option for these and similar reef species. Visual underwater assessments showed a clear spatial separation between adults and juveniles of both species, suggesting a recruitment of juveniles into shallow water followed by a gradual outward migration into deeper water with age. Denslty estlmates of C. laticeps in the Noordhoek (0.0064 fish/m²) and Tsitslkamma (0.0254 fish/m²) areas were significantly different and were attributed to exploitation. Within area differences in density were also significantly correlated with temperature) depth and substratum relief. Similar comparisons for C. cristiceps were not possible because these fish shied away from divers. Habitat preferences were mirrored in the diets of both species which fed on a wide variety of reef associated benthic animals. Size of prey appeared to be gape limited, only larger fish being able to manipulate prey. Quantitative differences between the diets of fish sampled in different areas were a reflection of differences in available prey and not related to the relative density of predators. The dietary characteristics recorded in these fish were typical of search hunters. An age and growth study based on the examination of sectioned otoliths showed that both species were slow growing and long lived. Maximum recorded ages were 18 and 22 years for C. laticeps and C. cristiceps respectively. Growth rates in exploited and unexploited areas were not significantly different although there was an indication that growth was slower in the exploited areas for C. cristiceps. Several inconsistencies in otolith interpretation and terminology were evaluated. Contrary to other South African studies the results suggested that somatic growth was fastest just after spawning. This coincided with the winter months and was reflected by the deposition of a hyaline zone in the otolith. Two independent estimates, the Pauli derivation and visually assessed length frequency analysis, produced the same value for total mortality of C. laticeps in the Tsitsikamma area (0.2). This result, together with a significant difference in the rate of fishing mortality at different areas along the coast suggested that the populations sampled were discrete and supported the postulate that both species were sedentary. Further support was obtained from a mark-recapture study. Although sample numbers were low the results showed no evidence of large scale migratory patterns in these fish. An investigation of the reproductive biology of both fish showed that they were protogynous hermaphrodites, all males being derived from females with no evidence for polyandry. Males were shown to be capable of mating with a number of females and this, together with monandry, monochromatism and a small testis size, suggested a polygynous mating system in both species. Observations of the courtship behaviour in captive c. laticeps also supported polygyny. A significant difference in the mean size at sex reversal between unexploited and exploited populations supported the postulate that sex reversal was independent of the size or age of the animal. The size at which sex reversal occurred appeared to be dependent on sex ratio. A detailed yield per recruit analysis of the response of the population to different levels of fishing mortality and size at recruitment (t [subscript]r), showed that sex reversing species were particularly vulnerable to over fishing. This was due to a dramatic decline in the number of surviving individuals past t[subscript]r which resulted in a skewing of the sex ratio towards females. Evidence is presented to show that current legislation is inadequately protecting the spawner stock of both species. In conclusion it is shown that longevity, sex reversal, restricted movements and the occupation of a demersal habitat make these species particularly vulnerable to over exploitation in a size- selective fishery. The sustained yield in the linefishery is suggested to be a result of distant recruitment from relatively unexploited populations rather than the adequacy of current conservation measures. The need to protect the spawner stock is emphasised and the use of marine reserves as an additional protection for sedentary reef-dwelling species is therefore advocated.
- Full Text:
- Date Issued: 1988
Biology and demography of the spotted grunter Pomadasys commersonnii (Haemulidae) in South African waters
- Authors: Webb, Garth Anthony
- Date: 2003
- Subjects: Grunts (Fishes) -- South Africa , Grunts (Fishes) -- South Africa -- Ecology
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5201 , http://hdl.handle.net/10962/d1003727 , Grunts (Fishes) -- South Africa , Grunts (Fishes) -- South Africa -- Ecology
- Description: The spotted grunter, Pomadasys commersonnii (Haemulidae), is an Indian Ocean coastal species, extending from India to False Bay but is absent from the central Indian Ocean islands. Its taste and texture has made it one of the most important line fish species and is caught by recreational and subsistence fisherman along the entire east coast of South Africa. Because of its inshore distribution, reduced catch rates and estuarine dependence the species was de-commercialised in 1992. Since then it has been investigated as a candidate species for mariculture. All previous work on the biology of the species was undertaken on fish collected in KwaZulu-Natal in the mid 1970's. All other information has been incidental and formed part of other ecological studies. All management plans for this species have been based on these data. To develop a more comprehensive management plan that incorporates the entire population of spotted grunter, it was deemed necessary to reassess the biology (including feeding biology, age and growth and reproductive biology) as well as the demography of the population throughout its distributional range in South African waters. Analysis of the diet of spotted grunter, collected in estuaries, indicates that crustaceans form the bulk of the prey selected. Amphipods, mysids and estuarine brachyura predominate the diet of fish < 300 mm TL. At 200 mm TL fish start to prey on anomurans, which are extracted from their burrows using the 'blowing' feeding mechanism. Anomurans, in particular Upogebia africana and Callianassa krausii, become the preferred prey of fish larger than 300 mm TL. The high degree of diet flexibility that spotted grunter exhibit means that the composition and abundance of the macrobenthos of a particular environment will dictate the diet of the species. Otolith growth zones were found to be deposited annually with the opaque zones being deposited during the austral summer (November - February). The optical definition of annual otolith growth rings differed significantly between geographic regions (namely: Western Cape, South Eastern Cape and KwaZulu-Natal). Growth (sex combined) offish in the Western Cape were best described by the specialised 3 parameter Von Bertalanffy with a relative error structure in the form : Lt = 753(1- e⁻°·¹⁵⁴ ⁽t +¹·⁶¹⁵⁾) while growth in the South Eastern Cape was best described by using the Schnute model with an absolute error structure in the form: Lt=177°·⁴¹⁶ + (676.2⁻¹·²⁶⁶ - 169.2⁻¹²⁶⁶)[1-e°·⁴¹⁶⁽t-t₁⁾ / 1-e°·⁴¹⁶⁽t₂-t₁⁾]¹/¹·²⁶⁶ whereas growth was best described in KwaZulu-Natal using the specialised 3 parameter Von Bertalanffy with a relative error structure in the form: Lt = 839(1-e°·¹⁷⁽t⁺°·⁴⁹⁾) In the South Eastern Cape, length at 50% maturity was found to be 305 mm TL for males. Since females with ripe & running or spent gonads were not found in the South Eastern Cape and since histological evidence suggests that females in the South Eastern Cape have spawned, it appears that spawning does not occur in the South Eastern Cape. These results suggest that adults are resident in the estuaries of the Western and South Eastern Cape and undertake the spring/summer, northward spawning migration to KwaZulu-Natal. After joining the resident spawner stock in KwaZulu-Natal and spawning in the offshore environment of KwaZulu-Natal, adults soon return to the southern regions of their distributional range. Juveniles recruit into KwaZulu-Natal estuaries at a length of 25 - 35 mm TL. A proportion of the eggs and larvae are transported southwards along the periphery of the western boundary Agulhas Current where juveniles (25 - 30 mm TL) recruit into the estuaries as far south as the Swartvlei estuary in the Western Cape. There is evidence to suggest that the fish, which occur in the Western Cape estuaries, have migrated there once they have attained sexual maturity further east.
- Full Text:
- Date Issued: 2003
- Authors: Webb, Garth Anthony
- Date: 2003
- Subjects: Grunts (Fishes) -- South Africa , Grunts (Fishes) -- South Africa -- Ecology
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5201 , http://hdl.handle.net/10962/d1003727 , Grunts (Fishes) -- South Africa , Grunts (Fishes) -- South Africa -- Ecology
- Description: The spotted grunter, Pomadasys commersonnii (Haemulidae), is an Indian Ocean coastal species, extending from India to False Bay but is absent from the central Indian Ocean islands. Its taste and texture has made it one of the most important line fish species and is caught by recreational and subsistence fisherman along the entire east coast of South Africa. Because of its inshore distribution, reduced catch rates and estuarine dependence the species was de-commercialised in 1992. Since then it has been investigated as a candidate species for mariculture. All previous work on the biology of the species was undertaken on fish collected in KwaZulu-Natal in the mid 1970's. All other information has been incidental and formed part of other ecological studies. All management plans for this species have been based on these data. To develop a more comprehensive management plan that incorporates the entire population of spotted grunter, it was deemed necessary to reassess the biology (including feeding biology, age and growth and reproductive biology) as well as the demography of the population throughout its distributional range in South African waters. Analysis of the diet of spotted grunter, collected in estuaries, indicates that crustaceans form the bulk of the prey selected. Amphipods, mysids and estuarine brachyura predominate the diet of fish < 300 mm TL. At 200 mm TL fish start to prey on anomurans, which are extracted from their burrows using the 'blowing' feeding mechanism. Anomurans, in particular Upogebia africana and Callianassa krausii, become the preferred prey of fish larger than 300 mm TL. The high degree of diet flexibility that spotted grunter exhibit means that the composition and abundance of the macrobenthos of a particular environment will dictate the diet of the species. Otolith growth zones were found to be deposited annually with the opaque zones being deposited during the austral summer (November - February). The optical definition of annual otolith growth rings differed significantly between geographic regions (namely: Western Cape, South Eastern Cape and KwaZulu-Natal). Growth (sex combined) offish in the Western Cape were best described by the specialised 3 parameter Von Bertalanffy with a relative error structure in the form : Lt = 753(1- e⁻°·¹⁵⁴ ⁽t +¹·⁶¹⁵⁾) while growth in the South Eastern Cape was best described by using the Schnute model with an absolute error structure in the form: Lt=177°·⁴¹⁶ + (676.2⁻¹·²⁶⁶ - 169.2⁻¹²⁶⁶)[1-e°·⁴¹⁶⁽t-t₁⁾ / 1-e°·⁴¹⁶⁽t₂-t₁⁾]¹/¹·²⁶⁶ whereas growth was best described in KwaZulu-Natal using the specialised 3 parameter Von Bertalanffy with a relative error structure in the form: Lt = 839(1-e°·¹⁷⁽t⁺°·⁴⁹⁾) In the South Eastern Cape, length at 50% maturity was found to be 305 mm TL for males. Since females with ripe & running or spent gonads were not found in the South Eastern Cape and since histological evidence suggests that females in the South Eastern Cape have spawned, it appears that spawning does not occur in the South Eastern Cape. These results suggest that adults are resident in the estuaries of the Western and South Eastern Cape and undertake the spring/summer, northward spawning migration to KwaZulu-Natal. After joining the resident spawner stock in KwaZulu-Natal and spawning in the offshore environment of KwaZulu-Natal, adults soon return to the southern regions of their distributional range. Juveniles recruit into KwaZulu-Natal estuaries at a length of 25 - 35 mm TL. A proportion of the eggs and larvae are transported southwards along the periphery of the western boundary Agulhas Current where juveniles (25 - 30 mm TL) recruit into the estuaries as far south as the Swartvlei estuary in the Western Cape. There is evidence to suggest that the fish, which occur in the Western Cape estuaries, have migrated there once they have attained sexual maturity further east.
- Full Text:
- Date Issued: 2003
Feeding ecology of Bathyclarias nyasensis (Siluroidei: Claridae) from Lake Malawi
- Kaunda, Emmanuel Kamlipe Watson Hawkins
- Authors: Kaunda, Emmanuel Kamlipe Watson Hawkins
- Date: 2001
- Subjects: Fishes -- Malawi Fishes -- Nyasa, Lake
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5282 , http://hdl.handle.net/10962/d1005126
- Description: In Malawi, fish contribute about 60-80% to the country's animal protein supply. The greater proportion (> 50%) comes from Lake Malawi. Bathyclarias nyasensis and other clariid catfish contribute up to > 20% of the total catches. Catches of Bathyclarias nyasensis in the inshore area of the south-east arm of Lake Malawi are declining and a management plan for the fishery is essentially lacking. There is paucity of biological data that precludes the use of any option to manage the species. The principal aim of the thesis was to define the ecological role B. nyasensis, the most abundant and common of the Bathyclarias species. By examining life history characteristics within a food web context, it was hypothesized that the study would provide an insight into the interrelationships between species, and, hence form the basis for the development of a rational exploitation strategy for the species. The study was undertaken in the south-east arm of Lake Malawi (9° 30'S, 14° 30'S). The principal objectives of the study were to investigate the feeding ecology of B. nyasensis by examining morphological characters and structures associated with feeding, diet of B. nyasensis, food assimilated in the species using carbon (∂¹³C) isotope analysis, daily food consumption rate for B. nyasensis; and to relate the feeding ecology to life history traits such as age, growth, and some aspects of the reproductive biology of B. nyasensis. The suitability of sectioned pectoral spines and sagittal otoliths to age B. nyasensis was assessed. Due to reabsorption of growth zones with increasing spine lumen diameter with fish size, and the relatively low number of spines that could be aged reliably, only otoliths were used. The maximum age for B. nyasensis was estimated at 14 vears. Growth was best was described by the four parameter Schnute mc: lt ={42+(81¹·⁸ - 42¹·⁸)x1-e⁻°·°⁵⁽t⁻¹⁾}¹/¹·⁸ over 1-e⁻⁽⁻°·°⁵⁾⁽¹¹⁾ for female, lt={41+(98¹·² - 41¹·²)x 1-e⁻°·°²⁽t⁻¹⁾}¹/¹·² over 1-e⁻⁽⁻°·°²⁾⁽¹³⁾ and for male fish. Age-at-50% maturity for females and males were estimated at 7 years and 4 years, respectively. Typically, fish grew rapidly in the first year, but slower during subsequent years. Smaller fish were found inshore while larger fish were found in offshore regions. It was hypothesised that the rapid growth in the first year and slower growth later is a consequence of change in diet from high quality and abundant food source to a more dilute food and that this may be associated with a shift in habitat. Morphological characters associated with feeding were used to predict the food and feeding behaviour of B. nyasensis. The size of premaxillary, vomerine, pharyngeal dental and palatine teeth and premaxillary and vomerine tooth plates suggested the capability of B. nyasensis to handle both large and small prey, with a propensity towards smaller prey in composition to C. gariepinus. The molariform teeth on the vomerine tooth plate suggested that molluscs form part of the diet. The relative gut length (1.27±0.24) suggested omnivory, with an ability to switch between planktivory and piscivory. Buccal cavity volume and filtering area changed with fish size at 500-600 mm TL upon which it was hypothesised that the fish diet changed to planktivory at this size. Detailed diet analysis provided information upon which the above hypotheses could be accepted. Percent Index of Relative Importance (%IRI) and a multi-way contingency table analysis based on log-linear models were used to analyse diet data. Results showed that B. nyasensis is omnivorous, but with a distinct ontogenetic dietary shift from piscivory to zooplanktivory at 500 - 600 mm TL. The increased buccal cavity volume at the same fish size therefore, suggests that B. nyasensis is well adapted to filter the dilute zooplankton resource. Increased foraging costs of feeding on zooplankton explained the slower growth of larger fish. The dietary shift was finally corroborated by results of the ∂¹³C isotope analysis. A polynomial equation described the change in carbon ratios with fish size: ∂¹³C = - 33.188 + 0.4997L - 0.0045 (total length)² (r² = 0.598, n = 12, p=0.022). The ontogenetic shift in diet was synchronised with a habitat shift postulated in life history studies. In the inshore region, B. nyasensis were predominantly piscivorous (apex predators), and were zooplanktivorous in the offshore region, thereby forming part of the pelagic food web in the latter region. After examining "bottom-up" and trophic cascade theories, it was postulated that perturbations of the B. nyasensis stock would be discernible both at the top and lower trophic levels. As a piscivore and therefore apex predator, effects of overfishing B. nyasensis in the inshore region could cascade to unpredictable ecological changes in inshore areas and, due to the ontogenetic habitat shift, in the offshore regions. Examples of trophic cascade phenomena are provided. On the basis of the feeding study, it was possible to reconstruct the pelagic food web of Lake Malawi. Apart from the lakefly Chaoborus edulis, B. nyasensis is the other predator that preys heavily on zooplankton in the pelagic zone. Perturbations of the B. nyasensis stock could affect size composition of zooplankton which in tum, could affect production of C. edulis, a resource for the top predators in the food web. The findings of the present study contributed to the ongoing debate of introducing a zooplanktivore into the pelagic zone of Lake Malawi. Proponents for the introductions have argued that zooplankton predation by fish is inferior to that of C. edulis. Introduction of a clupeid zooplankton was proposed as a strategy to boost fish production in the lake. The zooplanktivore would either out-compete or prey on C. edulis to extinction. Opponents to this view argued that zooplankton biomass in the pelagic region was too low to support introductions and that the fish biomass in the pelagic region may have been underestimated. Results from the present study suggest that planktivorous fish (including B. nyasensis) might not be inferior to C. edulis in utilising the zooplankton resource; B. nyasensis is well adapted to utilise the dilute zooplankton resource, and by omitting B. nyasensis from previous studies, overall zooplankton predation by fish may have been underestimated by between 7 - 33%. On the basis of the theoretical migratory life history cycle of B. nyasensis, it is recommended that the current interest in increasing fishing effort in offshore areas should proceed with caution. Ecological changes that may have occurred in the inshore areas due to overfishing have probably not been noticed: as the offshore zone has never been fished. The latter zone may have acted as a stock refuge area. Higher fishing intensity in the offshore areas could lead to serious ecological imbalances and instability. The study has shown that life history characteristics studied in the context of the food web, and in the absence of other fisheries information and/or data, strongly advocates the precautionary principle to managing changes in exploitation patterns.
- Full Text:
- Date Issued: 2001
- Authors: Kaunda, Emmanuel Kamlipe Watson Hawkins
- Date: 2001
- Subjects: Fishes -- Malawi Fishes -- Nyasa, Lake
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5282 , http://hdl.handle.net/10962/d1005126
- Description: In Malawi, fish contribute about 60-80% to the country's animal protein supply. The greater proportion (> 50%) comes from Lake Malawi. Bathyclarias nyasensis and other clariid catfish contribute up to > 20% of the total catches. Catches of Bathyclarias nyasensis in the inshore area of the south-east arm of Lake Malawi are declining and a management plan for the fishery is essentially lacking. There is paucity of biological data that precludes the use of any option to manage the species. The principal aim of the thesis was to define the ecological role B. nyasensis, the most abundant and common of the Bathyclarias species. By examining life history characteristics within a food web context, it was hypothesized that the study would provide an insight into the interrelationships between species, and, hence form the basis for the development of a rational exploitation strategy for the species. The study was undertaken in the south-east arm of Lake Malawi (9° 30'S, 14° 30'S). The principal objectives of the study were to investigate the feeding ecology of B. nyasensis by examining morphological characters and structures associated with feeding, diet of B. nyasensis, food assimilated in the species using carbon (∂¹³C) isotope analysis, daily food consumption rate for B. nyasensis; and to relate the feeding ecology to life history traits such as age, growth, and some aspects of the reproductive biology of B. nyasensis. The suitability of sectioned pectoral spines and sagittal otoliths to age B. nyasensis was assessed. Due to reabsorption of growth zones with increasing spine lumen diameter with fish size, and the relatively low number of spines that could be aged reliably, only otoliths were used. The maximum age for B. nyasensis was estimated at 14 vears. Growth was best was described by the four parameter Schnute mc: lt ={42+(81¹·⁸ - 42¹·⁸)x1-e⁻°·°⁵⁽t⁻¹⁾}¹/¹·⁸ over 1-e⁻⁽⁻°·°⁵⁾⁽¹¹⁾ for female, lt={41+(98¹·² - 41¹·²)x 1-e⁻°·°²⁽t⁻¹⁾}¹/¹·² over 1-e⁻⁽⁻°·°²⁾⁽¹³⁾ and for male fish. Age-at-50% maturity for females and males were estimated at 7 years and 4 years, respectively. Typically, fish grew rapidly in the first year, but slower during subsequent years. Smaller fish were found inshore while larger fish were found in offshore regions. It was hypothesised that the rapid growth in the first year and slower growth later is a consequence of change in diet from high quality and abundant food source to a more dilute food and that this may be associated with a shift in habitat. Morphological characters associated with feeding were used to predict the food and feeding behaviour of B. nyasensis. The size of premaxillary, vomerine, pharyngeal dental and palatine teeth and premaxillary and vomerine tooth plates suggested the capability of B. nyasensis to handle both large and small prey, with a propensity towards smaller prey in composition to C. gariepinus. The molariform teeth on the vomerine tooth plate suggested that molluscs form part of the diet. The relative gut length (1.27±0.24) suggested omnivory, with an ability to switch between planktivory and piscivory. Buccal cavity volume and filtering area changed with fish size at 500-600 mm TL upon which it was hypothesised that the fish diet changed to planktivory at this size. Detailed diet analysis provided information upon which the above hypotheses could be accepted. Percent Index of Relative Importance (%IRI) and a multi-way contingency table analysis based on log-linear models were used to analyse diet data. Results showed that B. nyasensis is omnivorous, but with a distinct ontogenetic dietary shift from piscivory to zooplanktivory at 500 - 600 mm TL. The increased buccal cavity volume at the same fish size therefore, suggests that B. nyasensis is well adapted to filter the dilute zooplankton resource. Increased foraging costs of feeding on zooplankton explained the slower growth of larger fish. The dietary shift was finally corroborated by results of the ∂¹³C isotope analysis. A polynomial equation described the change in carbon ratios with fish size: ∂¹³C = - 33.188 + 0.4997L - 0.0045 (total length)² (r² = 0.598, n = 12, p=0.022). The ontogenetic shift in diet was synchronised with a habitat shift postulated in life history studies. In the inshore region, B. nyasensis were predominantly piscivorous (apex predators), and were zooplanktivorous in the offshore region, thereby forming part of the pelagic food web in the latter region. After examining "bottom-up" and trophic cascade theories, it was postulated that perturbations of the B. nyasensis stock would be discernible both at the top and lower trophic levels. As a piscivore and therefore apex predator, effects of overfishing B. nyasensis in the inshore region could cascade to unpredictable ecological changes in inshore areas and, due to the ontogenetic habitat shift, in the offshore regions. Examples of trophic cascade phenomena are provided. On the basis of the feeding study, it was possible to reconstruct the pelagic food web of Lake Malawi. Apart from the lakefly Chaoborus edulis, B. nyasensis is the other predator that preys heavily on zooplankton in the pelagic zone. Perturbations of the B. nyasensis stock could affect size composition of zooplankton which in tum, could affect production of C. edulis, a resource for the top predators in the food web. The findings of the present study contributed to the ongoing debate of introducing a zooplanktivore into the pelagic zone of Lake Malawi. Proponents for the introductions have argued that zooplankton predation by fish is inferior to that of C. edulis. Introduction of a clupeid zooplankton was proposed as a strategy to boost fish production in the lake. The zooplanktivore would either out-compete or prey on C. edulis to extinction. Opponents to this view argued that zooplankton biomass in the pelagic region was too low to support introductions and that the fish biomass in the pelagic region may have been underestimated. Results from the present study suggest that planktivorous fish (including B. nyasensis) might not be inferior to C. edulis in utilising the zooplankton resource; B. nyasensis is well adapted to utilise the dilute zooplankton resource, and by omitting B. nyasensis from previous studies, overall zooplankton predation by fish may have been underestimated by between 7 - 33%. On the basis of the theoretical migratory life history cycle of B. nyasensis, it is recommended that the current interest in increasing fishing effort in offshore areas should proceed with caution. Ecological changes that may have occurred in the inshore areas due to overfishing have probably not been noticed: as the offshore zone has never been fished. The latter zone may have acted as a stock refuge area. Higher fishing intensity in the offshore areas could lead to serious ecological imbalances and instability. The study has shown that life history characteristics studied in the context of the food web, and in the absence of other fisheries information and/or data, strongly advocates the precautionary principle to managing changes in exploitation patterns.
- Full Text:
- Date Issued: 2001
An investigation into the dietary requirements of Oreochromis Mossambicus fry and the formulation and preparation of a dry food for use in aquaculture
- Authors: Thorpe, Peter Stuart
- Date: 1989
- Subjects: Cichlids , Tilapia -- Nutrition , Aquaculture
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5184 , http://hdl.handle.net/10962/d1001960 , Cichlids , Tilapia -- Nutrition , Aquaculture
- Description: The need for developing a dry feed which satisfied the nutrient requirements of Oreochromis mossambicus fry was identified. The spawning and grading techniques which resulted in a higher fecundity and ensured uniformity within and between samples of fry are discussed. Preparation techniques were developed which met the physical requirements of fry feeds. An analysis of the flesh and yolk sac fry for amino acid composition was undertaken. The initial test diet was based on these results, as well as on the natural feeding ecology of the species. Feeding trials were undertaken and growth responses monitored to determine the optimum levels of the various dietary components. A feed was developed which gave superior growth to that obtained with natural food organisms. This feed consisted of Torula yeast (47%), Weider Super Protein (44.39%), Vitamin supplement (0.45% - multivitamin), additional vitamin C supplement (0.16%), Spirulina (5%), and methionine supplement (3%). An optimum particle size range for O. mossambicus fry (between 5 & 25mm) was established at 125-200um. The fry should be fed the following feed ratios depending on age: 30.4% body weight/day up to day 5, 30.6% body weight/day up to day 10 and 25.1% body weight/day up to day 15. An optimum feeding frequency of 8/10hr day was recommended. A feed conversion ratio of 1.24:1 and a protein efficiency ratio of 0.682:1 was obtained. Digestable energy of the feed was determined at 16.1 kj/g feed. The established dry feed is suitable for use in commercial hatcheries, and as a formulation for further research on the intensive rearing of O. mossambicus fry.
- Full Text:
- Date Issued: 1989
- Authors: Thorpe, Peter Stuart
- Date: 1989
- Subjects: Cichlids , Tilapia -- Nutrition , Aquaculture
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5184 , http://hdl.handle.net/10962/d1001960 , Cichlids , Tilapia -- Nutrition , Aquaculture
- Description: The need for developing a dry feed which satisfied the nutrient requirements of Oreochromis mossambicus fry was identified. The spawning and grading techniques which resulted in a higher fecundity and ensured uniformity within and between samples of fry are discussed. Preparation techniques were developed which met the physical requirements of fry feeds. An analysis of the flesh and yolk sac fry for amino acid composition was undertaken. The initial test diet was based on these results, as well as on the natural feeding ecology of the species. Feeding trials were undertaken and growth responses monitored to determine the optimum levels of the various dietary components. A feed was developed which gave superior growth to that obtained with natural food organisms. This feed consisted of Torula yeast (47%), Weider Super Protein (44.39%), Vitamin supplement (0.45% - multivitamin), additional vitamin C supplement (0.16%), Spirulina (5%), and methionine supplement (3%). An optimum particle size range for O. mossambicus fry (between 5 & 25mm) was established at 125-200um. The fry should be fed the following feed ratios depending on age: 30.4% body weight/day up to day 5, 30.6% body weight/day up to day 10 and 25.1% body weight/day up to day 15. An optimum feeding frequency of 8/10hr day was recommended. A feed conversion ratio of 1.24:1 and a protein efficiency ratio of 0.682:1 was obtained. Digestable energy of the feed was determined at 16.1 kj/g feed. The established dry feed is suitable for use in commercial hatcheries, and as a formulation for further research on the intensive rearing of O. mossambicus fry.
- Full Text:
- Date Issued: 1989
The taxonomy and life history of the blue stingray, Dasyatis marmorata capensis (Batoidea : Dasyatidae) from Southern Africa
- Cowley, Paul D (Paul Denfer), 1964-
- Authors: Cowley, Paul D (Paul Denfer), 1964-
- Date: 1990
- Subjects: Stingrays -- Africa, Southern Dasyatidae
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5183 , http://hdl.handle.net/10962/d1001959
- Description: The blue stingray, Dasyatis marmorata capensis is an important recreational shore angling species, especially amongst tournament anglers, in South Africa. The taxonomy and aspects of the biology of this species were investigated from specimens collected between March 1987 and May 1989, from various localities along the southern African coastline. Morphological comparisons were made between Dasyatis pastinaca, Dasyatis marmorata and the local variety. The results indicated that the local variety is different to D. pastinaca in colouration, disc length size, and vertebral and radial counts. The proposed new sub-species Dasyatis marmorata capensis is closest to Dasyatis marmorata of the north-eastern Atlantic coast, but differs in snout angle, disc length and snout to vent length. Analysis of catch per unit of effort data revealed that the blue stingray exhibits distinct seasonality. Adult and sub-adult fish undertake an annual onshore/offshore migration and invade the inshore region between spring and summer, while juveniles remain offshore and utilize this zone as a nursery area. Length-at-age and growth rates for both sexes were estimated from bands formed seasonally in the vertebral centra. The derived von Bertalanffy growth parameters indicated that females reach a greater asymptotic size (913.8 mm DW) and have a slower growth rate (K = -0.07) than males (531.8 mm DW, K = -0.17). Growth rates of juvenile captive specimens compared favourably with back calculated values, at approximately 65 mm/year, for the first year of growth. Dasyatis marmorata capensis is an aplacental viviparous elasmobranch with a well defined annual breeding cycle. Young are released , at a size of 170-200 mm DW, from October to December after a gestation period of approximately nine months. An average fecundity of 3.07 was calculated . Examination of gonadal development indicated that sexual maturity is first attained at a size of 408 and 500 mm DW in males and females, respectively, corresponding to ages 4.5 and 7 years. Size-related food habits were determined independently from three depth zones. The major prey items for each size class were directly related to the abundance of the infauna of the different depth zones. Polychaeta, amphipoda and stomatopoda were the major prey items in the offshore zone. In the nearshore zone, the dominant prey species were Balanoglossus capensis and Callianassa spp .. The high incidence of empty stomachs and the presence of prey items which occur in greatest abundance beyond the surf zone indicated that this zone is not utilized for the purposes of feeding.
- Full Text:
- Date Issued: 1990
- Authors: Cowley, Paul D (Paul Denfer), 1964-
- Date: 1990
- Subjects: Stingrays -- Africa, Southern Dasyatidae
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5183 , http://hdl.handle.net/10962/d1001959
- Description: The blue stingray, Dasyatis marmorata capensis is an important recreational shore angling species, especially amongst tournament anglers, in South Africa. The taxonomy and aspects of the biology of this species were investigated from specimens collected between March 1987 and May 1989, from various localities along the southern African coastline. Morphological comparisons were made between Dasyatis pastinaca, Dasyatis marmorata and the local variety. The results indicated that the local variety is different to D. pastinaca in colouration, disc length size, and vertebral and radial counts. The proposed new sub-species Dasyatis marmorata capensis is closest to Dasyatis marmorata of the north-eastern Atlantic coast, but differs in snout angle, disc length and snout to vent length. Analysis of catch per unit of effort data revealed that the blue stingray exhibits distinct seasonality. Adult and sub-adult fish undertake an annual onshore/offshore migration and invade the inshore region between spring and summer, while juveniles remain offshore and utilize this zone as a nursery area. Length-at-age and growth rates for both sexes were estimated from bands formed seasonally in the vertebral centra. The derived von Bertalanffy growth parameters indicated that females reach a greater asymptotic size (913.8 mm DW) and have a slower growth rate (K = -0.07) than males (531.8 mm DW, K = -0.17). Growth rates of juvenile captive specimens compared favourably with back calculated values, at approximately 65 mm/year, for the first year of growth. Dasyatis marmorata capensis is an aplacental viviparous elasmobranch with a well defined annual breeding cycle. Young are released , at a size of 170-200 mm DW, from October to December after a gestation period of approximately nine months. An average fecundity of 3.07 was calculated . Examination of gonadal development indicated that sexual maturity is first attained at a size of 408 and 500 mm DW in males and females, respectively, corresponding to ages 4.5 and 7 years. Size-related food habits were determined independently from three depth zones. The major prey items for each size class were directly related to the abundance of the infauna of the different depth zones. Polychaeta, amphipoda and stomatopoda were the major prey items in the offshore zone. In the nearshore zone, the dominant prey species were Balanoglossus capensis and Callianassa spp .. The high incidence of empty stomachs and the presence of prey items which occur in greatest abundance beyond the surf zone indicated that this zone is not utilized for the purposes of feeding.
- Full Text:
- Date Issued: 1990
Aspects of the biology of the musselcracker, Sparodon Durbanensis, and the bronze bream, Pachymetopon grande, (Pisces : Sparidae), with notes on the Eastern Cape recreational rock-angling and spear fisheries
- Authors: Clarke, John Ross
- Date: 1988
- Subjects: Sparidea -- South Africa Sparodon Durbanensis Bream fishing
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5190 , http://hdl.handle.net/10962/d1001966
- Description: The musselcracker, Soarodon durbanensis, and the bronze bream, Pachymetopon arande, are important components of rock-angling and spearfishing catches In the Eastern Cape. Aspects of their biology, including age and growth, reproduction, nursery areas and feeding, were Investigated to provide a basis for the rational exploitation of the resource. Both species are slow-growing and long-lived. Growth in S. durbanensls was described by the von Bertalanffy growth equation: Lt(mm) = 1021.2(1-e⁻°.°⁹°°⁽t ⁻° .⁷°⁹⁾). The von Bertalanffy growth model did not describe the total data set in P. grande, but by excluding the zero and one year old fish the data were described by: Lt(mm) = 461.1(1-e⁻°.¹⁵³⁽t ⁺¹.⁶⁴°⁾). Detailed histological examination of gonadal development showed that S. durbanensis and P. grande are rudimentary hermaphrodites. Both species had restricted breeding seasons which coincided with peak exploitation. Results indicated that both species are group spawners with pelagic eggs. Size at 50% maturity in S. durbanensis and P. grande was determined at 350 and 300mm fork length, respectively, corresponding to ages of 4.5 and 5.5 years. Intertidal pools functioned as nursery areas for S. durbanensis during the first year, whereas subtidal waters were more important for larger juveniles. P. grande juveniles were commonly observed on subtidal weed beds to a depth of 10m. A size-related change in the diet of S. durbanensis was observed. The juveniles were omnivorous, feeding predominantly on gastropods, echinoids and chlorophytes. The adults fed on a variety of large, reef-associated invertebrates. P. grande were omnivorous, with macroalgae being the principal dietary component. This species appears to utilise the storage and extracellular carbohydrates of the macroalgae. Results showed that macroalgal degradation by gut endosymbionts and the utilisation of macroalgal epibionts does not appear to occur in this species. A detailed analysis of catch composition and catch per unit of effort In the Port Elizabeth rock-angling fishery and the Eastern Cape spearfishery emphasised the recreational importance of S. durbanensis and P. grande. The localised distribution, slow growth and late maturation in both species results in their being susceptible to over-exploitation in these size-selective fisheries. Current legislation protecting these fish is discussed in relation to the findings of this study and additional management measures are proposed.
- Full Text:
- Date Issued: 1988
- Authors: Clarke, John Ross
- Date: 1988
- Subjects: Sparidea -- South Africa Sparodon Durbanensis Bream fishing
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5190 , http://hdl.handle.net/10962/d1001966
- Description: The musselcracker, Soarodon durbanensis, and the bronze bream, Pachymetopon arande, are important components of rock-angling and spearfishing catches In the Eastern Cape. Aspects of their biology, including age and growth, reproduction, nursery areas and feeding, were Investigated to provide a basis for the rational exploitation of the resource. Both species are slow-growing and long-lived. Growth in S. durbanensls was described by the von Bertalanffy growth equation: Lt(mm) = 1021.2(1-e⁻°.°⁹°°⁽t ⁻° .⁷°⁹⁾). The von Bertalanffy growth model did not describe the total data set in P. grande, but by excluding the zero and one year old fish the data were described by: Lt(mm) = 461.1(1-e⁻°.¹⁵³⁽t ⁺¹.⁶⁴°⁾). Detailed histological examination of gonadal development showed that S. durbanensis and P. grande are rudimentary hermaphrodites. Both species had restricted breeding seasons which coincided with peak exploitation. Results indicated that both species are group spawners with pelagic eggs. Size at 50% maturity in S. durbanensis and P. grande was determined at 350 and 300mm fork length, respectively, corresponding to ages of 4.5 and 5.5 years. Intertidal pools functioned as nursery areas for S. durbanensis during the first year, whereas subtidal waters were more important for larger juveniles. P. grande juveniles were commonly observed on subtidal weed beds to a depth of 10m. A size-related change in the diet of S. durbanensis was observed. The juveniles were omnivorous, feeding predominantly on gastropods, echinoids and chlorophytes. The adults fed on a variety of large, reef-associated invertebrates. P. grande were omnivorous, with macroalgae being the principal dietary component. This species appears to utilise the storage and extracellular carbohydrates of the macroalgae. Results showed that macroalgal degradation by gut endosymbionts and the utilisation of macroalgal epibionts does not appear to occur in this species. A detailed analysis of catch composition and catch per unit of effort In the Port Elizabeth rock-angling fishery and the Eastern Cape spearfishery emphasised the recreational importance of S. durbanensis and P. grande. The localised distribution, slow growth and late maturation in both species results in their being susceptible to over-exploitation in these size-selective fisheries. Current legislation protecting these fish is discussed in relation to the findings of this study and additional management measures are proposed.
- Full Text:
- Date Issued: 1988
The dynamics of a subtropical lake fishery in central Mozambique
- Authors: Weyl, Olaf L F
- Date: 1999
- Subjects: Fisheries -- Chicamba, Lake Fisheries -- Mozambique
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5212 , http://hdl.handle.net/10962/d1004785
- Description: Fisheries in African reservoirs are typically multi -species and in most cases the fish resource is harvested with a number of gears. These characteristics complicate their management and the development of management procedures. Typically, long time series of data on catch and effort and length- or age-based catch are not available for these fisheries. This precludes the use of data intensive methods such as multi-species virtual population analysis. The principal aim of this thesis was to develop a management procedure for African reservoir fisheries that takes into account the pertinent biological characteristics of the target species and accounts for the multi-species and multi-gear irIteractions in such fisheries. An opportunity availed itself to undertake this work on Lake ChicaIllba (19°08'S 33°08'E) a man-made hydroelectric dam in subtropical Mozambique (Manica province). The specific objectives of this study were: to obtain locality specific biological parameters for the target species in Lake ChicaIllba; to assess gear utilisation trends in the fishery through the determination of gear-selectivity, catch rate and effort for each of the principal gears used in the fishery; to assess the fishery using traditional per-recruit models and to test existing and new per-recruit models that account for the multi-species and multi-gear nature of the fishery and to determine the adequacy of each of these approaches in the determination of suitable target reference point (TRP) exploitation rates. The three principal specIes in Lake Chicamba are the introduced largemouth bass, Micropterus salmoides, and two cichlids the Mozambique tilapia, Oreochromis mossambicus and the redbreast tilapia, Tilapia rendalli. Sectioned otoliths were used for age and growth determination. Marginal zone analysis showed that annulus formation in all three species occurred during winter. The maximum-recorded age was 5 years for M. salmoides, 16 years for T. rendalli and 10 years for O. mossambicus. Growth of the three species was best described by the 3 parameter von Bertalanffy growth model as ℓa = 465.51 (1 - e⁻ₑ·ₑ⁷⁵⁽a⁺⁰·⁰⁰⁹⁾ mm FL for M salmoides; ℓa = 238.74 (1 - e⁻⁰⁶³⁶⁽a⁺⁰·⁹⁰⁵⁾) mm TL for T. rendalli; and ℓa = 266.06 (1 - e⁻⁰⁷⁹⁰⁴⁽a⁺⁰·²⁶⁹⁾) mm TL for 0. mossambicus. Female T. rendalli attained 50%-maturity at 2.89 years, while O. mossambicus matured at 2.83 years and M. salmoides at 0.9 years. Both cichlid species spawned throughout summer while M. salmoides had a very short spawning season from August to September. The total annual mortality rate (Z) for M. salmoides in Lake Chicamba was 1.27 yr⁻¹, the mean empirical estimate of natural mortality (M) was 0.73 yr⁻¹, and fishing mortality (F) was calculated at 0.54 yr⁻¹. For T. rendalli Z = 0.31 yr⁻¹, M = 0.20 yr⁻¹, F = 0.11 yr⁻¹ and for 0. mossambicus Z= 0.62 yr⁻¹, M= 0.38 yr⁻¹, F= 0.24 yr⁻¹. The three species exhibited reproductive traits, which implied a high reliance of recruitment on spawner stock (nest guarding in T. rendalli and M. salmoides and mouthbrooding in O. mossambicus). For this reason it was decided that the cichlid fisheries should be managed using TRPs which maintained the spawner biomass-per-recruit at 50% (FSB50) of pristine levels. However, based on good evidence it was hypothesised that the high rate of fishing mortality helped to maintain the fast growth rate of M. salmoides. It was, therefore, decided to manage this species at a TRP of F SB40. The three most important fishing sectors were the gill-net, seine-net and hook-and-line fisheries. The total catch for 1996 was 223 t. The gill nets selected all three species at a size/age approximating 50%-maturity while the seine-net and hook-and-line fisheries selected mainly juvenile fishes. There was strong evidence to suggest that seine net fishing also disrupted spawning. It was shown that the 'traditional' single-species per-recruit models were unsuitable to assess multi-species and multi-gear reservoir fisheries. Since existing multi-species/multifishery yield-per-recruit models were not capable of defining FsB(x) TRPs, a new multispecies/ multi-fishery spawner-biomass-per-recruit approach was developed. This approach allowed for the simulation of the response of spawner biomass-per-recruit to changes in effort in the three fishery sectors, simultaneously. The models showed that the spawner biomass-per-recruit, at current effort levels, was higher than the suggested TRP for the three species. However, it was shown that an increase of 10% in current total effort would reduce spawner biomass-per-recruit to below the recommended TRP levels. With the closure of the seine-net fishery, gill-net effort could be increased to 338 fishers (340 for management purposes) and effort in the hook-and-line fishery could be increased by 30% before the TRP was reached. To maintain the fish stocks above TRP levels, effort control was considered to be the most effective management method. The main recommendations for Lake Chicamba were to close the seine-net fishery, to limit the gill-net fishery to 340 fishers (using 137-m long x 3-m deep gill nets) and to maintain the open access nature of the hook-and-line fishery. The multi-species/multi-fishery per-recruit approach allows for the meaningful simulation of various scenarios and provides relatively robust management options. In the absence of long time series of effort and age- or length-based catch data, this approach was considered as the most suitable assessment method for multi-species/multi-gear African reservoir fisheries.
- Full Text:
- Date Issued: 1999
- Authors: Weyl, Olaf L F
- Date: 1999
- Subjects: Fisheries -- Chicamba, Lake Fisheries -- Mozambique
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5212 , http://hdl.handle.net/10962/d1004785
- Description: Fisheries in African reservoirs are typically multi -species and in most cases the fish resource is harvested with a number of gears. These characteristics complicate their management and the development of management procedures. Typically, long time series of data on catch and effort and length- or age-based catch are not available for these fisheries. This precludes the use of data intensive methods such as multi-species virtual population analysis. The principal aim of this thesis was to develop a management procedure for African reservoir fisheries that takes into account the pertinent biological characteristics of the target species and accounts for the multi-species and multi-gear irIteractions in such fisheries. An opportunity availed itself to undertake this work on Lake ChicaIllba (19°08'S 33°08'E) a man-made hydroelectric dam in subtropical Mozambique (Manica province). The specific objectives of this study were: to obtain locality specific biological parameters for the target species in Lake ChicaIllba; to assess gear utilisation trends in the fishery through the determination of gear-selectivity, catch rate and effort for each of the principal gears used in the fishery; to assess the fishery using traditional per-recruit models and to test existing and new per-recruit models that account for the multi-species and multi-gear nature of the fishery and to determine the adequacy of each of these approaches in the determination of suitable target reference point (TRP) exploitation rates. The three principal specIes in Lake Chicamba are the introduced largemouth bass, Micropterus salmoides, and two cichlids the Mozambique tilapia, Oreochromis mossambicus and the redbreast tilapia, Tilapia rendalli. Sectioned otoliths were used for age and growth determination. Marginal zone analysis showed that annulus formation in all three species occurred during winter. The maximum-recorded age was 5 years for M. salmoides, 16 years for T. rendalli and 10 years for O. mossambicus. Growth of the three species was best described by the 3 parameter von Bertalanffy growth model as ℓa = 465.51 (1 - e⁻ₑ·ₑ⁷⁵⁽a⁺⁰·⁰⁰⁹⁾ mm FL for M salmoides; ℓa = 238.74 (1 - e⁻⁰⁶³⁶⁽a⁺⁰·⁹⁰⁵⁾) mm TL for T. rendalli; and ℓa = 266.06 (1 - e⁻⁰⁷⁹⁰⁴⁽a⁺⁰·²⁶⁹⁾) mm TL for 0. mossambicus. Female T. rendalli attained 50%-maturity at 2.89 years, while O. mossambicus matured at 2.83 years and M. salmoides at 0.9 years. Both cichlid species spawned throughout summer while M. salmoides had a very short spawning season from August to September. The total annual mortality rate (Z) for M. salmoides in Lake Chicamba was 1.27 yr⁻¹, the mean empirical estimate of natural mortality (M) was 0.73 yr⁻¹, and fishing mortality (F) was calculated at 0.54 yr⁻¹. For T. rendalli Z = 0.31 yr⁻¹, M = 0.20 yr⁻¹, F = 0.11 yr⁻¹ and for 0. mossambicus Z= 0.62 yr⁻¹, M= 0.38 yr⁻¹, F= 0.24 yr⁻¹. The three species exhibited reproductive traits, which implied a high reliance of recruitment on spawner stock (nest guarding in T. rendalli and M. salmoides and mouthbrooding in O. mossambicus). For this reason it was decided that the cichlid fisheries should be managed using TRPs which maintained the spawner biomass-per-recruit at 50% (FSB50) of pristine levels. However, based on good evidence it was hypothesised that the high rate of fishing mortality helped to maintain the fast growth rate of M. salmoides. It was, therefore, decided to manage this species at a TRP of F SB40. The three most important fishing sectors were the gill-net, seine-net and hook-and-line fisheries. The total catch for 1996 was 223 t. The gill nets selected all three species at a size/age approximating 50%-maturity while the seine-net and hook-and-line fisheries selected mainly juvenile fishes. There was strong evidence to suggest that seine net fishing also disrupted spawning. It was shown that the 'traditional' single-species per-recruit models were unsuitable to assess multi-species and multi-gear reservoir fisheries. Since existing multi-species/multifishery yield-per-recruit models were not capable of defining FsB(x) TRPs, a new multispecies/ multi-fishery spawner-biomass-per-recruit approach was developed. This approach allowed for the simulation of the response of spawner biomass-per-recruit to changes in effort in the three fishery sectors, simultaneously. The models showed that the spawner biomass-per-recruit, at current effort levels, was higher than the suggested TRP for the three species. However, it was shown that an increase of 10% in current total effort would reduce spawner biomass-per-recruit to below the recommended TRP levels. With the closure of the seine-net fishery, gill-net effort could be increased to 338 fishers (340 for management purposes) and effort in the hook-and-line fishery could be increased by 30% before the TRP was reached. To maintain the fish stocks above TRP levels, effort control was considered to be the most effective management method. The main recommendations for Lake Chicamba were to close the seine-net fishery, to limit the gill-net fishery to 340 fishers (using 137-m long x 3-m deep gill nets) and to maintain the open access nature of the hook-and-line fishery. The multi-species/multi-fishery per-recruit approach allows for the meaningful simulation of various scenarios and provides relatively robust management options. In the absence of long time series of effort and age- or length-based catch data, this approach was considered as the most suitable assessment method for multi-species/multi-gear African reservoir fisheries.
- Full Text:
- Date Issued: 1999
The nutritional requirements of Haliotis midae and development of a practical diet for abalone aquaculture
- Authors: Britz, Peter Jacobus
- Date: 1996
- Subjects: Abalones -- Nutrition Abalone culture
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5307 , http://hdl.handle.net/10962/d1005152
- Description: The available literature on abalone nutrition was synthesised and the prospects for developing a complete pelleted dry feed for Haliotis midae evaluated. The similar body compositions, digestive structures, enzyme activity, acceptance of a wide variety of feed ingredients and comparable growth performance on formulated diets of various abalone species suggests that they have similar nutritional requirements. Abalone also appear to be similar to other farmed monogastric animals in that digestion is primarily extracelluar and they possess a large, muscular crop and stomach. Abalone energy metabolism is carbohydrate based. They are capable of digesting high levels of dietary protein but their ability to utilise fat is limited. Abalone have been shown to consume predictable amounts of dry feed which is efficiently converted into body weight. Efficiency indices of utilisation of formulated feeds, such as feed conversion ratio, protein efficiency ratio, net protein utilisation and apparent digestibility have been successfully applied to abalone. It was concluded that the prospects for developing complete diets for H. midae using a conventional animal feed science model were good. The ability of H. midae to utilise a range of proteins which had potential for inclusion in practical diets was investigated. Five protein rich ingredients, namely, casein, fishmeal, soya oil cake, Spirulina, and torula yeast, were fed to H. midae in semi-purified diets. Two algal diets, fresh Plocamium corallorhiza and dried Ecklonia maxima were fed as controls. Abalone fed on fishmeal and Spirulina based diets displayed significantly higher growth rates than diets containing soya oil cake, torula yeast, casein and E. maxima. Growth rates of abalone fed with P. corallorhiza were significantly lower than all other diets. All artificial diets yield lower feed conversion ratios (0.7-1.8) and higher protein efficiency ratios (3.3-6.5) ratios than the seaweed control diets (FCR = 2.8-3.4; PER = 2.2-3.0). The results indicated that fishmeal and Spirulina were the most suitable proteins for inclusion in practical diets for H. midae. The effect of protein level on growth rate and nutritional indices was evaluated by feeding starch bound, fishmeal based diets containing 27, 32, 37, 42 and 47% protein to juvenile H. midae. Weight gain was positively related to the level of dietary protein, increasing by 18% between 27% and 47% protein. Protein efficiency ratio declining from 3.2 to 2.3 with an increasing dietary protein content. Feed consumption rate was approximately 1% of body weight per day for all diets. Post-weaning abalone (ca. lOmm shell length, 0.2g) differed from larger juveniles (ca. 35mm, 8g) in their response to varying proportions of dietary protein and energy. The smaller animals appeared to have a lower protein requirement and poorer ability to utilise lipid than the larger juveniles. Proximate analyses revealed that the levels of protein, lipid and carbohydrate in abalone soft tissue increased with increasing dietary levels of these nutrients. Larger juveniles contained significantly higher levels of protein and carbohydrate, but lower levels of lipid, ash and moisture, than the smaller post-weaning abalone. The assimilation efficiency of [U-¹⁴C]-arginine by H. midae fed diets enriched with [U-¹⁴C]-arginine was only 0.45%. Furthennore, supplementation of diets with graded levels of crystalline arginine did not have any effect on growth rates. It was concluded that the prospects for defining the quantitative amino acid requirements of H. midae using crystalline amino acids are not promising. Rates of gastric evacuation and enzyme secretion were monitored in juvenile H. midae fed an extruded, fishmeal based dry feed. Gut fullness peaked 6h after feed was offered and the bulk of feed consumed was digested within 24 h. Enzyme secretion appeared to begin with the onset of feeding and continued for at least 6h after peak gut fullness was attained. Protease activity increased significantly following ingestion but amylase activity was maintained at a more or less constant level. A low level of lipase activity was observed suggesting that the ability of H. midae to digest fat is limited. The growth rate of H. midae fed an extruded, fishmeal based feed increased with increasing temperature between 12°C and 20°C. Between 20°C and 24°C a marked decline in growth rate accompanied by a deterioration in feed conversion and increased mortality was observed. Consumption of a dry pelleted feed was shown to be a function of body size and temperature. Based on these data a model which predicts a daily ration for H. midae was developed. The present study showed that H. midae efficiently utilises extruded dry feeds containing conventional feed ingredients. Although technical difficulties were encountered in measuring apparent digestibility, it was concluded that the prospects for developing practical diets according to established nutritional principles are promising.
- Full Text:
- Date Issued: 1996
- Authors: Britz, Peter Jacobus
- Date: 1996
- Subjects: Abalones -- Nutrition Abalone culture
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5307 , http://hdl.handle.net/10962/d1005152
- Description: The available literature on abalone nutrition was synthesised and the prospects for developing a complete pelleted dry feed for Haliotis midae evaluated. The similar body compositions, digestive structures, enzyme activity, acceptance of a wide variety of feed ingredients and comparable growth performance on formulated diets of various abalone species suggests that they have similar nutritional requirements. Abalone also appear to be similar to other farmed monogastric animals in that digestion is primarily extracelluar and they possess a large, muscular crop and stomach. Abalone energy metabolism is carbohydrate based. They are capable of digesting high levels of dietary protein but their ability to utilise fat is limited. Abalone have been shown to consume predictable amounts of dry feed which is efficiently converted into body weight. Efficiency indices of utilisation of formulated feeds, such as feed conversion ratio, protein efficiency ratio, net protein utilisation and apparent digestibility have been successfully applied to abalone. It was concluded that the prospects for developing complete diets for H. midae using a conventional animal feed science model were good. The ability of H. midae to utilise a range of proteins which had potential for inclusion in practical diets was investigated. Five protein rich ingredients, namely, casein, fishmeal, soya oil cake, Spirulina, and torula yeast, were fed to H. midae in semi-purified diets. Two algal diets, fresh Plocamium corallorhiza and dried Ecklonia maxima were fed as controls. Abalone fed on fishmeal and Spirulina based diets displayed significantly higher growth rates than diets containing soya oil cake, torula yeast, casein and E. maxima. Growth rates of abalone fed with P. corallorhiza were significantly lower than all other diets. All artificial diets yield lower feed conversion ratios (0.7-1.8) and higher protein efficiency ratios (3.3-6.5) ratios than the seaweed control diets (FCR = 2.8-3.4; PER = 2.2-3.0). The results indicated that fishmeal and Spirulina were the most suitable proteins for inclusion in practical diets for H. midae. The effect of protein level on growth rate and nutritional indices was evaluated by feeding starch bound, fishmeal based diets containing 27, 32, 37, 42 and 47% protein to juvenile H. midae. Weight gain was positively related to the level of dietary protein, increasing by 18% between 27% and 47% protein. Protein efficiency ratio declining from 3.2 to 2.3 with an increasing dietary protein content. Feed consumption rate was approximately 1% of body weight per day for all diets. Post-weaning abalone (ca. lOmm shell length, 0.2g) differed from larger juveniles (ca. 35mm, 8g) in their response to varying proportions of dietary protein and energy. The smaller animals appeared to have a lower protein requirement and poorer ability to utilise lipid than the larger juveniles. Proximate analyses revealed that the levels of protein, lipid and carbohydrate in abalone soft tissue increased with increasing dietary levels of these nutrients. Larger juveniles contained significantly higher levels of protein and carbohydrate, but lower levels of lipid, ash and moisture, than the smaller post-weaning abalone. The assimilation efficiency of [U-¹⁴C]-arginine by H. midae fed diets enriched with [U-¹⁴C]-arginine was only 0.45%. Furthennore, supplementation of diets with graded levels of crystalline arginine did not have any effect on growth rates. It was concluded that the prospects for defining the quantitative amino acid requirements of H. midae using crystalline amino acids are not promising. Rates of gastric evacuation and enzyme secretion were monitored in juvenile H. midae fed an extruded, fishmeal based dry feed. Gut fullness peaked 6h after feed was offered and the bulk of feed consumed was digested within 24 h. Enzyme secretion appeared to begin with the onset of feeding and continued for at least 6h after peak gut fullness was attained. Protease activity increased significantly following ingestion but amylase activity was maintained at a more or less constant level. A low level of lipase activity was observed suggesting that the ability of H. midae to digest fat is limited. The growth rate of H. midae fed an extruded, fishmeal based feed increased with increasing temperature between 12°C and 20°C. Between 20°C and 24°C a marked decline in growth rate accompanied by a deterioration in feed conversion and increased mortality was observed. Consumption of a dry pelleted feed was shown to be a function of body size and temperature. Based on these data a model which predicts a daily ration for H. midae was developed. The present study showed that H. midae efficiently utilises extruded dry feeds containing conventional feed ingredients. Although technical difficulties were encountered in measuring apparent digestibility, it was concluded that the prospects for developing practical diets according to established nutritional principles are promising.
- Full Text:
- Date Issued: 1996
A life-history approach to the biology of Oreochromis mossambicus (Pisces : Cichlidae) in the Eastern Cape, South Africa
- Authors: James, Nick, 1955-
- Date: 1990
- Subjects: Cichlids -- South Africa -- Eastern Cape
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5187 , http://hdl.handle.net/10962/d1001963
- Description: The phenotypic plasticity of the cichlid Oreochromis mossambicus was examined in fresh and brackish water populations living in the eastern Cape near the southern extremity of the natural range. The primary goal of the study was to examine the wide variety of life-history styles which were expressed in relation to the varying environmental conditions. Four natural populations were studied during 1986 and 1987, and some experimental work under artificial conditions was carried out at the Rhodes University fishfarm during 1988. This comparative approach gave insights into the complex interactions between biotic and abiotic factors which are exploited by this hardy species. O. mossambicus displayed a wide variety of life-history tactics under harsh environmental conditions. The growth of juvenile fish was rapid and mortality of the young-of-the-year was high. Age at first onset of sexual maturity ranged from one to three years. Under harsh conditions growth rate was markedly reduced after maturity, which occurred during the second or third summer and growth disparity between the sexes was more pronounced. Longevity was found to vary from five to ten years for different populations. Under more favourable conditions (resembling the typical "floodplain" habitats of this species at lower latitudes) very rapid juvenile growth, and a high maximum mass of over 2300 g for both sexes was iii achieved. Sexual maturity under these conditions could be delayed until the third or fourth summer of life. O. mossambicus demonstrated an ability to withstand and even benefit from fluctuating water levels, and adjusted its life style according to the prevailing conditions: at times of high water level recruitment was low and sexual maturity delayed. During low water periods reproductive rates increased. Q. mossambicus showed a distinct preference for gentle gradients and well-vegetated habitats for feeding, nesting and refuge. During the study winter temperatures were near and at some times below the generally accepted lower limits for this species. This implies that these indigenous populations have a degree of cold tolerance. A further implication is that Q. mossambicus has the ability to thrive under adverse conditions which may partially explain its wide and successful colonisation of water bodies far from its natural range, and its obvious fisheries potential.
- Full Text:
- Date Issued: 1990
- Authors: James, Nick, 1955-
- Date: 1990
- Subjects: Cichlids -- South Africa -- Eastern Cape
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5187 , http://hdl.handle.net/10962/d1001963
- Description: The phenotypic plasticity of the cichlid Oreochromis mossambicus was examined in fresh and brackish water populations living in the eastern Cape near the southern extremity of the natural range. The primary goal of the study was to examine the wide variety of life-history styles which were expressed in relation to the varying environmental conditions. Four natural populations were studied during 1986 and 1987, and some experimental work under artificial conditions was carried out at the Rhodes University fishfarm during 1988. This comparative approach gave insights into the complex interactions between biotic and abiotic factors which are exploited by this hardy species. O. mossambicus displayed a wide variety of life-history tactics under harsh environmental conditions. The growth of juvenile fish was rapid and mortality of the young-of-the-year was high. Age at first onset of sexual maturity ranged from one to three years. Under harsh conditions growth rate was markedly reduced after maturity, which occurred during the second or third summer and growth disparity between the sexes was more pronounced. Longevity was found to vary from five to ten years for different populations. Under more favourable conditions (resembling the typical "floodplain" habitats of this species at lower latitudes) very rapid juvenile growth, and a high maximum mass of over 2300 g for both sexes was iii achieved. Sexual maturity under these conditions could be delayed until the third or fourth summer of life. O. mossambicus demonstrated an ability to withstand and even benefit from fluctuating water levels, and adjusted its life style according to the prevailing conditions: at times of high water level recruitment was low and sexual maturity delayed. During low water periods reproductive rates increased. Q. mossambicus showed a distinct preference for gentle gradients and well-vegetated habitats for feeding, nesting and refuge. During the study winter temperatures were near and at some times below the generally accepted lower limits for this species. This implies that these indigenous populations have a degree of cold tolerance. A further implication is that Q. mossambicus has the ability to thrive under adverse conditions which may partially explain its wide and successful colonisation of water bodies far from its natural range, and its obvious fisheries potential.
- Full Text:
- Date Issued: 1990
An assessment of the monkfish resource of Namibia
- Authors: Maartens, Lima
- Date: 2000
- Subjects: Lophius -- Namibia Lophius
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5231 , http://hdl.handle.net/10962/d1005074
- Description: Two monkfish species are caught off the coast of Namibia. These are Lophius vomerinus Valenciennes (1837) and L. vaillanti Regan (1903). L. vomerinus is the more abundant of the two and contributes approximately 99% to the total Namibian monkfish landings. The Namibian monkfish fishery is one of the largest and most valuable of its kind in the world. Two fishing fleets harvest the resource, viz. the monkfish and sole fleet and the hake directed fleet. In 1998 and 1999, monkfish were also harvested by one gillnet vessel that had an experimental right. In this study, the biology, abundance and exploitation patterns of monkfish were investigated and used as inputs into a stock assessment framework as a contribution towards the development of a management procedure for this species. Ground and sectioned otoliths and sectioned illicia were used to age Lophius vomerinus. The illicia provided more precise estimates of age, and growth was subsequently modelled using these age estimates. Growth in length and weight was best described by the threeparameter specialised von Bertalanffy model as Lt (cm TL) = 72.29 (1-e⁻°·¹⁴⁽t⁺°·³°⁾) and Wt (kg) = 3.96 (1-e⁻°·¹⁸⁽t⁺°·¹°⁾)²·⁹⁹ for males, and Lt (cm TL) = 111.98 (1-e⁻°·°⁸⁽t⁺°·³⁶⁾) and Wt (kg) = 6.92 (1-e⁻°·¹⁸⁽t⁻°·⁶⁴⁾)³·°³ for females. L. vomerinus is a slow-growing species with a life span in excess of 10 years. Reproductive activity was shown to occur throughout the year, with a slight peak during the winter months. The adult sex ratio of fish > 37 cm TL was biased towards females. Length-at-50% sexual maturity was attained at 32.1 cm TL for females and 23.7 cm TL for males. The rate of natural mortality was estimated at 0.15 year⁻¹, while fishing mortality rates during the 1970s, 1980s and 1990s were estimated at 0.01 year⁻¹, 0.04 year⁻¹ and 0.14 year⁻¹, respectively. Gillnetting for monkfish (300 mm stretched mesh) was highly efficient with a moderate bycatch of around 20% during the two years of operation. The main bycatch species were red crab, spider crab, squalid sharks, rays and Cape and Deep-water hake. The mean length of the monkfish caught in gillnets (67 cm TL) was significantly larger than the monkfish landed by the trawlers (38 cm TL) and less than 1% of immature fish were landed. Gillnet catch-per-unit-effort for monkfish fluctuated between 0.03 and 0.67 kg.day⁻¹.50 m net panel⁻¹, with a soak time of between one and sixteen days. More than 50% (by weight) of monkfish landed by monkfish and sole trawlers, consisted of fish below 36 cm TL. There was a significant increase in catches of juvenile monkfish during 1997 and 1998 in comparison to the period 1994 to 1996. Various types of rigid sorting grids were tested to release juvenile monkfish below 32 cm TL. Five grid designs were tested. These included an “Ex-it” grid with horizontal bars spaced at 55 mm, single grids with vertical and horizontal bars spaced at 55 mm and grids with circular openings of 110 and 168 mm in diameter. The most efficient design was the grid with circular openings of 110 cm in diameter, which ensured the escape of 66% of monkfish smaller than 31 cm TL. However, studies need to be undertaken to quantify the survival of released fish and to test the feasibility of using grid sorters on commercial monkfish and sole trawling gear. The monkfish resource was assessed by means of length cohort analyses, the Thompson and Bell predictive model and by way of a deterministic age-structured production modelling approach. The length cohort analysis models were sensitive to the rate of natural mortality and insensitive to changes in the terminal fishing mortality rate. These biases may, however, not be serious provided that estimates of abundance are used to reflect relative changes. Fish ranging between 26 and 59 cm TL are the most heavily exploited. The Thompson and Bell model predicted that the monkfish resource is exploited above MSY -levels and a reduction of approximately 40% in fishing effort would provide a higher yield. Yield-per-recruit ranged between 10 000 and 14 000 tonnes. Results should, however, be treated with caution, as the condition of steady state was not satisfied. The age-structured production model was tuned using trends in catch-per-unit-effort data, estimated by Generalised Linear Modeling, as well as relative abundance indices calculated from hake biomass surveys. The model was found to be sensitive to both the ‘steepness’ parameter h and estimates of natural mortality. The ‘depletion’ level of the monkfish resource is currently estimated to be 49%. Estimated coefficients of variation were high (> 63%) due to the lack of a consistent trend within the abundance indices to tune the model. Overall productivity of the monkfish resource was estimated to be approximately 16%, similar to other southern African demersal resources. Results of the risk analyses suggest that catches in excess of 7 000 tonnes may be unsustainable and that catches of 5 000 or 6 000 tonnes would decrease the risk of stock collapse and possibly lead to a recovery in the stock. Monkfish management strategies were reviewed and these were considered in relation to the results of this study. The following management recommendations were made: to follow the precautionary approach and implement a total allowable catch for monkfish; to implement rigid sorting grids as these would be the most appropriate way in which to reduce catches of juvenile monkfish; to restrict soak time, depth of operation and implement means to reduce ‘ghost fishing’ by gillnetting and finally, to develop a management procedure for Namibian monkfish with the main objective being the sustainable exploitation of the resource.
- Full Text:
- Date Issued: 2000
- Authors: Maartens, Lima
- Date: 2000
- Subjects: Lophius -- Namibia Lophius
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5231 , http://hdl.handle.net/10962/d1005074
- Description: Two monkfish species are caught off the coast of Namibia. These are Lophius vomerinus Valenciennes (1837) and L. vaillanti Regan (1903). L. vomerinus is the more abundant of the two and contributes approximately 99% to the total Namibian monkfish landings. The Namibian monkfish fishery is one of the largest and most valuable of its kind in the world. Two fishing fleets harvest the resource, viz. the monkfish and sole fleet and the hake directed fleet. In 1998 and 1999, monkfish were also harvested by one gillnet vessel that had an experimental right. In this study, the biology, abundance and exploitation patterns of monkfish were investigated and used as inputs into a stock assessment framework as a contribution towards the development of a management procedure for this species. Ground and sectioned otoliths and sectioned illicia were used to age Lophius vomerinus. The illicia provided more precise estimates of age, and growth was subsequently modelled using these age estimates. Growth in length and weight was best described by the threeparameter specialised von Bertalanffy model as Lt (cm TL) = 72.29 (1-e⁻°·¹⁴⁽t⁺°·³°⁾) and Wt (kg) = 3.96 (1-e⁻°·¹⁸⁽t⁺°·¹°⁾)²·⁹⁹ for males, and Lt (cm TL) = 111.98 (1-e⁻°·°⁸⁽t⁺°·³⁶⁾) and Wt (kg) = 6.92 (1-e⁻°·¹⁸⁽t⁻°·⁶⁴⁾)³·°³ for females. L. vomerinus is a slow-growing species with a life span in excess of 10 years. Reproductive activity was shown to occur throughout the year, with a slight peak during the winter months. The adult sex ratio of fish > 37 cm TL was biased towards females. Length-at-50% sexual maturity was attained at 32.1 cm TL for females and 23.7 cm TL for males. The rate of natural mortality was estimated at 0.15 year⁻¹, while fishing mortality rates during the 1970s, 1980s and 1990s were estimated at 0.01 year⁻¹, 0.04 year⁻¹ and 0.14 year⁻¹, respectively. Gillnetting for monkfish (300 mm stretched mesh) was highly efficient with a moderate bycatch of around 20% during the two years of operation. The main bycatch species were red crab, spider crab, squalid sharks, rays and Cape and Deep-water hake. The mean length of the monkfish caught in gillnets (67 cm TL) was significantly larger than the monkfish landed by the trawlers (38 cm TL) and less than 1% of immature fish were landed. Gillnet catch-per-unit-effort for monkfish fluctuated between 0.03 and 0.67 kg.day⁻¹.50 m net panel⁻¹, with a soak time of between one and sixteen days. More than 50% (by weight) of monkfish landed by monkfish and sole trawlers, consisted of fish below 36 cm TL. There was a significant increase in catches of juvenile monkfish during 1997 and 1998 in comparison to the period 1994 to 1996. Various types of rigid sorting grids were tested to release juvenile monkfish below 32 cm TL. Five grid designs were tested. These included an “Ex-it” grid with horizontal bars spaced at 55 mm, single grids with vertical and horizontal bars spaced at 55 mm and grids with circular openings of 110 and 168 mm in diameter. The most efficient design was the grid with circular openings of 110 cm in diameter, which ensured the escape of 66% of monkfish smaller than 31 cm TL. However, studies need to be undertaken to quantify the survival of released fish and to test the feasibility of using grid sorters on commercial monkfish and sole trawling gear. The monkfish resource was assessed by means of length cohort analyses, the Thompson and Bell predictive model and by way of a deterministic age-structured production modelling approach. The length cohort analysis models were sensitive to the rate of natural mortality and insensitive to changes in the terminal fishing mortality rate. These biases may, however, not be serious provided that estimates of abundance are used to reflect relative changes. Fish ranging between 26 and 59 cm TL are the most heavily exploited. The Thompson and Bell model predicted that the monkfish resource is exploited above MSY -levels and a reduction of approximately 40% in fishing effort would provide a higher yield. Yield-per-recruit ranged between 10 000 and 14 000 tonnes. Results should, however, be treated with caution, as the condition of steady state was not satisfied. The age-structured production model was tuned using trends in catch-per-unit-effort data, estimated by Generalised Linear Modeling, as well as relative abundance indices calculated from hake biomass surveys. The model was found to be sensitive to both the ‘steepness’ parameter h and estimates of natural mortality. The ‘depletion’ level of the monkfish resource is currently estimated to be 49%. Estimated coefficients of variation were high (> 63%) due to the lack of a consistent trend within the abundance indices to tune the model. Overall productivity of the monkfish resource was estimated to be approximately 16%, similar to other southern African demersal resources. Results of the risk analyses suggest that catches in excess of 7 000 tonnes may be unsustainable and that catches of 5 000 or 6 000 tonnes would decrease the risk of stock collapse and possibly lead to a recovery in the stock. Monkfish management strategies were reviewed and these were considered in relation to the results of this study. The following management recommendations were made: to follow the precautionary approach and implement a total allowable catch for monkfish; to implement rigid sorting grids as these would be the most appropriate way in which to reduce catches of juvenile monkfish; to restrict soak time, depth of operation and implement means to reduce ‘ghost fishing’ by gillnetting and finally, to develop a management procedure for Namibian monkfish with the main objective being the sustainable exploitation of the resource.
- Full Text:
- Date Issued: 2000
Aspects of the biology, ecology and population dynamics of Galeichthys feliceps (Valenciennes) and G. ater (Castelnau) (Pisces: Ariidae) off the south-east coast of South Africa
- Authors: Tilney, Robin Lewis
- Date: 1991
- Subjects: Catfishes -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5221 , http://hdl.handle.net/10962/d1005064
- Description: This thesis represents a detailed investigation into aspects of the biology, ecology and population dynamics of two endemic ariid species, Galeichthys feliceps and G. ater, off the southeast coast of South Africa. The two species are exploited as a by-catch in the commercial ski-boat fishery off Port Alfred, a fishery dominated by highly fecund sparid and sciaenid species. They collectively constitute approximately 10% of the total annual catch in terms of landed mass G. feliceps outnumber G. ater in the catches by a ratio of 3:1. The investigation was designed to provide the biological data required for stock assessment and to determine optimum management strategies for the two populations. The implications of their K-selected life-history styles for exploitation received particular attention. While the two species were sympatric and had similar depth distributions they were found to be allopatric with respect to their foraging habitats. G. feliceps foraged over sandy and muddy substrata in marine and estuarine environments. G. ater fed only on reef-associated species and did not utilise estuaries. Their feeding-associated morphologies were identical and both species preyed primarily on crustaceans (brachyuran crabs and isopods), echiurids, molluscs and polychaetes. The diet of G. ater was broader in terms of the number of species consumed. The two species are mouth-brooders with low fecundity. G. feliceps and G. ater produced a mean of 49 and 32 eggs each, per annum. The buccal incubation period was determined to be in the region of 140 days for G. feliceps. Embryos hatched after approximately 75-80 days and the young began exogenous feeding thereafter. The young fed intra-buccally on detritus provided by the parent. Adult buccal mucus may also have been used as a food source. Young were released at a total length of ± 55mm. Adult males ceased feeding whilst mouth-brooding. Body musculature, abdominal fat and liver reserves provided energy for basal metabolism and males lost approximately 28% of their body mass during buccal incubation. Females expended less reproductive energy than males. Catches were dominated by mature fish (76% in G. feliceps and 97% in G. ater). Females were significantly more abundant in catches during the spawning and mouth-brooding period. The female to male sex ratios were 1.65:1 and 2.23:1 for G. feliceps and G. ater respectively. Age and growth studies revealed that the two species mature at advanced ages (10 and 9 years for G. feliceps and 9 and 7 years for G. ater males and females respectively). They are long-lived, reaching ages in excess of 18 years in G. feliceps and in excess of 15 years in G. ater. Females live longer than males and grow larger. Yield-per-recruit and spawner biomass-per-recruit analyses demonstrated that G. ater were exploited below FO.1 at a level where spawner biomass-per-recruit was reduced to between 45% and 65% of the unexploited level. The G. ater stock was not adversely affected by current levels of fishing effort. For G. feliceps, both sexes were exploited beyond F₀.₁ where spawner biomass-per-recruit was reduced to between 30% and 22% of the unexploited level. G. feliceps were shown to be sensitive to relatively low levels of exploitation, a phenomenon attributed to their highly Kselected life-history style. Should the species become targeted for in the future, effort restrictions in the form of a closed season during the spawning and mouth-brooding period would prove effective in reducing effort and conserving the population sex ratio.
- Full Text:
- Date Issued: 1991
- Authors: Tilney, Robin Lewis
- Date: 1991
- Subjects: Catfishes -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5221 , http://hdl.handle.net/10962/d1005064
- Description: This thesis represents a detailed investigation into aspects of the biology, ecology and population dynamics of two endemic ariid species, Galeichthys feliceps and G. ater, off the southeast coast of South Africa. The two species are exploited as a by-catch in the commercial ski-boat fishery off Port Alfred, a fishery dominated by highly fecund sparid and sciaenid species. They collectively constitute approximately 10% of the total annual catch in terms of landed mass G. feliceps outnumber G. ater in the catches by a ratio of 3:1. The investigation was designed to provide the biological data required for stock assessment and to determine optimum management strategies for the two populations. The implications of their K-selected life-history styles for exploitation received particular attention. While the two species were sympatric and had similar depth distributions they were found to be allopatric with respect to their foraging habitats. G. feliceps foraged over sandy and muddy substrata in marine and estuarine environments. G. ater fed only on reef-associated species and did not utilise estuaries. Their feeding-associated morphologies were identical and both species preyed primarily on crustaceans (brachyuran crabs and isopods), echiurids, molluscs and polychaetes. The diet of G. ater was broader in terms of the number of species consumed. The two species are mouth-brooders with low fecundity. G. feliceps and G. ater produced a mean of 49 and 32 eggs each, per annum. The buccal incubation period was determined to be in the region of 140 days for G. feliceps. Embryos hatched after approximately 75-80 days and the young began exogenous feeding thereafter. The young fed intra-buccally on detritus provided by the parent. Adult buccal mucus may also have been used as a food source. Young were released at a total length of ± 55mm. Adult males ceased feeding whilst mouth-brooding. Body musculature, abdominal fat and liver reserves provided energy for basal metabolism and males lost approximately 28% of their body mass during buccal incubation. Females expended less reproductive energy than males. Catches were dominated by mature fish (76% in G. feliceps and 97% in G. ater). Females were significantly more abundant in catches during the spawning and mouth-brooding period. The female to male sex ratios were 1.65:1 and 2.23:1 for G. feliceps and G. ater respectively. Age and growth studies revealed that the two species mature at advanced ages (10 and 9 years for G. feliceps and 9 and 7 years for G. ater males and females respectively). They are long-lived, reaching ages in excess of 18 years in G. feliceps and in excess of 15 years in G. ater. Females live longer than males and grow larger. Yield-per-recruit and spawner biomass-per-recruit analyses demonstrated that G. ater were exploited below FO.1 at a level where spawner biomass-per-recruit was reduced to between 45% and 65% of the unexploited level. The G. ater stock was not adversely affected by current levels of fishing effort. For G. feliceps, both sexes were exploited beyond F₀.₁ where spawner biomass-per-recruit was reduced to between 30% and 22% of the unexploited level. G. feliceps were shown to be sensitive to relatively low levels of exploitation, a phenomenon attributed to their highly Kselected life-history style. Should the species become targeted for in the future, effort restrictions in the form of a closed season during the spawning and mouth-brooding period would prove effective in reducing effort and conserving the population sex ratio.
- Full Text:
- Date Issued: 1991
Determination of the optimum environmental requirements of juvenile marine fish : the development of a protocol
- Authors: Deacon, Neil
- Date: 1997
- Subjects: Mariculture Fishes -- Ecology Grunts (Fishes) -- Ecology Pomadasys -- Ecology
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5254 , http://hdl.handle.net/10962/d1005097
- Description: The spotted grunter, Pomodasys commersonnii, has been identified as a candidate species for mariculture in South Africa due to its high market demand and apparent biological suitability for culture. In common with most other potential mariculture species the spotted grunter has an estuarine juvenile phase. In this respect, the difficulties encountered in determining the optimum environmental requirements of spotted grunter are applicable to the majority of other potential mariculture species. Due the variability of estuarine habitats determination of the optimum environmental requirements of estuarine species under laboratory conditions are a prerequisite to subsequent evaluation of aquaculture potential. Therefore, using the spotted grunter as a representative of a typical potent ial mariculture species, the aim of this study was to contribute to the protocol for evaluating the environmental requirements of potential aquaculture species. The growth of fish is dependent on the relationship between food intake, metabolism and environmental factors. In this relationship, environmental factors do not act on growth per se, but rather act through metabolism on growth. Consequently, the environmental factors affecting the growth of a fish species can be classified into functional categories according to their respective influence on metabolic processes. Five functional categories are recognised, namely: controlling, limiting, masking, directive and lethal factors. In this study, the functional categories were sequenced to develop a theoretical protocol for determining the optimum environmental requirements of potential mariculture species under artificial conditions. It was hypothesised that the correct sequence for experimentally determining the optimum environmental requirements of a species should correspond to theoretical protocol. The hypothesis was tested from the experimental investigation into the individual effects of temperature, salinity, light intensity, photoperiod and food intake on growth performance of juvenile spotted grunter. The investigations quantified the relative effects of controlling, masking, directive and limiting factors on growth performance. The results of the studies were statistically compared to obtain a ranking of the effects of the environmental factors (e.g. temperature, salinity, etc.) on growth performance of juvenile spotted grunter. The statistical ranking facilitated the formulation of a second protocol for sequentially determining the optimum environmental requirements of a species. The second protocol was derived purely from the experimental data. Based on the corroboration between the theoretical protocol and that formulated from the experimental investigations, the hypothesis was accepted. Confirmation of the protocol for examining the effects of controlling, limiting, masking and directive factors provided a basis for the development of a preliminary experimental sequence for determining the optimum environmental requirements of juvenile marine fish. In the development of the experimental sequence, the protocol was interpreted in conjunction with the experimental methods used to determine the effects of environmental factors on the growth of juvenile spotted grunter. The experimental sequence provides a logical frame work within which the optimum environmental requirements of other marine fish species can be determined. In addition, evaluation of growth performance by this method provides a basis for comparison of the mariculture potential between species.
- Full Text:
- Date Issued: 1997
- Authors: Deacon, Neil
- Date: 1997
- Subjects: Mariculture Fishes -- Ecology Grunts (Fishes) -- Ecology Pomadasys -- Ecology
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5254 , http://hdl.handle.net/10962/d1005097
- Description: The spotted grunter, Pomodasys commersonnii, has been identified as a candidate species for mariculture in South Africa due to its high market demand and apparent biological suitability for culture. In common with most other potential mariculture species the spotted grunter has an estuarine juvenile phase. In this respect, the difficulties encountered in determining the optimum environmental requirements of spotted grunter are applicable to the majority of other potential mariculture species. Due the variability of estuarine habitats determination of the optimum environmental requirements of estuarine species under laboratory conditions are a prerequisite to subsequent evaluation of aquaculture potential. Therefore, using the spotted grunter as a representative of a typical potent ial mariculture species, the aim of this study was to contribute to the protocol for evaluating the environmental requirements of potential aquaculture species. The growth of fish is dependent on the relationship between food intake, metabolism and environmental factors. In this relationship, environmental factors do not act on growth per se, but rather act through metabolism on growth. Consequently, the environmental factors affecting the growth of a fish species can be classified into functional categories according to their respective influence on metabolic processes. Five functional categories are recognised, namely: controlling, limiting, masking, directive and lethal factors. In this study, the functional categories were sequenced to develop a theoretical protocol for determining the optimum environmental requirements of potential mariculture species under artificial conditions. It was hypothesised that the correct sequence for experimentally determining the optimum environmental requirements of a species should correspond to theoretical protocol. The hypothesis was tested from the experimental investigation into the individual effects of temperature, salinity, light intensity, photoperiod and food intake on growth performance of juvenile spotted grunter. The investigations quantified the relative effects of controlling, masking, directive and limiting factors on growth performance. The results of the studies were statistically compared to obtain a ranking of the effects of the environmental factors (e.g. temperature, salinity, etc.) on growth performance of juvenile spotted grunter. The statistical ranking facilitated the formulation of a second protocol for sequentially determining the optimum environmental requirements of a species. The second protocol was derived purely from the experimental data. Based on the corroboration between the theoretical protocol and that formulated from the experimental investigations, the hypothesis was accepted. Confirmation of the protocol for examining the effects of controlling, limiting, masking and directive factors provided a basis for the development of a preliminary experimental sequence for determining the optimum environmental requirements of juvenile marine fish. In the development of the experimental sequence, the protocol was interpreted in conjunction with the experimental methods used to determine the effects of environmental factors on the growth of juvenile spotted grunter. The experimental sequence provides a logical frame work within which the optimum environmental requirements of other marine fish species can be determined. In addition, evaluation of growth performance by this method provides a basis for comparison of the mariculture potential between species.
- Full Text:
- Date Issued: 1997
Aspects of the nutritional physiology and dietary requirements of juvenile and adult sharptooth catfish, Clarias Gariepinus (Pisces : clariidae)
- Authors: Uys, Wynand
- Date: 1990
- Subjects: Clariidae Catfishes Catfishes -- Nutrition-requirements Clarias
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5225 , http://hdl.handle.net/10962/d1005068
- Description: Past and current research on the biology and culture of the sharptooth catfish (Clarias gariepinus) has stimulated the development of its commercial production. At the inception of this project in 1985, it was decided that the development of an optimal dry feed was the most important step in developing a more complete culture technology for the species. The objectives of the project were, therefore, to investigate aspects of the nutritional physiology and dietary requirements of the species to provide the necessary information for the formulation of optimum economical feeds. A review of the natural feeding biology of the species provided valuable information with regard to its food preferences. From this information, and by estimating the nutritional composition of its preferred natural diet, it could be concluded that the animal requires a relatively high dietary protein content. This conclusion was substantiated by studies of the functional morphology of the digestive system (gross and fine structure). The characteristics of the digestive system and external anatomy were found to be typical of an opportunistic, omnivorous predator. The ontogenetic development of the digestive system is relatively fast, and enables the utilization of a variety of diets efficiently from an early stage. Since the ability of an animal to digest a given substance is predominantly dependent on the presence of appropriate enzymes the quantification and characterization of C. gariepinus digestive enzyme activities were investigated on an experimental basis. This work revealed that the animal has relatively high levels of digestive enzyme activities, and that its proteolytic enzyme activities correspond with those of other carnivorous fishes, while its starch digesting abilities correspond with those of specialized herbivores. Also, the high levels of lysozyme and alkaline phosphatase indicate an adaptation to detritivory, making this fish a truly opportunistic omnivore. Evidence was found to suggest that sharptooth catfish are able to re-absorb secreted digestive enzymes in the posterior section of its intestinal tract. It was shown that digestive enzyme activities were induced by food intake, and that no inherent rhythmicity in digestive activities seemed to occur. The development of digestive enzyme systems in the early life history of this animal was found to be also extremely rapid (complete within 10-16 days after hatching). It was also shown that artificial dry feed stimulates higher levels of digestive enzyme activities in larvae and juveniles than natural food. Although increased cellulase activity was found in larvae and juveniles that were fed on live zooplankton, the relative contribution of exogenous prey enzymes to protein digestion could not be ascertained. Specific nutritional requirements were investigated by means of feeding trials. The results of these trials confirmed the above indications regarding the dietary requirements of sharptooth catfish. It was concluded that for optimal growth and production performance, this animal requires a dietary protein content of 40-42%, a dietary lipid content of 10-12% and that diets should contain 14-16 kJ/g digestible energy. The optimum protein-to-energy ratio was found to be 26-29mg protein per kJ of digestible energy. Dietary requirements are evidently the same for juveniles and adults, except that younger fish require higher relative feeding levels. Finally, practical feed formulations were evaluated in terms of their economic feasibility. The results of these trials are presented along with recommendations on the formulation of commercial feeds for C. gariepinus. A computer program with which to implement least cost feed formulation by means of linear programming is provided.
- Full Text:
- Date Issued: 1990
- Authors: Uys, Wynand
- Date: 1990
- Subjects: Clariidae Catfishes Catfishes -- Nutrition-requirements Clarias
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5225 , http://hdl.handle.net/10962/d1005068
- Description: Past and current research on the biology and culture of the sharptooth catfish (Clarias gariepinus) has stimulated the development of its commercial production. At the inception of this project in 1985, it was decided that the development of an optimal dry feed was the most important step in developing a more complete culture technology for the species. The objectives of the project were, therefore, to investigate aspects of the nutritional physiology and dietary requirements of the species to provide the necessary information for the formulation of optimum economical feeds. A review of the natural feeding biology of the species provided valuable information with regard to its food preferences. From this information, and by estimating the nutritional composition of its preferred natural diet, it could be concluded that the animal requires a relatively high dietary protein content. This conclusion was substantiated by studies of the functional morphology of the digestive system (gross and fine structure). The characteristics of the digestive system and external anatomy were found to be typical of an opportunistic, omnivorous predator. The ontogenetic development of the digestive system is relatively fast, and enables the utilization of a variety of diets efficiently from an early stage. Since the ability of an animal to digest a given substance is predominantly dependent on the presence of appropriate enzymes the quantification and characterization of C. gariepinus digestive enzyme activities were investigated on an experimental basis. This work revealed that the animal has relatively high levels of digestive enzyme activities, and that its proteolytic enzyme activities correspond with those of other carnivorous fishes, while its starch digesting abilities correspond with those of specialized herbivores. Also, the high levels of lysozyme and alkaline phosphatase indicate an adaptation to detritivory, making this fish a truly opportunistic omnivore. Evidence was found to suggest that sharptooth catfish are able to re-absorb secreted digestive enzymes in the posterior section of its intestinal tract. It was shown that digestive enzyme activities were induced by food intake, and that no inherent rhythmicity in digestive activities seemed to occur. The development of digestive enzyme systems in the early life history of this animal was found to be also extremely rapid (complete within 10-16 days after hatching). It was also shown that artificial dry feed stimulates higher levels of digestive enzyme activities in larvae and juveniles than natural food. Although increased cellulase activity was found in larvae and juveniles that were fed on live zooplankton, the relative contribution of exogenous prey enzymes to protein digestion could not be ascertained. Specific nutritional requirements were investigated by means of feeding trials. The results of these trials confirmed the above indications regarding the dietary requirements of sharptooth catfish. It was concluded that for optimal growth and production performance, this animal requires a dietary protein content of 40-42%, a dietary lipid content of 10-12% and that diets should contain 14-16 kJ/g digestible energy. The optimum protein-to-energy ratio was found to be 26-29mg protein per kJ of digestible energy. Dietary requirements are evidently the same for juveniles and adults, except that younger fish require higher relative feeding levels. Finally, practical feed formulations were evaluated in terms of their economic feasibility. The results of these trials are presented along with recommendations on the formulation of commercial feeds for C. gariepinus. A computer program with which to implement least cost feed formulation by means of linear programming is provided.
- Full Text:
- Date Issued: 1990
Development of an artificial weaning diet for the South African abalone, Haliotis midae (Haliotidae: Gastropoda)
- Authors: Knauer, Jens
- Date: 1994
- Subjects: Abalone culture -- Research -- South Africa , Ichthyology , Abalones
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5204 , http://hdl.handle.net/10962/d1004099 , Abalone culture -- Research -- South Africa , Ichthyology , Abalones
- Description: An adequate supply of diatoms during the weaning stage (generally 5 - 10 mm shell length (SL)) is one of the primary constraints to the commercial culture of the South African abalone, Haliotis midae. Because of the seriousness of the problem, a project aimed at the development of an artificial weaning diet was initiated. Initially, the chemical composition (proximate composition, amino acid, fatty acid and mineral element profile) of juvenile H. midae was analyzed, as a general lack of such information was identified in a review. Due to the lack of knowledge on the nutritional requirements of H. midae, the formulation of the weaning diet was based on the essential amino acid (EAA) pattern of the shucked tissue, and the known nutrient requirements of haliotids. Subsequently, a water stable gel and pellet form of the diet were developed. The best water stability of a gel was obtained with a 1:3 agar/gelatine mixture which retained 70.7 ± 2.7 % of its dry weight after 24 h. Starch based pellets, however, retained 89.0 ± 0.6 % of their dry weight after 24 h. In a comparative growth trial, pellets produced a significantly better increase in SL and weight than gels after only 15 days. This was probably due to the better water stability of pellets, which resulted in a better nutritional quality than in gels. The feeding behaviour on both forms of the diet did not differ. Activity patterns were exclusively nocturnal and feeding frequency was consistently low. The percentage composition of the pelleted weaning diet, on a dry weight basis, was 5 % casein, 15 % gelatine, 15 % fish meal, 10 % Spirulina spp., 2.5 % fish oil, 2.5 % sunflower oil, 21.0 % dextrin, 23.0 % starch, 4.0 % of a mineral and 2.0 % of a vitamin mixture. The correlation coefficient between the EAA pattern of H. midae and the dietary EAA pattern was r⁷= 0.8989. Pellets were fed to juveniles in a 30 day growth trial to study the effect of photoperiod (12, 16, 20 and 23 hours of darkness) on growth and general nutritional parameters. A comparative experiment feeding diatoms was conducted under a 12hL: 12hD light regime at the same time. The SL and weight of the juveniles did not increase significantly with an increase in hours of darkness. The growth of juveniles fed on pellets did not differ significantly from those fed on diatoms. Percentage feed consumption (PFC), percentage feeding rate (PFR), feed conversion ratio (FCR), protein efficiency ratio (PER) and percentage protein deposited (PPD) were determined for the animals fed on pellets. None of the parameters were significantly affected by photoperiod. However, there were trends in that PFC increased with longer periods of darkness, while PPD decreased. The FCRs (0.44 ± 0.04 to 0.60 ± 0.19) and PERs (5.06 ± 1.74 to 6.64 ± 0.77) indicated that juveniles used the feed, and in particular the protein, very efficiently. Photoperiod did not have an effect on the specific activity of the digestive enzymes amylase, protease and lipase. The specific activity of amylase in the juveniles fed on diatoms was significantly higher than in the pellet fed groups. This was surprising as the main carbohydrate of diatoms is the ß-(l-3) glucan chrysolaminarin, and not starch, a ß-(l-4) glucan. Protease specific activity, on the other hand, was significantly higher in the pellet fed groups, indicating an ability to adapt to the high protein content in the artificial diet (35.48 %), compared to diatoms which had a protein content of 5 %. The specific activity of lipase did not differ significantly between groups, probably because of a similar lipid concentration (5 - 10 %) in diatoms and pellets. Finally, the effect of stocking density, ranging from 1250 to 10,000 juveniles/m2, on the growth of juveniles was evaluated. A model of hatchery productivity was developed based on this investigation. Hatchery productivity was defined as the number of juveniles per unit space reared through to the grow-out stage per unit time. The model predicted that maximum productivity would be achieved at a stocking density of 10,000 juveniles/m2. The results have shown that H. midae can be successfully weaned on an artificial diet, as the growth on the diet was not significantly different to growth obtained on diatoms. Long-term growth trials are needed to confirm these results. The importance of standardized experiments on the nutritional requirements and digestibility of abalone was emphasized. The importance of improved artificial diets, optimal culture conditions, as well as the application of biotechnological techniques to further abalone aquaculture was highlighted.
- Full Text:
- Date Issued: 1994
- Authors: Knauer, Jens
- Date: 1994
- Subjects: Abalone culture -- Research -- South Africa , Ichthyology , Abalones
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5204 , http://hdl.handle.net/10962/d1004099 , Abalone culture -- Research -- South Africa , Ichthyology , Abalones
- Description: An adequate supply of diatoms during the weaning stage (generally 5 - 10 mm shell length (SL)) is one of the primary constraints to the commercial culture of the South African abalone, Haliotis midae. Because of the seriousness of the problem, a project aimed at the development of an artificial weaning diet was initiated. Initially, the chemical composition (proximate composition, amino acid, fatty acid and mineral element profile) of juvenile H. midae was analyzed, as a general lack of such information was identified in a review. Due to the lack of knowledge on the nutritional requirements of H. midae, the formulation of the weaning diet was based on the essential amino acid (EAA) pattern of the shucked tissue, and the known nutrient requirements of haliotids. Subsequently, a water stable gel and pellet form of the diet were developed. The best water stability of a gel was obtained with a 1:3 agar/gelatine mixture which retained 70.7 ± 2.7 % of its dry weight after 24 h. Starch based pellets, however, retained 89.0 ± 0.6 % of their dry weight after 24 h. In a comparative growth trial, pellets produced a significantly better increase in SL and weight than gels after only 15 days. This was probably due to the better water stability of pellets, which resulted in a better nutritional quality than in gels. The feeding behaviour on both forms of the diet did not differ. Activity patterns were exclusively nocturnal and feeding frequency was consistently low. The percentage composition of the pelleted weaning diet, on a dry weight basis, was 5 % casein, 15 % gelatine, 15 % fish meal, 10 % Spirulina spp., 2.5 % fish oil, 2.5 % sunflower oil, 21.0 % dextrin, 23.0 % starch, 4.0 % of a mineral and 2.0 % of a vitamin mixture. The correlation coefficient between the EAA pattern of H. midae and the dietary EAA pattern was r⁷= 0.8989. Pellets were fed to juveniles in a 30 day growth trial to study the effect of photoperiod (12, 16, 20 and 23 hours of darkness) on growth and general nutritional parameters. A comparative experiment feeding diatoms was conducted under a 12hL: 12hD light regime at the same time. The SL and weight of the juveniles did not increase significantly with an increase in hours of darkness. The growth of juveniles fed on pellets did not differ significantly from those fed on diatoms. Percentage feed consumption (PFC), percentage feeding rate (PFR), feed conversion ratio (FCR), protein efficiency ratio (PER) and percentage protein deposited (PPD) were determined for the animals fed on pellets. None of the parameters were significantly affected by photoperiod. However, there were trends in that PFC increased with longer periods of darkness, while PPD decreased. The FCRs (0.44 ± 0.04 to 0.60 ± 0.19) and PERs (5.06 ± 1.74 to 6.64 ± 0.77) indicated that juveniles used the feed, and in particular the protein, very efficiently. Photoperiod did not have an effect on the specific activity of the digestive enzymes amylase, protease and lipase. The specific activity of amylase in the juveniles fed on diatoms was significantly higher than in the pellet fed groups. This was surprising as the main carbohydrate of diatoms is the ß-(l-3) glucan chrysolaminarin, and not starch, a ß-(l-4) glucan. Protease specific activity, on the other hand, was significantly higher in the pellet fed groups, indicating an ability to adapt to the high protein content in the artificial diet (35.48 %), compared to diatoms which had a protein content of 5 %. The specific activity of lipase did not differ significantly between groups, probably because of a similar lipid concentration (5 - 10 %) in diatoms and pellets. Finally, the effect of stocking density, ranging from 1250 to 10,000 juveniles/m2, on the growth of juveniles was evaluated. A model of hatchery productivity was developed based on this investigation. Hatchery productivity was defined as the number of juveniles per unit space reared through to the grow-out stage per unit time. The model predicted that maximum productivity would be achieved at a stocking density of 10,000 juveniles/m2. The results have shown that H. midae can be successfully weaned on an artificial diet, as the growth on the diet was not significantly different to growth obtained on diatoms. Long-term growth trials are needed to confirm these results. The importance of standardized experiments on the nutritional requirements and digestibility of abalone was emphasized. The importance of improved artificial diets, optimal culture conditions, as well as the application of biotechnological techniques to further abalone aquaculture was highlighted.
- Full Text:
- Date Issued: 1994
The influence of oceanographic conditions and culture methods on the dynamics of mussel farming in Saldanha Bay, South Africa
- Authors: Heasman, Kevin Gerald
- Date: 1996
- Subjects: Mytilidae -- South Africa , Mussel culture -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5290 , http://hdl.handle.net/10962/d1005134 , Mytilidae -- South Africa , Mussel culture -- South Africa
- Description: The principal aim of this study was to establish the biological and environmental parameters governing the successful and sustainable cultivation of mussels in Saldanha Bay. The environmental study investigated seston, chlorophyll-a and particulate organic matter (POM) levels, water temperature dissolved oxygen and salinity levels in the bay and water flow in and around the rafts. The biological part of the study investigated the efficiency of food extraction, growth rates, mussel condition, fouling and production and yield on a rope, raft and farm scale. Saldanha Bay is well suited for the culture of mussels, particularly Mytilus galloprovincialis and Choromytilus meridionalis. Water temperature and salinity in Saldanha Bay were found to be near optimal for mussel culture. POM and chlorophyll-a levels were found to be high due to primary production resulting from the nutrient rich upwelled water outside Saldanha Bay. The mean levels of chlorophyll-a (8,6μg/l) represent 6%, by mass, of the total POM. On a bay scale the POM remained above the mussels maximum requirements (pseudofaeces threshold) during the study period. Mussels showed a preference for the phytoplankton portion of the POM. Approximately 40% of the chlorophyll-a was extracted from the water by the mussel farm. The efficiency of food extraction increased with mussel age. Rafts with seed mussels younger than 2 months, 3 to 4 months, 5 to 6 months and older than 6 months extracted 32%, 55%, 85% and 92% of the available chlorophyll-a respectively. An increase of rope spacing on the rafts resulted in 37% more chlorophyll-a and 30% more particle volume reaching the lee of the raft. Ambient water currents in the bay show flow rates of up to 22cm per second. However, on entering a raft with a rope spacing of 60cm, the water flow is attenuated by 90%. Increasing the rope spacing to 90cm resulted in a water flow attenuation of 72%. The increase in rope spacing ensures that the mussels in the centre of the raft are feeding on food levels close to, or above, the pseudofaeces level. Mussel growth rate at a rope spacing of 90cm is significantly improved as a result of the increased food delivery. There are other factors, however that effect mussel growth. Growth rates were found to be better in summer than in winter. The reduced winter growth rate is possibly due to competition with the maturing fouling organisms which settle in mid to late summer. Fouling by mussel spat and Ciona intestinalis is seasonal, occurring from December to May. C.intestinalis is prevalent in the centre of the farm and rafts as low energy waters are preferred by this species. Mussel spat settles mainly on the periphery of the farm and the rafts. Competition with fouling organisms reduces growth and increases mortality of the cultured mussels. Results indicate that the present spacing of rafts, (1 raft per hectare) is adequate under existing conditions. Any new farms should maintain batches of 50 rafts with channels between them to ensure water current penetration into the furthest reaches of the farm. Rope spacing on the rafts should be increased to between 60cm and 90cm. Mussel density should be regulated according to mussel size and fouling should be controlled to maintain yields.
- Full Text:
- Date Issued: 1996
- Authors: Heasman, Kevin Gerald
- Date: 1996
- Subjects: Mytilidae -- South Africa , Mussel culture -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5290 , http://hdl.handle.net/10962/d1005134 , Mytilidae -- South Africa , Mussel culture -- South Africa
- Description: The principal aim of this study was to establish the biological and environmental parameters governing the successful and sustainable cultivation of mussels in Saldanha Bay. The environmental study investigated seston, chlorophyll-a and particulate organic matter (POM) levels, water temperature dissolved oxygen and salinity levels in the bay and water flow in and around the rafts. The biological part of the study investigated the efficiency of food extraction, growth rates, mussel condition, fouling and production and yield on a rope, raft and farm scale. Saldanha Bay is well suited for the culture of mussels, particularly Mytilus galloprovincialis and Choromytilus meridionalis. Water temperature and salinity in Saldanha Bay were found to be near optimal for mussel culture. POM and chlorophyll-a levels were found to be high due to primary production resulting from the nutrient rich upwelled water outside Saldanha Bay. The mean levels of chlorophyll-a (8,6μg/l) represent 6%, by mass, of the total POM. On a bay scale the POM remained above the mussels maximum requirements (pseudofaeces threshold) during the study period. Mussels showed a preference for the phytoplankton portion of the POM. Approximately 40% of the chlorophyll-a was extracted from the water by the mussel farm. The efficiency of food extraction increased with mussel age. Rafts with seed mussels younger than 2 months, 3 to 4 months, 5 to 6 months and older than 6 months extracted 32%, 55%, 85% and 92% of the available chlorophyll-a respectively. An increase of rope spacing on the rafts resulted in 37% more chlorophyll-a and 30% more particle volume reaching the lee of the raft. Ambient water currents in the bay show flow rates of up to 22cm per second. However, on entering a raft with a rope spacing of 60cm, the water flow is attenuated by 90%. Increasing the rope spacing to 90cm resulted in a water flow attenuation of 72%. The increase in rope spacing ensures that the mussels in the centre of the raft are feeding on food levels close to, or above, the pseudofaeces level. Mussel growth rate at a rope spacing of 90cm is significantly improved as a result of the increased food delivery. There are other factors, however that effect mussel growth. Growth rates were found to be better in summer than in winter. The reduced winter growth rate is possibly due to competition with the maturing fouling organisms which settle in mid to late summer. Fouling by mussel spat and Ciona intestinalis is seasonal, occurring from December to May. C.intestinalis is prevalent in the centre of the farm and rafts as low energy waters are preferred by this species. Mussel spat settles mainly on the periphery of the farm and the rafts. Competition with fouling organisms reduces growth and increases mortality of the cultured mussels. Results indicate that the present spacing of rafts, (1 raft per hectare) is adequate under existing conditions. Any new farms should maintain batches of 50 rafts with channels between them to ensure water current penetration into the furthest reaches of the farm. Rope spacing on the rafts should be increased to between 60cm and 90cm. Mussel density should be regulated according to mussel size and fouling should be controlled to maintain yields.
- Full Text:
- Date Issued: 1996
Towards the development of species-specific fish production models for small reservoirs in Southern Africa
- Authors: Potts, Warren M
- Date: 2004
- Subjects: Freshwater fishes -- Africa, Southern Fish populations -- Africa, Southern Fisheries -- Africa, Southern Sustainable fisheries -- Africa, Southern Labeo -- Reproduction Cyprinidae -- Africa, Southern
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5330 , http://hdl.handle.net/10962/d1005176
- Description: The fish populations in small southern African reservoirs are largely unexploited and there is potential for fisheries development. However, the development of sustainable fisheries requires reliable estimates of potential yield or production. Empirical models that have been developed to predict fish production only apply to large water bodies and only predict total fish production, not the production of individual species. Small reservoirs generally have few commercially important species and therefore species-specific fish production models are an alternative approach. The small reservoirs of the Eastern Cape are dominated by the moggel (Labeo umbratus). The principal objectives of this thesis were to gain an understanding of the ecology of small reservoirs and the function of moggel in these systems. This information was used to design a research approach to rapidly develop species-specific models for small reservoirs in southern Africa. The limnology of two small reservoirs was compared. During the study period the reservoirs were turbid and showed a warm, monomictic pattern of thermal stratification. Anthropogenic pressure in the reservoir catchments appeared to be the overriding factor increasing the nutrient input to the reservoirs and consequently, influencing the biomass of algae in the reservoirs. The feeding biology of moggel in Katriver and Laing reservoirs was examined. Moggel are detrivorous and successfully digested diatoms. The slower growth rate of moggel in Katriver reservoir was attributed to the poorer nutritional value of the diet as a consequence of the lower concentration of diatoms in the detrital aggregate. The reproductive characteristics of moggel were examined in four reservoirs. Moggel were able to reproduce successfully in the reservoirs. This could be attributed to their r-selected reproductive strategy, with a high fecundity and an extended spawning season and their ability to spawn in a floodplain environment. Differences in recruitment success between years were found to be a consequence of the timing and duration of seasonal rains. The number of mature females in a population and the availability of suitable spawning habitat influenced recruitment success. The life history of the moggel in five reservoirs was compared. Growth appeared to be related to food availability, while mortality was lower in the populations where food was abundant and there were less predatory species. Size and age at maturity were not affected by environmental factors, but were dependent on growth and mortality. Three contrasting methods were used to estimate moggel gillnet selectivity. The Sechin, direct fyke net method and length-structured model all yielded similar results and correction factors obtained from the selectivity study were applied to the gillnet data to estimate the fish population size and structure in each reservoir. Using information from the life history and selectivity studies, the biomass and production of five moggel populations was estimated and related to abiotic and biotic factors in the reservoirs and their catchments. Moggel biomass and production was dependent on the biomass of algae, which was dependent on morphoedaphic characteristics of the reservoirs. Small, shallow reservoirs with a reasonable amount of human habitation in their catchments would sustain the highest algal biomass and provided they had adequate spawning habitat would also have the highest moggel biomass and production. The future research requirements for small reservoir fisheries are outlined and include a three-year program to develop a species-specific production model for any of the dominant species.
- Full Text:
- Date Issued: 2004
- Authors: Potts, Warren M
- Date: 2004
- Subjects: Freshwater fishes -- Africa, Southern Fish populations -- Africa, Southern Fisheries -- Africa, Southern Sustainable fisheries -- Africa, Southern Labeo -- Reproduction Cyprinidae -- Africa, Southern
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5330 , http://hdl.handle.net/10962/d1005176
- Description: The fish populations in small southern African reservoirs are largely unexploited and there is potential for fisheries development. However, the development of sustainable fisheries requires reliable estimates of potential yield or production. Empirical models that have been developed to predict fish production only apply to large water bodies and only predict total fish production, not the production of individual species. Small reservoirs generally have few commercially important species and therefore species-specific fish production models are an alternative approach. The small reservoirs of the Eastern Cape are dominated by the moggel (Labeo umbratus). The principal objectives of this thesis were to gain an understanding of the ecology of small reservoirs and the function of moggel in these systems. This information was used to design a research approach to rapidly develop species-specific models for small reservoirs in southern Africa. The limnology of two small reservoirs was compared. During the study period the reservoirs were turbid and showed a warm, monomictic pattern of thermal stratification. Anthropogenic pressure in the reservoir catchments appeared to be the overriding factor increasing the nutrient input to the reservoirs and consequently, influencing the biomass of algae in the reservoirs. The feeding biology of moggel in Katriver and Laing reservoirs was examined. Moggel are detrivorous and successfully digested diatoms. The slower growth rate of moggel in Katriver reservoir was attributed to the poorer nutritional value of the diet as a consequence of the lower concentration of diatoms in the detrital aggregate. The reproductive characteristics of moggel were examined in four reservoirs. Moggel were able to reproduce successfully in the reservoirs. This could be attributed to their r-selected reproductive strategy, with a high fecundity and an extended spawning season and their ability to spawn in a floodplain environment. Differences in recruitment success between years were found to be a consequence of the timing and duration of seasonal rains. The number of mature females in a population and the availability of suitable spawning habitat influenced recruitment success. The life history of the moggel in five reservoirs was compared. Growth appeared to be related to food availability, while mortality was lower in the populations where food was abundant and there were less predatory species. Size and age at maturity were not affected by environmental factors, but were dependent on growth and mortality. Three contrasting methods were used to estimate moggel gillnet selectivity. The Sechin, direct fyke net method and length-structured model all yielded similar results and correction factors obtained from the selectivity study were applied to the gillnet data to estimate the fish population size and structure in each reservoir. Using information from the life history and selectivity studies, the biomass and production of five moggel populations was estimated and related to abiotic and biotic factors in the reservoirs and their catchments. Moggel biomass and production was dependent on the biomass of algae, which was dependent on morphoedaphic characteristics of the reservoirs. Small, shallow reservoirs with a reasonable amount of human habitation in their catchments would sustain the highest algal biomass and provided they had adequate spawning habitat would also have the highest moggel biomass and production. The future research requirements for small reservoir fisheries are outlined and include a three-year program to develop a species-specific production model for any of the dominant species.
- Full Text:
- Date Issued: 2004
A comparison of the aquaculture potential of Clarias gariepinus (Burchell, 1822) and its hybrid with Heterobranchus longifilis valenciennes, 1840 in Southern Africa
- Authors: Oellermann, Lawrence Keith
- Date: 1996
- Subjects: Aquaculture -- South Africa Clarias Clariidae Catfishes -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5246 , http://hdl.handle.net/10962/d1005089
- Description: The aim of this study was to evaluate the aquaculture potential of a hybrid between the two largest African silurid catfish, Heterobranchus longifilis (HL) and Clarias gariepinus (CG) . A morphometric, meristic and karyological study showed that the hybrid shared some of the physical characteristics of both C. gariepinus and H. longifilis, while its chromosome complement and fundamental number (2N = 54, FN = 95), was intermediate between that of H. longifilis (2N = 52, FN = 92) and C. gariepinus (2N = 56, FN = 97). The HLxCG cross could therefore be regarded as a true hybrid. For the characters tested, no morphological or karyological differences were apparent between the HLxCG hybrids produced in West Africa and those produced in southern Africa. In southern Africa, the HL♂xCG♀ cross had higher fertilization and hatching rates than pure strain H. longifilis, C. gariepinus or the reverse cross. There was no difference in the survival of the HL♂xCG♀ hybrid larvae and C. gariepinus larvae up to the onset of exogenous feeding. Pure strain H. longifilis juveniles had a faster growth rate than the H. longifilis ♂ x C. gariepinus ♀ juveniles, but the hybrid always grew at a faster rate than C. gariepinus or the reverse cross. The HL♂xCG♀ cross was consequently chosen as the hybrid with the greatest potential for siluroid aquaculture in southern Africa. The HL♂xCG♀ hybrid showed evidence of partial gonadic, gametic and post-zygotic sterility in both sexes. The hybrid was not completely sterile, as it was artificially induced to spawn, and a small number of viable F₂ hybrid and F₁ hybrid x C. gariepinus larvae were produced. However, in the light of its probable reproductive strategy, it is highly unlikely that the hybrid would pose an ecological risk to the southern African region. The H. longifilis ♂ x C. gariepinus ♀ hybrid was compared to C. gariepinus for selected water quality preferences and tolerances. The hybrid had a wider temperature preference (28 °C to 34 °C) than C. gariepinus (28 °C to 30 °C), but appeared to be more dependent on aerial respiration than C. gariepinus. The air-breathing frequency of hybrid fish began to increase at dissolved oxygen concentrations below 3 .8 mg.ℓ⁻¹, while C. gariepinus only showed an increase in air-breathing frequency at concentrations below 3.0 mg.ℓ¹. The hybrid was more tolerant of un-ionised ammonia (96-hour LC₅₀ = 9.1 mg.ℓ⁻¹) than C. gariepinus (96-hour LC₅₀ = 6.5 mg.ℓ⁻¹), but their 96-hour LC₅₀ salinity tolerances were similar (10.8-11.0 g.ℓ⁻¹). The H. longifilis ♂ x C. gariepinus ♀ hybrid had a higher fillet yield (43.9 %) than C. gariepinus (38.9 %), but the crude protein content and amino acid profile of the two groups were similar. Catfish are traditionally grown in earthen ponds under semi-intensive conditions in southern Africa, at around 4 kg of fish per cubic meter of water (kg.m⁻³). However, the hybrid could tolerate densities of up to 415 kg of fish per cubic metre of water, if the water was exchanged hourly (kg.m⁻³.hr⁻¹), and the density at which yield was optimised was rounded off to 400 kg.m³.hr⁻¹. The high threshold density and water quality tolerances of the H. longifilis ♂ x C. gariepinus ♀ hybrid indicates that it is ideally suited for highly intensive aquaculture. It was concluded that it would be more productive to farm the HL♂xCG♀ hybrid on an intensive basis in southern Africa, than it would be to farm C. gariepinus in the traditional manner.
- Full Text:
- Date Issued: 1996
- Authors: Oellermann, Lawrence Keith
- Date: 1996
- Subjects: Aquaculture -- South Africa Clarias Clariidae Catfishes -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5246 , http://hdl.handle.net/10962/d1005089
- Description: The aim of this study was to evaluate the aquaculture potential of a hybrid between the two largest African silurid catfish, Heterobranchus longifilis (HL) and Clarias gariepinus (CG) . A morphometric, meristic and karyological study showed that the hybrid shared some of the physical characteristics of both C. gariepinus and H. longifilis, while its chromosome complement and fundamental number (2N = 54, FN = 95), was intermediate between that of H. longifilis (2N = 52, FN = 92) and C. gariepinus (2N = 56, FN = 97). The HLxCG cross could therefore be regarded as a true hybrid. For the characters tested, no morphological or karyological differences were apparent between the HLxCG hybrids produced in West Africa and those produced in southern Africa. In southern Africa, the HL♂xCG♀ cross had higher fertilization and hatching rates than pure strain H. longifilis, C. gariepinus or the reverse cross. There was no difference in the survival of the HL♂xCG♀ hybrid larvae and C. gariepinus larvae up to the onset of exogenous feeding. Pure strain H. longifilis juveniles had a faster growth rate than the H. longifilis ♂ x C. gariepinus ♀ juveniles, but the hybrid always grew at a faster rate than C. gariepinus or the reverse cross. The HL♂xCG♀ cross was consequently chosen as the hybrid with the greatest potential for siluroid aquaculture in southern Africa. The HL♂xCG♀ hybrid showed evidence of partial gonadic, gametic and post-zygotic sterility in both sexes. The hybrid was not completely sterile, as it was artificially induced to spawn, and a small number of viable F₂ hybrid and F₁ hybrid x C. gariepinus larvae were produced. However, in the light of its probable reproductive strategy, it is highly unlikely that the hybrid would pose an ecological risk to the southern African region. The H. longifilis ♂ x C. gariepinus ♀ hybrid was compared to C. gariepinus for selected water quality preferences and tolerances. The hybrid had a wider temperature preference (28 °C to 34 °C) than C. gariepinus (28 °C to 30 °C), but appeared to be more dependent on aerial respiration than C. gariepinus. The air-breathing frequency of hybrid fish began to increase at dissolved oxygen concentrations below 3 .8 mg.ℓ⁻¹, while C. gariepinus only showed an increase in air-breathing frequency at concentrations below 3.0 mg.ℓ¹. The hybrid was more tolerant of un-ionised ammonia (96-hour LC₅₀ = 9.1 mg.ℓ⁻¹) than C. gariepinus (96-hour LC₅₀ = 6.5 mg.ℓ⁻¹), but their 96-hour LC₅₀ salinity tolerances were similar (10.8-11.0 g.ℓ⁻¹). The H. longifilis ♂ x C. gariepinus ♀ hybrid had a higher fillet yield (43.9 %) than C. gariepinus (38.9 %), but the crude protein content and amino acid profile of the two groups were similar. Catfish are traditionally grown in earthen ponds under semi-intensive conditions in southern Africa, at around 4 kg of fish per cubic meter of water (kg.m⁻³). However, the hybrid could tolerate densities of up to 415 kg of fish per cubic metre of water, if the water was exchanged hourly (kg.m⁻³.hr⁻¹), and the density at which yield was optimised was rounded off to 400 kg.m³.hr⁻¹. The high threshold density and water quality tolerances of the H. longifilis ♂ x C. gariepinus ♀ hybrid indicates that it is ideally suited for highly intensive aquaculture. It was concluded that it would be more productive to farm the HL♂xCG♀ hybrid on an intensive basis in southern Africa, than it would be to farm C. gariepinus in the traditional manner.
- Full Text:
- Date Issued: 1996
The fishes of Tristan Da Cunha and Gough Island (South Atlantic), and the effects of environmental seasonality on the biology of selected species
- Authors: Andrew, Timothy Gilbert
- Date: 1993
- Subjects: Fishes -- Tristan da Cunha Fishes -- Gough Island Fishes -- Conservation
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5284 , http://hdl.handle.net/10962/d1005128
- Description: This study focuses on the taxonomy, biogeography and biological aspects of the fishes of Tristan da Cunha and Gough Island in the cental South Atlantic Ocean. Oceanographic measurements were undertaken to determine the position and nature of the Subtropical Convergence (STC) in the vicinity of the islands. The Tristan da Cunha group is thought to be situated on the northern edge of the STC while Gough Island is situated on the southern edge of the front. The seasonal environmental cycle at the islands is characterised by an annual sea surface temperature fluctuation of approximately 5 °C and an annual change in stratification of the water column. It is suggested that this increase in stratification at the STC, brought about in the summer by insolation warming the surface layers of the ocean, enhances primary production. The STC is identified as a unique habitat for fishes and as an important barrier to dispersal of species in the Southern Ocean. The present study has produced 18 new distributional records from the shelf waters of Tristan da Cunha and Gough Island. The fish fauna known from the shelf waters of these islands is reviewed, and a diagnosis, synonymy, and in some cases an illustration are provided. For certain species, brief notes on their biology, relative abundance and seasonal distribution are included. An analysis of the ichthyofaunal relationships between shallow water areas in the vicinity of the STC has resulted in the identification of a characteristic neritic STC ichthyofauna. The circumglobal distribution of many of these species is thought to be a consequence of a dispersive pelagic phase in their life-cycles. Biological processes in fishes at these islands are temporally and spatially affected by seasonal changes in oceanographic conditions. Seasonal sea temperature variation was identified as being an indicator of other oceanographic phenomena that may affect processes such as growth, reproduction and feeding in fishes at the islands. Most species have a summer spawning season and growth rate increased during the summer months. It was hypothesised that enhanced primary production and a related increase in food quality and availability during the summer were the major factors temporally regulating the growth and reproduction of fishes at the islands. Twenty families of neritic fishes have been recorded in the shelf waters of Tristan da Cunhaand Gough Island. Most families are represented by only one species. Because of this a wide range of life-history strategies are apparent in the fauna. Nevertheless, there are a number of common life-history characteristics shared by most of the neritic species. Firstly, the early lifehistory of many species included an extended pelagic phase which might have facilitated colonisation throughout the STC zone in the past, thereby regulating extant species diversity in this region. Secondly, the common occurrence of an extended summer breeding season and iteroparity suggested that these characters also contributed to survival in what is considered to be an unpredictable environment. It would appear that these common characteristics hold the key to the success of many fishes in the STC zone. This study has contributed to our understanding of oceanic island ecosystems and has revealed distributional patterns of fishes that were unknown previously. By focusing on the biological processes of selected species the regulatory role played by local environmental conditions became apparent. The results of this study are also relevant to proposed conservation measures for Tristan da Cunha and Gough Island.
- Full Text:
- Date Issued: 1993
- Authors: Andrew, Timothy Gilbert
- Date: 1993
- Subjects: Fishes -- Tristan da Cunha Fishes -- Gough Island Fishes -- Conservation
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5284 , http://hdl.handle.net/10962/d1005128
- Description: This study focuses on the taxonomy, biogeography and biological aspects of the fishes of Tristan da Cunha and Gough Island in the cental South Atlantic Ocean. Oceanographic measurements were undertaken to determine the position and nature of the Subtropical Convergence (STC) in the vicinity of the islands. The Tristan da Cunha group is thought to be situated on the northern edge of the STC while Gough Island is situated on the southern edge of the front. The seasonal environmental cycle at the islands is characterised by an annual sea surface temperature fluctuation of approximately 5 °C and an annual change in stratification of the water column. It is suggested that this increase in stratification at the STC, brought about in the summer by insolation warming the surface layers of the ocean, enhances primary production. The STC is identified as a unique habitat for fishes and as an important barrier to dispersal of species in the Southern Ocean. The present study has produced 18 new distributional records from the shelf waters of Tristan da Cunha and Gough Island. The fish fauna known from the shelf waters of these islands is reviewed, and a diagnosis, synonymy, and in some cases an illustration are provided. For certain species, brief notes on their biology, relative abundance and seasonal distribution are included. An analysis of the ichthyofaunal relationships between shallow water areas in the vicinity of the STC has resulted in the identification of a characteristic neritic STC ichthyofauna. The circumglobal distribution of many of these species is thought to be a consequence of a dispersive pelagic phase in their life-cycles. Biological processes in fishes at these islands are temporally and spatially affected by seasonal changes in oceanographic conditions. Seasonal sea temperature variation was identified as being an indicator of other oceanographic phenomena that may affect processes such as growth, reproduction and feeding in fishes at the islands. Most species have a summer spawning season and growth rate increased during the summer months. It was hypothesised that enhanced primary production and a related increase in food quality and availability during the summer were the major factors temporally regulating the growth and reproduction of fishes at the islands. Twenty families of neritic fishes have been recorded in the shelf waters of Tristan da Cunhaand Gough Island. Most families are represented by only one species. Because of this a wide range of life-history strategies are apparent in the fauna. Nevertheless, there are a number of common life-history characteristics shared by most of the neritic species. Firstly, the early lifehistory of many species included an extended pelagic phase which might have facilitated colonisation throughout the STC zone in the past, thereby regulating extant species diversity in this region. Secondly, the common occurrence of an extended summer breeding season and iteroparity suggested that these characters also contributed to survival in what is considered to be an unpredictable environment. It would appear that these common characteristics hold the key to the success of many fishes in the STC zone. This study has contributed to our understanding of oceanic island ecosystems and has revealed distributional patterns of fishes that were unknown previously. By focusing on the biological processes of selected species the regulatory role played by local environmental conditions became apparent. The results of this study are also relevant to proposed conservation measures for Tristan da Cunha and Gough Island.
- Full Text:
- Date Issued: 1993