Occurrence of mugilid and sparid fishes in Zostera capensis and bare sediment habitats of the Knysna Estuary
- Authors: Pollard, Melissa
- Date: 2018
- Subjects: Gray mullets South Africa Knysna Lagoon , Sparidae South Africa Knysna Lagoon , Seagrasses South Africa Knysna Lagoon , Eelgrass South Africa Knysna Lagoon , Gray mullets Habitat South Africa Knysna Lagoon , Sparidae Habitat South Africa Knysna Lagoon , Underwater videography in wildlife monitoring , Seining , Zostera capensis Setchel
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/54979 , vital:26641
- Description: Seagrass meadows are regarded as one of the most unique and valuable ecosystems in the biosphere, primarily because of the variety of services that they provide. Seagrass meadows serve as nursery grounds for many species and often play an important role in the juvenile stages of economically and recreationally important fish. Zostera capensis Setchell is one of the most dominant submerged macrophytes and the most common seagrass in South African estuaries and is often referred to as eelgrass. Zostera capensis meadows occupy a large area within the Knysna Estuary but little is known about their importance to associated fish assemblages. With Z. capensis meadows being under increased pressure from anthropogenic influences, it is important to establish which fish species and families utilize these habitats and what role they play in the ecology of fish assemblages within the Knysna Estuary. The primary objective of this study was to compare the use of Z. capensis and adjacent bare sediment areas by mainly large juvenile and subadult sparids and mugilids using different techniques. The two main hypotheses were as follows; 1) Mugilidae are likely to be more dominant in the unvegetated areas of the estuary littoral and Sparidae are likely to predominate within the Z. capensis bed areas of the estuary littoral. 2) The non-destructive underwater video monitoring method would yield similar fish composition data to seine netting sampling of the identical sites. Both hypotheses were assessed using data collected during this study and the analysis of historical unpublished data. Overall, Mugilidae were more abundant at bare unvegetated areas where they did most of their foraging. Sparidae were more abundant in the Z. capensis beds, which was also the habitat where they primarily foraged. With regards to the comparison of two different sampling methods, namely underwater video monitoring and seine netting, similar patterns arose with regards to the fish species observed in camera footage and those captured in the seine net, although the abundances were not always comparable.
- Full Text:
- Authors: Pollard, Melissa
- Date: 2018
- Subjects: Gray mullets South Africa Knysna Lagoon , Sparidae South Africa Knysna Lagoon , Seagrasses South Africa Knysna Lagoon , Eelgrass South Africa Knysna Lagoon , Gray mullets Habitat South Africa Knysna Lagoon , Sparidae Habitat South Africa Knysna Lagoon , Underwater videography in wildlife monitoring , Seining , Zostera capensis Setchel
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/54979 , vital:26641
- Description: Seagrass meadows are regarded as one of the most unique and valuable ecosystems in the biosphere, primarily because of the variety of services that they provide. Seagrass meadows serve as nursery grounds for many species and often play an important role in the juvenile stages of economically and recreationally important fish. Zostera capensis Setchell is one of the most dominant submerged macrophytes and the most common seagrass in South African estuaries and is often referred to as eelgrass. Zostera capensis meadows occupy a large area within the Knysna Estuary but little is known about their importance to associated fish assemblages. With Z. capensis meadows being under increased pressure from anthropogenic influences, it is important to establish which fish species and families utilize these habitats and what role they play in the ecology of fish assemblages within the Knysna Estuary. The primary objective of this study was to compare the use of Z. capensis and adjacent bare sediment areas by mainly large juvenile and subadult sparids and mugilids using different techniques. The two main hypotheses were as follows; 1) Mugilidae are likely to be more dominant in the unvegetated areas of the estuary littoral and Sparidae are likely to predominate within the Z. capensis bed areas of the estuary littoral. 2) The non-destructive underwater video monitoring method would yield similar fish composition data to seine netting sampling of the identical sites. Both hypotheses were assessed using data collected during this study and the analysis of historical unpublished data. Overall, Mugilidae were more abundant at bare unvegetated areas where they did most of their foraging. Sparidae were more abundant in the Z. capensis beds, which was also the habitat where they primarily foraged. With regards to the comparison of two different sampling methods, namely underwater video monitoring and seine netting, similar patterns arose with regards to the fish species observed in camera footage and those captured in the seine net, although the abundances were not always comparable.
- Full Text:
Predation by alien largemouth bass, Micropterus salmoides Lacepéde 1802 (Centrarchidae: Perciformes), on indigenous marine fish species in the Kowie System, South Africa
- Authors: Magoro, Mandla Leon
- Date: 2014
- Subjects: Largemouth bass -- South Africa -- Port Alfred , Largemouth bass -- Food -- South Africa -- Port Alfred , Estuarine fishes -- South Africa -- Port Alfred , Estuarine fishes -- Predators of , Centrarchidae , Fishes -- Effect of human beings on
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5851 , http://hdl.handle.net/10962/d1011939 , Largemouth bass -- South Africa -- Port Alfred , Largemouth bass -- Food -- South Africa -- Port Alfred , Estuarine fishes -- South Africa -- Port Alfred , Estuarine fishes -- Predators of , Centrarchidae , Fishes -- Effect of human beings on
- Description: Estuaries serve as nursery areas for a large number of estuary-associated fish species. Some of these taxa also use river catchments as nursery areas. During the upstream migration of this latter group, the juveniles are prone to predation by native and alien predatory fish inhabiting the system. The rate of invasion of ecosystems by alien organisms can be directly linked to anthropogenic influences, including both intentional and unintentional introductions by alien organisms into new regions. The largemouth bass, Micropterus salmoides, is a facultative piscivorous fish that has been successfully introduced worldwide for the main purpose of sport fishing. Where introduced, it has been found to negatively impact native fish and invertebrate species through predation, competitive exclusion and displacement of indigenous fish species. The aim of this thesis was to investigate the predatory impact of largemouth bass on the estuary-associated Cape moony Monodactylus falciformis, Cape stumpnose Rhabdosargus holubi and freshwater mullet Myxus capensis in the lower Kowie River of the Eastern Cape Province, South Africa. A combination of approaches was employed during this study. Stomach contents, stable isotopes and fatty acid analyses were employed for the reconstruction of the diet of largemouth bass. Acoustic telemetry was used to elucidate largemouth bass movements, particularly their ability to enter the upper reaches of the Kowie Estuary. Stomach contents and stable isotopes analyses showed that M. salmoides exhibit an ontogenic shift in diet, with small and medium sized individuals consuming the higher proportion of fish prey, while large sized individuals mostly consumed invertebrates such as crabs (Potamonautes sidneyi) and Odonata larvae, while consuming only a small proportion of estuary-associated fish. Fatty acid analysis only showed a direct connection between the fatty acid profiles of largemouth bass and those of M. capensis and M. falciformis. The acoustic telemetry results indicated that some M. salmoides individuals periodically move into the upper reaches of the estuary following river flood events. The results obtained from all these approaches highlight the risk posed by largemouth bass introductions on indigenous fish species, particularly those that enter the areas occupied by these top predators.
- Full Text:
- Authors: Magoro, Mandla Leon
- Date: 2014
- Subjects: Largemouth bass -- South Africa -- Port Alfred , Largemouth bass -- Food -- South Africa -- Port Alfred , Estuarine fishes -- South Africa -- Port Alfred , Estuarine fishes -- Predators of , Centrarchidae , Fishes -- Effect of human beings on
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5851 , http://hdl.handle.net/10962/d1011939 , Largemouth bass -- South Africa -- Port Alfred , Largemouth bass -- Food -- South Africa -- Port Alfred , Estuarine fishes -- South Africa -- Port Alfred , Estuarine fishes -- Predators of , Centrarchidae , Fishes -- Effect of human beings on
- Description: Estuaries serve as nursery areas for a large number of estuary-associated fish species. Some of these taxa also use river catchments as nursery areas. During the upstream migration of this latter group, the juveniles are prone to predation by native and alien predatory fish inhabiting the system. The rate of invasion of ecosystems by alien organisms can be directly linked to anthropogenic influences, including both intentional and unintentional introductions by alien organisms into new regions. The largemouth bass, Micropterus salmoides, is a facultative piscivorous fish that has been successfully introduced worldwide for the main purpose of sport fishing. Where introduced, it has been found to negatively impact native fish and invertebrate species through predation, competitive exclusion and displacement of indigenous fish species. The aim of this thesis was to investigate the predatory impact of largemouth bass on the estuary-associated Cape moony Monodactylus falciformis, Cape stumpnose Rhabdosargus holubi and freshwater mullet Myxus capensis in the lower Kowie River of the Eastern Cape Province, South Africa. A combination of approaches was employed during this study. Stomach contents, stable isotopes and fatty acid analyses were employed for the reconstruction of the diet of largemouth bass. Acoustic telemetry was used to elucidate largemouth bass movements, particularly their ability to enter the upper reaches of the Kowie Estuary. Stomach contents and stable isotopes analyses showed that M. salmoides exhibit an ontogenic shift in diet, with small and medium sized individuals consuming the higher proportion of fish prey, while large sized individuals mostly consumed invertebrates such as crabs (Potamonautes sidneyi) and Odonata larvae, while consuming only a small proportion of estuary-associated fish. Fatty acid analysis only showed a direct connection between the fatty acid profiles of largemouth bass and those of M. capensis and M. falciformis. The acoustic telemetry results indicated that some M. salmoides individuals periodically move into the upper reaches of the estuary following river flood events. The results obtained from all these approaches highlight the risk posed by largemouth bass introductions on indigenous fish species, particularly those that enter the areas occupied by these top predators.
- Full Text:
Structure and functioning of fish assemblages in two South African estuaries, with emphasis on the presence and absence of aquatic macrophyte beds
- Authors: Sheppard, Jill Nicole
- Date: 2010
- Subjects: Estuaries -- South Africa -- Eastern Cape , Estuarine ecology -- South Africa -- Eastern Cape
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5321 , http://hdl.handle.net/10962/d1005166 , Estuaries -- South Africa -- Eastern Cape , Estuarine ecology -- South Africa -- Eastern Cape
- Description: Temporarily open/closed estuaries (TOCEs) are the dominant estuary type in South Africa. These systems are often characterized by extensive beds of submerged macrophytes, which form important foraging and shelter habitats for fishes, especially for estuary-dependent fish species such as the Cape stumpnose Rhabdosargus holubi and Cape moony Monodactylus falciformis that are commonly associated with them. A loss of submerged macrophytes from an estuary has been shown to affect the fish community as well as reducing overall system productivity. The TOC East Kleinemonde Estuary, situated in the warm-temperate biogeographic region of South Africa has been subject to an ongoing long-term fish monitoring project since 1995. During the period 1995 to 2002, this estuary contained large beds of the submerged macrophytes Ruppia cirrhosa and Potamogeton pectinatus. However, subsequent to a major flood event in 2003 these macrophytes have been largely absent from this system. The effect of the loss of submerged macrophytes on the East Kleinemonde fish assemblage was investigated through an analysis of seine and gill net catch data. Seine net catches for a 12 year period, encompassing six years of macrophyte presence and six years of macrophyte senescence, revealed changes in the relative abundance of certain fish species. Vegetation-associated species such as R. holubi and M. falciformis decreased in abundance whereas sediment-associated species, especially members of the family Mugilidae, increased in abundance following loss of the macrophytes in this estuary. The critically endangered pipefish Syngnathus watermeyeri was only recorded in catches during years in which macrophyte beds were present. In addition to the analysis of catch data, the importance of macrophytes as a primary energy source for selected estuarine fishes was explored through the analysis of carbon and nitrogen stable isotopes. Prior to conducting these analyses, common methodological practices to address the presence of carbonates and lipids within isotope samples were evaluated. A subset of samples were either acid washed to remove carbonates, or lipids were removed according to the method of Bligh and Dyer (1959) as both of these compounds have been shown to affect stable carbon and nitrogen isotope ratios. The suitability of the lipid normalization models of Fry (2002) and Post et al. (2007) for samples of estuarine fish muscle were also tested. Based on this evaluation both models are suitable for use with estuarine fish muscle tissue, however since neither carbonate nor lipid content of any of the samples used in this study was high all samples were left untreated in the following analysis. Carbon isotope ratios from a wide range of fish species collected from the East Kleinemonde Estuary during the macrophyte-senescent phase were compared with individuals of the same species from the neighbouring West Kleinemonde Estuary (where extensive beds of R. cirrhosa and P. pectinatus were present) and revealed the influence of submerged macrophyte material in the diet of fishes in the latter system. However, it was apparent that these plants are not directly consumed but rather contribute to the detrital pool that forms a food source for most invertebrate and some fish species. The most significant source of carbon for East Kleinemonde fishes during the macrophyte senescent phase appeared to have a more depleted origin; probably from benthic or pelagic microalgae. In conclusion, while the importance of macrophyte beds as shelter and foraging habitats for estuarine fishes are well documented, their role in terms of the structuring and functioning of fish assemblages in TOCEs remains somewhat uncertain. The findings of this study were possibly masked by the resilience of vegetation-associated species to the loss of this habitat, as well as by life history characteristics of species such as R. holubi that allow their numerical dominance despite habitat change. Nonetheless, macrophyte senescence in the East Kleinemonde Estuary resulted in the loss of at least one species and the reduced abundance of vegetation-associated species, probably reflective of reduced food resources and/or increased vulnerability to predation. As a result, beds of submerged macrophytes are an important habitat within TOCEs.
- Full Text:
- Authors: Sheppard, Jill Nicole
- Date: 2010
- Subjects: Estuaries -- South Africa -- Eastern Cape , Estuarine ecology -- South Africa -- Eastern Cape
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5321 , http://hdl.handle.net/10962/d1005166 , Estuaries -- South Africa -- Eastern Cape , Estuarine ecology -- South Africa -- Eastern Cape
- Description: Temporarily open/closed estuaries (TOCEs) are the dominant estuary type in South Africa. These systems are often characterized by extensive beds of submerged macrophytes, which form important foraging and shelter habitats for fishes, especially for estuary-dependent fish species such as the Cape stumpnose Rhabdosargus holubi and Cape moony Monodactylus falciformis that are commonly associated with them. A loss of submerged macrophytes from an estuary has been shown to affect the fish community as well as reducing overall system productivity. The TOC East Kleinemonde Estuary, situated in the warm-temperate biogeographic region of South Africa has been subject to an ongoing long-term fish monitoring project since 1995. During the period 1995 to 2002, this estuary contained large beds of the submerged macrophytes Ruppia cirrhosa and Potamogeton pectinatus. However, subsequent to a major flood event in 2003 these macrophytes have been largely absent from this system. The effect of the loss of submerged macrophytes on the East Kleinemonde fish assemblage was investigated through an analysis of seine and gill net catch data. Seine net catches for a 12 year period, encompassing six years of macrophyte presence and six years of macrophyte senescence, revealed changes in the relative abundance of certain fish species. Vegetation-associated species such as R. holubi and M. falciformis decreased in abundance whereas sediment-associated species, especially members of the family Mugilidae, increased in abundance following loss of the macrophytes in this estuary. The critically endangered pipefish Syngnathus watermeyeri was only recorded in catches during years in which macrophyte beds were present. In addition to the analysis of catch data, the importance of macrophytes as a primary energy source for selected estuarine fishes was explored through the analysis of carbon and nitrogen stable isotopes. Prior to conducting these analyses, common methodological practices to address the presence of carbonates and lipids within isotope samples were evaluated. A subset of samples were either acid washed to remove carbonates, or lipids were removed according to the method of Bligh and Dyer (1959) as both of these compounds have been shown to affect stable carbon and nitrogen isotope ratios. The suitability of the lipid normalization models of Fry (2002) and Post et al. (2007) for samples of estuarine fish muscle were also tested. Based on this evaluation both models are suitable for use with estuarine fish muscle tissue, however since neither carbonate nor lipid content of any of the samples used in this study was high all samples were left untreated in the following analysis. Carbon isotope ratios from a wide range of fish species collected from the East Kleinemonde Estuary during the macrophyte-senescent phase were compared with individuals of the same species from the neighbouring West Kleinemonde Estuary (where extensive beds of R. cirrhosa and P. pectinatus were present) and revealed the influence of submerged macrophyte material in the diet of fishes in the latter system. However, it was apparent that these plants are not directly consumed but rather contribute to the detrital pool that forms a food source for most invertebrate and some fish species. The most significant source of carbon for East Kleinemonde fishes during the macrophyte senescent phase appeared to have a more depleted origin; probably from benthic or pelagic microalgae. In conclusion, while the importance of macrophyte beds as shelter and foraging habitats for estuarine fishes are well documented, their role in terms of the structuring and functioning of fish assemblages in TOCEs remains somewhat uncertain. The findings of this study were possibly masked by the resilience of vegetation-associated species to the loss of this habitat, as well as by life history characteristics of species such as R. holubi that allow their numerical dominance despite habitat change. Nonetheless, macrophyte senescence in the East Kleinemonde Estuary resulted in the loss of at least one species and the reduced abundance of vegetation-associated species, probably reflective of reduced food resources and/or increased vulnerability to predation. As a result, beds of submerged macrophytes are an important habitat within TOCEs.
- Full Text:
A preliminary investigation into the use of biomarkers and a fish community index to assess estuarine health in selected Eastern Cape estuaries
- Authors: Richardson, Naomi
- Date: 2008
- Subjects: Fish communities -- South Africa -- Eastern Cape Estuarine fishes -- South Africa -- Eastern Cape Estuarine health -- South Africa -- Eastern Cape Estuaries -- South Africa -- Eastern Cape -- Management Coastal zone management -- South Africa -- Eastern Cape Environmental monitoring -- South Africa -- Eastern Cape Estuarine ecology -- South Africa -- Eastern Cape
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5348 , http://hdl.handle.net/10962/d1007480
- Description: The aims of this study were to determine the potential use of biomarkers at multiple levels of biological organisation together with a fish community bioindicator to assess the estuarine health status of three temporarily open/closed estuaries. The estuaries investigated were the East Kleinemonde (EK), Old Woman's (OW) and Mtana (MTN), all of which are situated in the Eastern Cape Province. Three biomarkers, the acetylcholinesterase (AChE) assay, lipid peroxidation (LPx) assay, liver histopathology and a condition factor were used to determine sub-organism health and one bioindicator, the Estuarine Fish Community Index (EFCI), was used as a bioindicator of community health. The estuarine-dependent marine species Rhabdosargus holubi was selected as an indicator species for the sub-organism level analyses. The results from the community analyses indicated that the EK and OW estuaries were in 'good' condition, while the MTN was found to be in 'moderate' condition. Histological analyses revealed that R. holubi from all three estuaries showed signs of pathological changes to the liver, with the fish from the MTN eliciting the highest occurrence of these changes. The LPx assay found that R. holubi from both the OW and MTN showed signs of oxidative damage in the liver tissue, but those from the EK did not appear to be affected. The AChE assay showed that only the fish from the OW had been affected by anticholinesterase compounds. A laboratory study was undertaken using R. holubi as a positive control for the AChE and LPx assay. The fish were exposed to 3 μg/L chlorpyrifos, a known cholinesterase inhibitor, for six hours and their tissues were examined for changes to LPx levels and AChE activities. AChE activity was significantly inhibited (Mann Whitney U test, Z = 3.65, n = 38, P < 0.001) by the exposure, but LPx levels were not significantly affected. A composite index incorporating the biomarkers at different biological levels of organisation was developed. The index was designed to assist managers and scientists to determine whether the ichthyofauna of a system was being affected by environmental stressors and what management interventions could be undertaken to ameliorate the water quality in an estuary. The index was applied to the three estuaries investigated during the present study and both the OW and MTN were assessed to be in need of immediate management intervention. The fish in the OW were found to be stressed at all the sub-organism levels measured and the reason for this was hypothesised to be as a result of golf course activities in this adjacent estuary. A number of management actions are proposed to reduce the sub-organism stress observed in the fish from the OW. The livers of fish from the MTN were shown to be under stress; however the causative agent of this stress was unknown because there is no formal development in the MTN catchment. However, a possible contaminant source is proposed and management interventions to alleviate the stress on the biota of the MTN are suggested. The EK does not require immediate management intervention, however, continuous routine monitoring is recommended to ensure that conditions do not deteriorate. Shortcomings of the index were outlined and a number of suggestions were made in terms of other measures of biological health which could be incorporated into the index.
- Full Text:
- Authors: Richardson, Naomi
- Date: 2008
- Subjects: Fish communities -- South Africa -- Eastern Cape Estuarine fishes -- South Africa -- Eastern Cape Estuarine health -- South Africa -- Eastern Cape Estuaries -- South Africa -- Eastern Cape -- Management Coastal zone management -- South Africa -- Eastern Cape Environmental monitoring -- South Africa -- Eastern Cape Estuarine ecology -- South Africa -- Eastern Cape
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5348 , http://hdl.handle.net/10962/d1007480
- Description: The aims of this study were to determine the potential use of biomarkers at multiple levels of biological organisation together with a fish community bioindicator to assess the estuarine health status of three temporarily open/closed estuaries. The estuaries investigated were the East Kleinemonde (EK), Old Woman's (OW) and Mtana (MTN), all of which are situated in the Eastern Cape Province. Three biomarkers, the acetylcholinesterase (AChE) assay, lipid peroxidation (LPx) assay, liver histopathology and a condition factor were used to determine sub-organism health and one bioindicator, the Estuarine Fish Community Index (EFCI), was used as a bioindicator of community health. The estuarine-dependent marine species Rhabdosargus holubi was selected as an indicator species for the sub-organism level analyses. The results from the community analyses indicated that the EK and OW estuaries were in 'good' condition, while the MTN was found to be in 'moderate' condition. Histological analyses revealed that R. holubi from all three estuaries showed signs of pathological changes to the liver, with the fish from the MTN eliciting the highest occurrence of these changes. The LPx assay found that R. holubi from both the OW and MTN showed signs of oxidative damage in the liver tissue, but those from the EK did not appear to be affected. The AChE assay showed that only the fish from the OW had been affected by anticholinesterase compounds. A laboratory study was undertaken using R. holubi as a positive control for the AChE and LPx assay. The fish were exposed to 3 μg/L chlorpyrifos, a known cholinesterase inhibitor, for six hours and their tissues were examined for changes to LPx levels and AChE activities. AChE activity was significantly inhibited (Mann Whitney U test, Z = 3.65, n = 38, P < 0.001) by the exposure, but LPx levels were not significantly affected. A composite index incorporating the biomarkers at different biological levels of organisation was developed. The index was designed to assist managers and scientists to determine whether the ichthyofauna of a system was being affected by environmental stressors and what management interventions could be undertaken to ameliorate the water quality in an estuary. The index was applied to the three estuaries investigated during the present study and both the OW and MTN were assessed to be in need of immediate management intervention. The fish in the OW were found to be stressed at all the sub-organism levels measured and the reason for this was hypothesised to be as a result of golf course activities in this adjacent estuary. A number of management actions are proposed to reduce the sub-organism stress observed in the fish from the OW. The livers of fish from the MTN were shown to be under stress; however the causative agent of this stress was unknown because there is no formal development in the MTN catchment. However, a possible contaminant source is proposed and management interventions to alleviate the stress on the biota of the MTN are suggested. The EK does not require immediate management intervention, however, continuous routine monitoring is recommended to ensure that conditions do not deteriorate. Shortcomings of the index were outlined and a number of suggestions were made in terms of other measures of biological health which could be incorporated into the index.
- Full Text:
Trends in fish community structure and recruitment in a temporarily open/closed South African estuary
- Authors: James, Nicola Caroline
- Date: 2007
- Subjects: Fish communities -- South Africa -- Eastern Cape Marine fishes -- South Africa -- Eastern Cape Estuarine fishes -- South Africa -- Eastern Cape Estuarine ecology -- South Africa -- Eastern Cape
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5332 , http://hdl.handle.net/10962/d1005178
- Description: Long-term interannual changes in richness, abundance, diversity and structure of the fish community in the temporarily open/closed East Kleinemonde Estuary, Eastern Cape, are described and the recruitment success of two estuary-dependent marine species assessed. In addition, laboratory experiments were conducted to assess the possible role of olfaction in the recruitment process of an estuary-dependent marine fish species. Multivariate analyses of the annual marine fish communites identified two distinct groups with more species recorded during years that succeeded spring (September to November) mouth opening events than in years following no mouth opening events in spring. Interannual community stability (IMD) and seriation (IMS) also increased from the ‘other’ to the ‘spring’ years. These results highlight the importance of the timing of mouth opening to the marine fish community in a temporarily open/closed estuary. This study also made use of long-term records of daily mouth state and linked them to the recruitment of distinct year-class cohorts in two spardis with contrasting lifehistory characteristics. Lithognathus lithognathus only recruited into the estuary in years when the mouth opened between late August and January. This was linked to the limited spawning season of this species and its inability to enter closed estuaries via wave overwash events. In contrast, recruitment by Rhabdosargus holubi juveniles appeared to be uninterrupted and was not determined by the seasonality of mouth opening. This species dominates the marine-spawning component of the East Kleinemonde Estuary and its success is attributed to an extended spawning season and its ability to recruit into estuaries during both overwash and open mouth conditions. Attraction of postflexion Rhabdosargus holubi larvae to estuary, surf zone and river water was also measured using a rectangular choice chamber. In two sets of experiments, conducted during peak recruitment periods, larvae from both the surf zone and estuary mouth region selected estuary water with a significantly higher frequency than sea water. Larvae collected in the mouth region showed a stronger preference for river water than those collected in the surf zone, thus suggesting that these fish are more attracted to freshwater influenced nursery areas once they have entered the estuary than those in the surf zone. Larvae collected in the marine environment also selected surf zone water with significantly higher frequencies than estuary water or offshore sea water, thus confirming the importance of the surf zone as an interim nursery area for postflexion R. holubi.
- Full Text:
- Authors: James, Nicola Caroline
- Date: 2007
- Subjects: Fish communities -- South Africa -- Eastern Cape Marine fishes -- South Africa -- Eastern Cape Estuarine fishes -- South Africa -- Eastern Cape Estuarine ecology -- South Africa -- Eastern Cape
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5332 , http://hdl.handle.net/10962/d1005178
- Description: Long-term interannual changes in richness, abundance, diversity and structure of the fish community in the temporarily open/closed East Kleinemonde Estuary, Eastern Cape, are described and the recruitment success of two estuary-dependent marine species assessed. In addition, laboratory experiments were conducted to assess the possible role of olfaction in the recruitment process of an estuary-dependent marine fish species. Multivariate analyses of the annual marine fish communites identified two distinct groups with more species recorded during years that succeeded spring (September to November) mouth opening events than in years following no mouth opening events in spring. Interannual community stability (IMD) and seriation (IMS) also increased from the ‘other’ to the ‘spring’ years. These results highlight the importance of the timing of mouth opening to the marine fish community in a temporarily open/closed estuary. This study also made use of long-term records of daily mouth state and linked them to the recruitment of distinct year-class cohorts in two spardis with contrasting lifehistory characteristics. Lithognathus lithognathus only recruited into the estuary in years when the mouth opened between late August and January. This was linked to the limited spawning season of this species and its inability to enter closed estuaries via wave overwash events. In contrast, recruitment by Rhabdosargus holubi juveniles appeared to be uninterrupted and was not determined by the seasonality of mouth opening. This species dominates the marine-spawning component of the East Kleinemonde Estuary and its success is attributed to an extended spawning season and its ability to recruit into estuaries during both overwash and open mouth conditions. Attraction of postflexion Rhabdosargus holubi larvae to estuary, surf zone and river water was also measured using a rectangular choice chamber. In two sets of experiments, conducted during peak recruitment periods, larvae from both the surf zone and estuary mouth region selected estuary water with a significantly higher frequency than sea water. Larvae collected in the mouth region showed a stronger preference for river water than those collected in the surf zone, thus suggesting that these fish are more attracted to freshwater influenced nursery areas once they have entered the estuary than those in the surf zone. Larvae collected in the marine environment also selected surf zone water with significantly higher frequencies than estuary water or offshore sea water, thus confirming the importance of the surf zone as an interim nursery area for postflexion R. holubi.
- Full Text:
Biogeography and community structure of fishes in South African estuaries
- Authors: Harrison, Trevor D
- Date: 2003
- Subjects: Estuarine ecology -- South Africa Estuarine fishes -- South Africa Fish communities -- South Africa Biogeography -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5235 , http://hdl.handle.net/10962/d1005078
- Description: The biogeography and community structure of the fishes of South African estuaries was investigated. In all, 109 systems were examined representing two broad types: temporarily closed and permanently open estuaries. Multivariate analyses of the fish communities identified three biogeographic regions. A cool-temperate region extended along the west and southwest coasts; a warmtemperate zone stretched along the south, southeast and east coasts and a subtropical region occurred along the east coast. The boundaries of these biogeographic regions were also delineated. The general physico-chemical characteristics of the estuaries within the three biogeographic regions also reflected regional differences in climate, rainfall and ocean conditions. Estuarine temperatures followed the trend for marine coastal waters, decreasing from subtropical estuaries toward cool-temperate systems. The low rainfall and runoff in the warm-temperate region together with high evaporation rates and strong seawater input resulted in higher salinities in these estuaries. These factors also accounted for the predominantly clearer waters in warm-temperate estuaries. The estuaries in the three biogeographic regions were also shown to contain somewhat distinctive fish assemblages. Temperature and salinity appeared to be the two main factors affecting the distribution and abundance of fishes in South African estuaries. Subtropical systems were characterised by fishes mostly of tropical origin as well as certain south coast endemic species. Warm-temperate estuaries were dominated by endemic taxa with some tropical species also present. The fish fauna of cooltemperate estuaries mostly comprised south coast endemic species with cosmopolitan and temperate taxa also present. Certain functional components of the ichthyofauna also exhibited slight differences between regions. Freshwater fishes were a major component of closed subtropical estuaries while estuarine resident species were more abundant in warm-temperate estuaries. Overall, estuarine-dependent marine species dominated the fish fauna of the estuaries in all biogeographic regions, signifying that South African estuaries perform a vital nursery function for this group of fishes. Slight differences were also apparent in the trophic structure of the fishes; these were related to environmental differences between regions. Zooplanktivores and fishes that feed on aquatic macrophytes/invertebrates assumed a relatively higher importance in warm-temperate systems. Overall, detritivores dominated the estuarine fish fauna in all regions, indicating that detritus forms the main energy source in South African estuaries.
- Full Text:
- Authors: Harrison, Trevor D
- Date: 2003
- Subjects: Estuarine ecology -- South Africa Estuarine fishes -- South Africa Fish communities -- South Africa Biogeography -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5235 , http://hdl.handle.net/10962/d1005078
- Description: The biogeography and community structure of the fishes of South African estuaries was investigated. In all, 109 systems were examined representing two broad types: temporarily closed and permanently open estuaries. Multivariate analyses of the fish communities identified three biogeographic regions. A cool-temperate region extended along the west and southwest coasts; a warmtemperate zone stretched along the south, southeast and east coasts and a subtropical region occurred along the east coast. The boundaries of these biogeographic regions were also delineated. The general physico-chemical characteristics of the estuaries within the three biogeographic regions also reflected regional differences in climate, rainfall and ocean conditions. Estuarine temperatures followed the trend for marine coastal waters, decreasing from subtropical estuaries toward cool-temperate systems. The low rainfall and runoff in the warm-temperate region together with high evaporation rates and strong seawater input resulted in higher salinities in these estuaries. These factors also accounted for the predominantly clearer waters in warm-temperate estuaries. The estuaries in the three biogeographic regions were also shown to contain somewhat distinctive fish assemblages. Temperature and salinity appeared to be the two main factors affecting the distribution and abundance of fishes in South African estuaries. Subtropical systems were characterised by fishes mostly of tropical origin as well as certain south coast endemic species. Warm-temperate estuaries were dominated by endemic taxa with some tropical species also present. The fish fauna of cooltemperate estuaries mostly comprised south coast endemic species with cosmopolitan and temperate taxa also present. Certain functional components of the ichthyofauna also exhibited slight differences between regions. Freshwater fishes were a major component of closed subtropical estuaries while estuarine resident species were more abundant in warm-temperate estuaries. Overall, estuarine-dependent marine species dominated the fish fauna of the estuaries in all biogeographic regions, signifying that South African estuaries perform a vital nursery function for this group of fishes. Slight differences were also apparent in the trophic structure of the fishes; these were related to environmental differences between regions. Zooplanktivores and fishes that feed on aquatic macrophytes/invertebrates assumed a relatively higher importance in warm-temperate systems. Overall, detritivores dominated the estuarine fish fauna in all regions, indicating that detritus forms the main energy source in South African estuaries.
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Dynamics of early stage fishes associated with selected warm temperate estuaries in South Africa
- Authors: Strydom, Nadine Amelia
- Date: 2002
- Subjects: Estuarine fishes -- South Africa Fishes -- Infancy -- South Africa Estuarine ecology -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5262 , http://hdl.handle.net/10962/d1005106
- Description: Early stage fishes, namely larvae and early juveniles, were collected from 12 estuaries and associated habitats in the warm temperate region of South Africa between July 1998 and December 2000. This study served to provide new information and expand on existing knowledge of early stage fish assemblages associated with various types of Eastern Cape estuaries. A total of 65 536 predominantly postflexion fish larvae were collected. Together with early juveniles, taxa comprised 72 species from 25 teleost fish families. Surf zones associated with two intermittently open estuaries showed that estuary-dependent marine species predominated in this zone. Early stage fishes responded positively during natural estuary opening events and concentrated along the estuary outflow plume, suggesting that cueing from estuary and/or river water may be taking place. A habitat study in the Swartkops Estuary, using light traps, showed that newly recruited larval fishes concentrated along the margins of the estuary and unlike their older juvenile counterparts, were poorly represented in eelgrass beds. A multi-estuary comparison, including seven permanently open and five intermittently open estuaries, indicated that early stage fish assemblages were more diverse than indicated by past investigations. Early stage fishes were also shown to concentrate in the mesohaline regions of these estuaries. Studies of estuaries with altered freshwater flow regimes were also included in the research. A regulated release of dam water in the euhaline Kromme Estuary failed to induce a cueing response from estuary-dependent marine fish larvae. This project showed that large amounts of freshwater are required to reverse the negative effects of river impoundments. The release study did show that estuary-resident fishes were sensitive to small natural pulses of freshwater and responded by spawning. However, excessive river supply through an inter-basin water transfer scheme was shown to have a flushing effect on the larvae and early juveniles of the estuary-resident Gilchristella aestuaria in the upper reaches of the Great Fish Estuary. This effect was evident in comparisons with the freshwater starved Kariega Estuary where concentrations of G. aestuaria, a species usually more abundant in freshwater rich estuaries, were considerably higher. This finding suggests that freshwater minima and maxima be considered when assessing and managing estuarine systems.
- Full Text:
- Authors: Strydom, Nadine Amelia
- Date: 2002
- Subjects: Estuarine fishes -- South Africa Fishes -- Infancy -- South Africa Estuarine ecology -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5262 , http://hdl.handle.net/10962/d1005106
- Description: Early stage fishes, namely larvae and early juveniles, were collected from 12 estuaries and associated habitats in the warm temperate region of South Africa between July 1998 and December 2000. This study served to provide new information and expand on existing knowledge of early stage fish assemblages associated with various types of Eastern Cape estuaries. A total of 65 536 predominantly postflexion fish larvae were collected. Together with early juveniles, taxa comprised 72 species from 25 teleost fish families. Surf zones associated with two intermittently open estuaries showed that estuary-dependent marine species predominated in this zone. Early stage fishes responded positively during natural estuary opening events and concentrated along the estuary outflow plume, suggesting that cueing from estuary and/or river water may be taking place. A habitat study in the Swartkops Estuary, using light traps, showed that newly recruited larval fishes concentrated along the margins of the estuary and unlike their older juvenile counterparts, were poorly represented in eelgrass beds. A multi-estuary comparison, including seven permanently open and five intermittently open estuaries, indicated that early stage fish assemblages were more diverse than indicated by past investigations. Early stage fishes were also shown to concentrate in the mesohaline regions of these estuaries. Studies of estuaries with altered freshwater flow regimes were also included in the research. A regulated release of dam water in the euhaline Kromme Estuary failed to induce a cueing response from estuary-dependent marine fish larvae. This project showed that large amounts of freshwater are required to reverse the negative effects of river impoundments. The release study did show that estuary-resident fishes were sensitive to small natural pulses of freshwater and responded by spawning. However, excessive river supply through an inter-basin water transfer scheme was shown to have a flushing effect on the larvae and early juveniles of the estuary-resident Gilchristella aestuaria in the upper reaches of the Great Fish Estuary. This effect was evident in comparisons with the freshwater starved Kariega Estuary where concentrations of G. aestuaria, a species usually more abundant in freshwater rich estuaries, were considerably higher. This finding suggests that freshwater minima and maxima be considered when assessing and managing estuarine systems.
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Ichthyofaunal community structures in different types of Eastern Cape estuaries
- Authors: Vorwerk, Paul D
- Date: 2001
- Subjects: Estuarine fishes -- South Africa -- Eastern Cape , Estuarine fishes -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5377 , http://hdl.handle.net/10962/d1016241
- Description: This investigation had three major objectives; firstly to document the estuarine fishes from several poorly studied Eastern Cape estuaries, secondly, to identify similarities and differences in the fish communities from permanently open and intermittently open estuaries, and thirdly, to establish which physico-chemical or environmental variables are responsible for the structuring of these communities. The study area incorporated 10 estuaries along a 70 km long stretch of the Eastern Cape coastline between the towns of Seafield (33° 32' 42" S, 27° 03' 05" E) in the south-east and Hamburg (33° 16' 45" S, 27° 29' 50" E) in the north-west. The systems investigated included two permanently open estuaries and eight intermittently open systems of varying sizes. Overall fish abundance and species richness in the study area revealed no significant seasonal variation. A contrast between open and closed estuaries was evident in the dominance of species from different estuarine dependence categories. Open estuaries comprised mainly category Ib, lla and IIb species, with a high proportion of category IV species. Closed estuaries were dominated by category IIa species, with few category IV species represented. Clear longitudinal trends in relative abundance were demonstrated for individual species. Margalefs species richness index for seine net catches, and a combination of the seine and gill net data, were significantly correlated with catchment size, mean annual runoff, estuarine area and linear length. Fish assemblages in the permanently open and intermittently open estuaries were significantly different (p<0.001) when analysed on a community and density basis. Similarly, the communities in large and small intermittently open estuaries differed significantly on a community (p=0.01) and density basis (p<0.001). These differences where caused by changes in abundance of the dominant estuarine resident and marine migrant species. The environmental variable that was dominant in accounting for these differences was the estuary mouth status. Estuarine resident species had smaller overall body lengths when compared with the marine migrant species. In addition, the estuarine resident species had smaller modal size classes in the closed estuaries when compared with the open estuaries. These differences may be linked to the foraging strategies of the species, as well as food availability and different levels of predation in the different estuary types. The marine migrant species did not show any trends in this respect although the closed systems had the largest individuals for all four dominant marine species. This finding may be linked to the inability of these species to breed in closed estuaries, thus channelling reproductive energy resources into somatic growth. In addition, prolonged periods of mouth closure prevent the emigration of large individuals back to the sea. Otter trawling was conducted in fiye estuaries and captured mostly demersal species. The otter trawl data confirmed the seine net community analysis, with significantly different communities identified in the permanently open and intermittently open estuaries (p=0.02), as well as in the small and large intermittently open estuaries (p=0.03). The environmental variables responsible for these community differences were mouth status and variables associated with estuarine size. The identification of estuary mouth status by this study as the most influential environmental factor is compounded by this "ariable affecting or being indicative of numerous other physico-chemical features. The large differences in the fish communities in different estuary types indicate the importance of each estuary type to various species. This should be taken into account before allowing any form of commercial fishing in these important nursery areas.
- Full Text:
- Authors: Vorwerk, Paul D
- Date: 2001
- Subjects: Estuarine fishes -- South Africa -- Eastern Cape , Estuarine fishes -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5377 , http://hdl.handle.net/10962/d1016241
- Description: This investigation had three major objectives; firstly to document the estuarine fishes from several poorly studied Eastern Cape estuaries, secondly, to identify similarities and differences in the fish communities from permanently open and intermittently open estuaries, and thirdly, to establish which physico-chemical or environmental variables are responsible for the structuring of these communities. The study area incorporated 10 estuaries along a 70 km long stretch of the Eastern Cape coastline between the towns of Seafield (33° 32' 42" S, 27° 03' 05" E) in the south-east and Hamburg (33° 16' 45" S, 27° 29' 50" E) in the north-west. The systems investigated included two permanently open estuaries and eight intermittently open systems of varying sizes. Overall fish abundance and species richness in the study area revealed no significant seasonal variation. A contrast between open and closed estuaries was evident in the dominance of species from different estuarine dependence categories. Open estuaries comprised mainly category Ib, lla and IIb species, with a high proportion of category IV species. Closed estuaries were dominated by category IIa species, with few category IV species represented. Clear longitudinal trends in relative abundance were demonstrated for individual species. Margalefs species richness index for seine net catches, and a combination of the seine and gill net data, were significantly correlated with catchment size, mean annual runoff, estuarine area and linear length. Fish assemblages in the permanently open and intermittently open estuaries were significantly different (p<0.001) when analysed on a community and density basis. Similarly, the communities in large and small intermittently open estuaries differed significantly on a community (p=0.01) and density basis (p<0.001). These differences where caused by changes in abundance of the dominant estuarine resident and marine migrant species. The environmental variable that was dominant in accounting for these differences was the estuary mouth status. Estuarine resident species had smaller overall body lengths when compared with the marine migrant species. In addition, the estuarine resident species had smaller modal size classes in the closed estuaries when compared with the open estuaries. These differences may be linked to the foraging strategies of the species, as well as food availability and different levels of predation in the different estuary types. The marine migrant species did not show any trends in this respect although the closed systems had the largest individuals for all four dominant marine species. This finding may be linked to the inability of these species to breed in closed estuaries, thus channelling reproductive energy resources into somatic growth. In addition, prolonged periods of mouth closure prevent the emigration of large individuals back to the sea. Otter trawling was conducted in fiye estuaries and captured mostly demersal species. The otter trawl data confirmed the seine net community analysis, with significantly different communities identified in the permanently open and intermittently open estuaries (p=0.02), as well as in the small and large intermittently open estuaries (p=0.03). The environmental variables responsible for these community differences were mouth status and variables associated with estuarine size. The identification of estuary mouth status by this study as the most influential environmental factor is compounded by this "ariable affecting or being indicative of numerous other physico-chemical features. The large differences in the fish communities in different estuary types indicate the importance of each estuary type to various species. This should be taken into account before allowing any form of commercial fishing in these important nursery areas.
- Full Text:
Ecology of the ichthyofauna in three temporarily open/closed estuaries on the Natal coast
- Authors: Harrison, Trevor D
- Date: 1994
- Subjects: Estuarine ecology -- South Africa -- KwaZulu-Natal , Estuarine animals -- South Africa -- Swartkops River Estuary , Fishes -- South Africa -- KwaZulu-Natal
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5269 , http://hdl.handle.net/10962/d1005113 , Estuarine ecology -- South Africa -- KwaZulu-Natal , Estuarine animals -- South Africa -- Swartkops River Estuary , Fishes -- South Africa -- KwaZulu-Natal
- Description: The ichthyofauna of three small Natal estuaries, the Mhlanga, Damba and Zotsha was sampled over a period of two years. A total of 68 kinds of fishes representing 24 families, 39 genera and 56 species were captured during this study. Forty seven kinds of fishes were recorded in the Mhlanga of which Gilchristella aestuaria,Oreochromis mossambicus, Valamugil cunnesius, Valamugil sp. and juvenile mugilids numerically dominated. In terms of biomass, O. mossambicus, V. cunnesius, Liza alata, Myxus capensis and Mugil cephalus dominated the ichthyofauna of the Mhlanga. In the Damba, 24 kinds of fishes were recorded. The most abundant fishes captured were Glossogobius callidus, M. capensis and O. mossambicus. M. capensis, M. cephalus, O. mossambicus and G. callidus dominated the fish biomass captured in the Damba. A total of 56 kinds of fishes were recorded in the Zotsha during this study. The ichthyofauna of the Zotsha was numerically dominated by juvenile mugilids, G. aestuaria, O. mossambicus, Rhabdosargus holubi, Terapon jarbua, Ambassis productus and G. callidus. The species which dominated the fish biomass in the Zotsha were O. mossambicus, L. alata, Valamugil robustus, Valamugil buchanani, M. capensis, M. cephalus and V. cunnesius. Classifying the species captured according to whether they were resident estuarine species, freshwater species, estuarine-dependent marine species and marine species revealed that the first three groups were all well represented in the systems. The only system in which marine species made any significant contribution to the ichthyofauna was the Zotsha. Oreochromis mossambicus was the dominant freshwater species in all three estuaries during this study. Gilchristella aestuaria and Glossogobius callidus were the principal estuarine species in the Mhlanga and the Damba respectively. Both G. aestuaria and G. callidus were the dominant estuarine species captured in the Zotsha. The principal estuarine-dependent marine fishes captured in the Mhlanga were V. cunnesius, Valamugil sp., juvenile mugilids, M. capensis, M. cephalus and L. alata. In the Damba, M. capensis and M. cephalus were the dominant estuarine-dependent marine species and in the Zotsha juvenile mugilids, R. holubi, T. jarbua, A . productus, M. capensis, V. cunnesius, V. robustus, M. cephalus and L. alata predominated. The results of this study indicate that the estuaries are dominated at different periods by different assemblages of fishes. This is linked to the spawning and migration patterns of the various species as well as the hydrological regime of each estuary. During the winter these systems are normally closed with relatively deep waters and high food resource and habitat availability. Freshwater and estuarine species mainly inhabit the upper reaches of the systems while estuarine-dependent marine species mainly occupy the middle and lower reaches and dominate the fish community. When these systems open with the onset of the spring/summer rains, adult and sub-adult estuarine-dependent marine species emigrate to the marine environment and juveniles begin recruiting into the systems. Spring is also the peak breeding period of resident estuarine and freshwater species, resulting in an increase in the contribution of these fishes to the overall population during this period. When closed estuaries open many of them drain and this results in the fishes concentrating in the lower reaches of the system where moderate water depths are present, thus further contributing to an increase in the proportion of freshwater and estuarine species in this region. The breaching of closed estuaries also results in a slump in food resources and habitat availability. Competition and possible increased vulnerability to avian predation (due to the shallow nature of the systems), may contribute to a decrease in the proportion of estuarine and freshwater species in summer. The prolonged spawning and recruitment of 0+ juveniles of estuarine-dependent marine species results in an increase in the proportion of these fishes present in the estuaries during summer. In autumn, these systems normally close, water levels rise and available food resources and habitat increase. This allows the redistribution of freshwater and estuarine species upstream, leaving estuarine-dependent marine species to dominate the middle and lower reaches. Although temporarily open/closed estuaries along the Natal coast may not be as diverse as permanently open estuaries in terms of their ichthyofauna, their importance must not be underestimated since by providing a continuous sequence of sheltered habitats along the coast they may contribute significantly to the viability of estuarine-dependent marine fish stocks.
- Full Text:
- Authors: Harrison, Trevor D
- Date: 1994
- Subjects: Estuarine ecology -- South Africa -- KwaZulu-Natal , Estuarine animals -- South Africa -- Swartkops River Estuary , Fishes -- South Africa -- KwaZulu-Natal
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5269 , http://hdl.handle.net/10962/d1005113 , Estuarine ecology -- South Africa -- KwaZulu-Natal , Estuarine animals -- South Africa -- Swartkops River Estuary , Fishes -- South Africa -- KwaZulu-Natal
- Description: The ichthyofauna of three small Natal estuaries, the Mhlanga, Damba and Zotsha was sampled over a period of two years. A total of 68 kinds of fishes representing 24 families, 39 genera and 56 species were captured during this study. Forty seven kinds of fishes were recorded in the Mhlanga of which Gilchristella aestuaria,Oreochromis mossambicus, Valamugil cunnesius, Valamugil sp. and juvenile mugilids numerically dominated. In terms of biomass, O. mossambicus, V. cunnesius, Liza alata, Myxus capensis and Mugil cephalus dominated the ichthyofauna of the Mhlanga. In the Damba, 24 kinds of fishes were recorded. The most abundant fishes captured were Glossogobius callidus, M. capensis and O. mossambicus. M. capensis, M. cephalus, O. mossambicus and G. callidus dominated the fish biomass captured in the Damba. A total of 56 kinds of fishes were recorded in the Zotsha during this study. The ichthyofauna of the Zotsha was numerically dominated by juvenile mugilids, G. aestuaria, O. mossambicus, Rhabdosargus holubi, Terapon jarbua, Ambassis productus and G. callidus. The species which dominated the fish biomass in the Zotsha were O. mossambicus, L. alata, Valamugil robustus, Valamugil buchanani, M. capensis, M. cephalus and V. cunnesius. Classifying the species captured according to whether they were resident estuarine species, freshwater species, estuarine-dependent marine species and marine species revealed that the first three groups were all well represented in the systems. The only system in which marine species made any significant contribution to the ichthyofauna was the Zotsha. Oreochromis mossambicus was the dominant freshwater species in all three estuaries during this study. Gilchristella aestuaria and Glossogobius callidus were the principal estuarine species in the Mhlanga and the Damba respectively. Both G. aestuaria and G. callidus were the dominant estuarine species captured in the Zotsha. The principal estuarine-dependent marine fishes captured in the Mhlanga were V. cunnesius, Valamugil sp., juvenile mugilids, M. capensis, M. cephalus and L. alata. In the Damba, M. capensis and M. cephalus were the dominant estuarine-dependent marine species and in the Zotsha juvenile mugilids, R. holubi, T. jarbua, A . productus, M. capensis, V. cunnesius, V. robustus, M. cephalus and L. alata predominated. The results of this study indicate that the estuaries are dominated at different periods by different assemblages of fishes. This is linked to the spawning and migration patterns of the various species as well as the hydrological regime of each estuary. During the winter these systems are normally closed with relatively deep waters and high food resource and habitat availability. Freshwater and estuarine species mainly inhabit the upper reaches of the systems while estuarine-dependent marine species mainly occupy the middle and lower reaches and dominate the fish community. When these systems open with the onset of the spring/summer rains, adult and sub-adult estuarine-dependent marine species emigrate to the marine environment and juveniles begin recruiting into the systems. Spring is also the peak breeding period of resident estuarine and freshwater species, resulting in an increase in the contribution of these fishes to the overall population during this period. When closed estuaries open many of them drain and this results in the fishes concentrating in the lower reaches of the system where moderate water depths are present, thus further contributing to an increase in the proportion of freshwater and estuarine species in this region. The breaching of closed estuaries also results in a slump in food resources and habitat availability. Competition and possible increased vulnerability to avian predation (due to the shallow nature of the systems), may contribute to a decrease in the proportion of estuarine and freshwater species in summer. The prolonged spawning and recruitment of 0+ juveniles of estuarine-dependent marine species results in an increase in the proportion of these fishes present in the estuaries during summer. In autumn, these systems normally close, water levels rise and available food resources and habitat increase. This allows the redistribution of freshwater and estuarine species upstream, leaving estuarine-dependent marine species to dominate the middle and lower reaches. Although temporarily open/closed estuaries along the Natal coast may not be as diverse as permanently open estuaries in terms of their ichthyofauna, their importance must not be underestimated since by providing a continuous sequence of sheltered habitats along the coast they may contribute significantly to the viability of estuarine-dependent marine fish stocks.
- Full Text:
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