The biology and management considerations of abundant large cyprinids in Lake le Roux, Orange River, South Africa
- Authors: Tómasson, Tumi
- Date: 1983
- Subjects: Barbus Labeo Cyprinidae Freshwater fishes -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5206 , http://hdl.handle.net/10962/d1004518
- Description: The biology of three large cyprinid fishes, Barbus holubi, B. kimberleyensis and Labeo capensis, was studied in a large turbid man-made lake on the Orange River, South Africa. The influence of environmental fluctuations on population dynamics was examined in relation to biological adaptations. On this basis inferences were made about the effect exploitation would have on the populations, and management alternatives were considered. Lake Ie Roux is situated in a semi- arid area, downstream from another large reservoir, Lake Verwoerd. In the pristine river, flooding is seasonal, and floodwaters carry a heavy silt load. Lake Verwoerd acts as a silt-trap and changes in turbidity in Lake Ie Roux (Secchi disc readings range from 15 to 160 cm) are influenced by hydrological management and are not necessarily seasonal. Water temperatures in the regulated river connecting the two lakes depend on the pattern of water release, and fluctuations in Iake levels depend on management practices. Reproduction was studied by examination of gonads and from the distribution of newly hatched juveniles. Age and growth was interpreted from the reading of scales and otoliths. Relative year class strength, dispersal and mortality were monitored using catch data from a regular, standardized gillnetting survey. The Barbus species spawn in the regulated river in spring or summer. Spawning is coordinated for a large part of the population and B. holubi spawn four to six weeks earlier than B. kimberleyensis. The juveniles are 'initially found along the shoreline, but later move into the pelagic zone and disperse throughout the lake. Year class strength is dependent on time of spawning and a late spawning results in a poor year class. For this reason, year class strength of B. kimberleyensis is generally poor, but there is a greater variation in year class strength of B. holubi, which is the dominant large Barbus. The two Barbus species exploit the pelagic zone, visually predating on zooplankton. When turbidity increases large scale mortalities occur, especially in B. holubi. Mortalities more than compensate for the reduction in carrying capacity, and the size of the reduced population may be inversely related to initial abundance. Growth rate is similarly affected by turbidity and density, and size at sexual maturity in B. holubi is reduced when growing conditions deteriorate. Labeo capensis do not form a homogenous population in Lake le Roux. Spawning occurs throughout the lake, but is erratic, probably depending on local rainfall. At each locality, more than one spawning may occur during spring and summer because of temporal variation in gonadal development. Juvenile survival appears to be mainly dependent on water level fluctuations, and strong year classes were formed when the lake was filling during the first two years. Subsequent year classes were weak. Subadult and adult B. capensis are herbivorous and depend on autochthonous production for food. They are relatively sedentary, but when turbidity increases fish smaller than 200 mm disperse, probably in response to food shortages. Mortality rates may be accelerated but mortalities are not as high as in B. holubi. The growth rate of B. capensis is variable and depends on turbidity and population density. Size at sexual maturity remained relatively constant during the study period. B. holubi has a relatively fixed reproduction cycle in Lake le Roux and exploitation is likely to dampen fluctuations in population density. Population growth and stability could be further promoted through hydrological management. B. kimberleyensis does not represent an exploitable population in Lake le Roux, although the large size obtained by this species (>3 kg) may be an attraction to anglers. B. capensis does not appear to have the potential to withstand sustained exploitation because of its erratic and generally poor reproductive success. However, the species is long-lived and can sustain relatively high densities under harsh conditions because of its relatively low mortality rates. It was concluded that harvesting of B. holubi must be flexible to promote maximum benefits to commercial and recreational fisheries, but B. capensis which is not an angling species, should chiefly be exploited when catches of B. holubi are poor, but left to recover in between. An alternative management strategy would be to commercially exploit the fish populations of several reservoirs. A monitoring programme could be established to decide when each reservoir is to be exploited to see to the interests of commercial and recreational fishing alike.
- Full Text:
- Authors: Tómasson, Tumi
- Date: 1983
- Subjects: Barbus Labeo Cyprinidae Freshwater fishes -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5206 , http://hdl.handle.net/10962/d1004518
- Description: The biology of three large cyprinid fishes, Barbus holubi, B. kimberleyensis and Labeo capensis, was studied in a large turbid man-made lake on the Orange River, South Africa. The influence of environmental fluctuations on population dynamics was examined in relation to biological adaptations. On this basis inferences were made about the effect exploitation would have on the populations, and management alternatives were considered. Lake Ie Roux is situated in a semi- arid area, downstream from another large reservoir, Lake Verwoerd. In the pristine river, flooding is seasonal, and floodwaters carry a heavy silt load. Lake Verwoerd acts as a silt-trap and changes in turbidity in Lake Ie Roux (Secchi disc readings range from 15 to 160 cm) are influenced by hydrological management and are not necessarily seasonal. Water temperatures in the regulated river connecting the two lakes depend on the pattern of water release, and fluctuations in Iake levels depend on management practices. Reproduction was studied by examination of gonads and from the distribution of newly hatched juveniles. Age and growth was interpreted from the reading of scales and otoliths. Relative year class strength, dispersal and mortality were monitored using catch data from a regular, standardized gillnetting survey. The Barbus species spawn in the regulated river in spring or summer. Spawning is coordinated for a large part of the population and B. holubi spawn four to six weeks earlier than B. kimberleyensis. The juveniles are 'initially found along the shoreline, but later move into the pelagic zone and disperse throughout the lake. Year class strength is dependent on time of spawning and a late spawning results in a poor year class. For this reason, year class strength of B. kimberleyensis is generally poor, but there is a greater variation in year class strength of B. holubi, which is the dominant large Barbus. The two Barbus species exploit the pelagic zone, visually predating on zooplankton. When turbidity increases large scale mortalities occur, especially in B. holubi. Mortalities more than compensate for the reduction in carrying capacity, and the size of the reduced population may be inversely related to initial abundance. Growth rate is similarly affected by turbidity and density, and size at sexual maturity in B. holubi is reduced when growing conditions deteriorate. Labeo capensis do not form a homogenous population in Lake le Roux. Spawning occurs throughout the lake, but is erratic, probably depending on local rainfall. At each locality, more than one spawning may occur during spring and summer because of temporal variation in gonadal development. Juvenile survival appears to be mainly dependent on water level fluctuations, and strong year classes were formed when the lake was filling during the first two years. Subsequent year classes were weak. Subadult and adult B. capensis are herbivorous and depend on autochthonous production for food. They are relatively sedentary, but when turbidity increases fish smaller than 200 mm disperse, probably in response to food shortages. Mortality rates may be accelerated but mortalities are not as high as in B. holubi. The growth rate of B. capensis is variable and depends on turbidity and population density. Size at sexual maturity remained relatively constant during the study period. B. holubi has a relatively fixed reproduction cycle in Lake le Roux and exploitation is likely to dampen fluctuations in population density. Population growth and stability could be further promoted through hydrological management. B. kimberleyensis does not represent an exploitable population in Lake le Roux, although the large size obtained by this species (>3 kg) may be an attraction to anglers. B. capensis does not appear to have the potential to withstand sustained exploitation because of its erratic and generally poor reproductive success. However, the species is long-lived and can sustain relatively high densities under harsh conditions because of its relatively low mortality rates. It was concluded that harvesting of B. holubi must be flexible to promote maximum benefits to commercial and recreational fisheries, but B. capensis which is not an angling species, should chiefly be exploited when catches of B. holubi are poor, but left to recover in between. An alternative management strategy would be to commercially exploit the fish populations of several reservoirs. A monitoring programme could be established to decide when each reservoir is to be exploited to see to the interests of commercial and recreational fishing alike.
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Nepticulidae (Lepidoptera) : a revised classification, and descriptions of some new taxa particularly from southern Africa
- Authors: Scoble, M J
- Date: 1982
- Subjects: Nepticulidae Nepticulidae--Africa, Southern
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5801 , http://hdl.handle.net/10962/d1006183
- Description: Previous classifications of the Nepticulidae were based on species from the northern hemisphere, primarily western Europe (including the United Kingdom) and North America. The revised classification presented in the present work was developed from a study of material primarily from southern Africa, but also from Australia, Europe, North America, and the Orient. Most of the descriptive section deals with the taxonomy of the Nepticulidae of southern Africa. Nevertheless, a guide to the supraspecific classification of the family was a major aim. Cladistic methods were used to assess genealogy as far as possible. The degree to which it has been elucidated is incorporated into the classification. As a result of the study, the Nepticulidae are divided into two subfamilies. One of these subfamilies is divided into two tribes. Fifteen genera and six subgenera are recognised. Two keys are presented, one to subfamilies and tribes and the other to genera and subgenera. The species from southern Africa are included in eight genera, three of which are new. One hundred and seventeen species from southern Africa are considered. Seventy- two new species are described in this work. Of these 40 have been published and 32 appear as manuscript names. For some species described by previous workers lectotypes have been designated where appropriate. The primary types of all the known species from southern Africa have been examined. Observations on adult structure are discussed from the viewpoint of phylogenetic relationships both within the Nepticulidae and between the family and other lepidopteran groups. This has led to a re-interpretation and expansion of some aspects of nepticulid morphology. Numerical phenetic methods, which include cluster analyses and an ordination technique (principal component analysis), were used to check my personal assessment of phena. The illustrations (phenograms and ordination diagrams) provide a visual summary of phenetic relationships of as wide a range of Nepticulidae as possible. Aspects of the nature of taxonomic characters, ancestor-descendant relationships, and homoplasy are discussed. Brief comments are made on distribution, and host-plant choice and phylogeny.
- Full Text:
- Authors: Scoble, M J
- Date: 1982
- Subjects: Nepticulidae Nepticulidae--Africa, Southern
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5801 , http://hdl.handle.net/10962/d1006183
- Description: Previous classifications of the Nepticulidae were based on species from the northern hemisphere, primarily western Europe (including the United Kingdom) and North America. The revised classification presented in the present work was developed from a study of material primarily from southern Africa, but also from Australia, Europe, North America, and the Orient. Most of the descriptive section deals with the taxonomy of the Nepticulidae of southern Africa. Nevertheless, a guide to the supraspecific classification of the family was a major aim. Cladistic methods were used to assess genealogy as far as possible. The degree to which it has been elucidated is incorporated into the classification. As a result of the study, the Nepticulidae are divided into two subfamilies. One of these subfamilies is divided into two tribes. Fifteen genera and six subgenera are recognised. Two keys are presented, one to subfamilies and tribes and the other to genera and subgenera. The species from southern Africa are included in eight genera, three of which are new. One hundred and seventeen species from southern Africa are considered. Seventy- two new species are described in this work. Of these 40 have been published and 32 appear as manuscript names. For some species described by previous workers lectotypes have been designated where appropriate. The primary types of all the known species from southern Africa have been examined. Observations on adult structure are discussed from the viewpoint of phylogenetic relationships both within the Nepticulidae and between the family and other lepidopteran groups. This has led to a re-interpretation and expansion of some aspects of nepticulid morphology. Numerical phenetic methods, which include cluster analyses and an ordination technique (principal component analysis), were used to check my personal assessment of phena. The illustrations (phenograms and ordination diagrams) provide a visual summary of phenetic relationships of as wide a range of Nepticulidae as possible. Aspects of the nature of taxonomic characters, ancestor-descendant relationships, and homoplasy are discussed. Brief comments are made on distribution, and host-plant choice and phylogeny.
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A study of phosphorus dynamics in the main basin of Swartvlei, September - December 1980
- Authors: Silberbauer, M J
- Date: 1981
- Subjects: Phosphorus -- Environmental aspects -- South Africa -- Swartvlei Lakes -- South Africa -- Swartvlei Limnology -- South Africa -- Swartvlei Aquatic ecology -- South Africa -- Swartvlei
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5016 , http://hdl.handle.net/10962/d1006146
- Description: Introduction: Phosphorus is an especially scarce element in the aquatic environment, yet it is of vital importance to living organisms: the pyrophosphate bond is the basis of biochemical energy transfer, and phosphate groups make up the backbone of the nucleic acids (Wetzel, 1975). An understanding of phosphorus movements and transformations is thus essential to a study of the biological functioning of a lake. The present report describes a study of phosphorus dynamics inSwartvlei, a lake with an extremely low pelagic primary production rate, in the order of 0,15 gC.m⁻².d⁻¹ (Robarts, 1973; Howard-Williams and Allanson, 1978). The littoral phosphorus dynamics of Swartvlei have been discussed in some detail (Howard-Williams, 1977; Howard-Williams and Allanson, 1978), and what follows is an attempt to quantify the changes in phosphorus concentration occurring in the deeper parts of the lake, which comprise some 57% of the total area.
- Full Text:
- Authors: Silberbauer, M J
- Date: 1981
- Subjects: Phosphorus -- Environmental aspects -- South Africa -- Swartvlei Lakes -- South Africa -- Swartvlei Limnology -- South Africa -- Swartvlei Aquatic ecology -- South Africa -- Swartvlei
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5016 , http://hdl.handle.net/10962/d1006146
- Description: Introduction: Phosphorus is an especially scarce element in the aquatic environment, yet it is of vital importance to living organisms: the pyrophosphate bond is the basis of biochemical energy transfer, and phosphate groups make up the backbone of the nucleic acids (Wetzel, 1975). An understanding of phosphorus movements and transformations is thus essential to a study of the biological functioning of a lake. The present report describes a study of phosphorus dynamics inSwartvlei, a lake with an extremely low pelagic primary production rate, in the order of 0,15 gC.m⁻².d⁻¹ (Robarts, 1973; Howard-Williams and Allanson, 1978). The littoral phosphorus dynamics of Swartvlei have been discussed in some detail (Howard-Williams, 1977; Howard-Williams and Allanson, 1978), and what follows is an attempt to quantify the changes in phosphorus concentration occurring in the deeper parts of the lake, which comprise some 57% of the total area.
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The classification and phylogeny of the Psocoptera
- Authors: Smithers, Courtenay Neville
- Date: 1970
- Subjects: Psocoptera , Insects -- Classification
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5884 , http://hdl.handle.net/10962/d1013232
- Description: This work provides a phylogenetic classification of the insect order PSOCOPTERA. Some of the problems involved, mainly arising from lack of adequate published data, are pointed out and work carried out to overcome them is indicated in a short introductory chapter (Chapter I). This consisted of accummulating data on the genera from published texts and illustrations, adding data from the study of fresh material or material held in collections and compiling generic definitions in adequate detail where possible. Chapter II gives a general description of the Psocoptera together with brief background information on their biology. As considerable changes are proposed in the classification of the order (in Chapter VII) the classification in use at present is set out for comparison to generic level and a brief history of systematic work on the order is given {Chapter III). The data necessary for a discussion of the phylogeny is presented in the series of definitions of genera and suprageneric groups in Chapter IV. Data on fossil forms is given in Chapter V. The principles of phylogenetic study are briefly discussed in Chapter VI and the important question of the relatively primitive or advanced condition of characters in the order is discussed. The monophyly of the order and the relationships between genera are established using Hennig's system and the results are set out in discussion and dendrogram. On the basis of the relationships so established a classification of the order is proposed which is considered to be practical and to reflect evolutionary history of the group. (Chapter. VII.) Comments on the distribution of the Psocoptera are made in Chapter VIII and it is suggested that, despite inadequacy of data, a consideration of the distributions supports the proposed classification in general terms. A general discussion follows and references and figures are included. An appendix provides a practical up-to-date key to the genera of the order.
- Full Text:
- Authors: Smithers, Courtenay Neville
- Date: 1970
- Subjects: Psocoptera , Insects -- Classification
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5884 , http://hdl.handle.net/10962/d1013232
- Description: This work provides a phylogenetic classification of the insect order PSOCOPTERA. Some of the problems involved, mainly arising from lack of adequate published data, are pointed out and work carried out to overcome them is indicated in a short introductory chapter (Chapter I). This consisted of accummulating data on the genera from published texts and illustrations, adding data from the study of fresh material or material held in collections and compiling generic definitions in adequate detail where possible. Chapter II gives a general description of the Psocoptera together with brief background information on their biology. As considerable changes are proposed in the classification of the order (in Chapter VII) the classification in use at present is set out for comparison to generic level and a brief history of systematic work on the order is given {Chapter III). The data necessary for a discussion of the phylogeny is presented in the series of definitions of genera and suprageneric groups in Chapter IV. Data on fossil forms is given in Chapter V. The principles of phylogenetic study are briefly discussed in Chapter VI and the important question of the relatively primitive or advanced condition of characters in the order is discussed. The monophyly of the order and the relationships between genera are established using Hennig's system and the results are set out in discussion and dendrogram. On the basis of the relationships so established a classification of the order is proposed which is considered to be practical and to reflect evolutionary history of the group. (Chapter. VII.) Comments on the distribution of the Psocoptera are made in Chapter VIII and it is suggested that, despite inadequacy of data, a consideration of the distributions supports the proposed classification in general terms. A general discussion follows and references and figures are included. An appendix provides a practical up-to-date key to the genera of the order.
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Aspects of the ecology of piscivorous birds of Lake Kyle Rhodesia
- Authors: Junor, F J R
- Date: 1969
- Subjects: Bird populations -- Zimbabwe , Water birds -- Zimbabwe
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5909 , http://hdl.handle.net/10962/d1015201
- Description: [From Introduction]. From an early period in the lake's history research was undataken to determine the potential of Kyle as a source of edible fish, In such a study, knowledge of the number and weight of fish consumed by piscivorous birds would seem to be an obvious consideration. Accordingly special investigation was undertaken into the food requirements of fish eating birds which live in the lake area. The method employed, on this occasion, to obtain the required information has differed radically from that more generally used by investigators working with similar objects in view. The common practise has been to examine the stomach contents of a considerable number of birds of the same species, which have been destroyed in order to make information available.
- Full Text:
- Authors: Junor, F J R
- Date: 1969
- Subjects: Bird populations -- Zimbabwe , Water birds -- Zimbabwe
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5909 , http://hdl.handle.net/10962/d1015201
- Description: [From Introduction]. From an early period in the lake's history research was undataken to determine the potential of Kyle as a source of edible fish, In such a study, knowledge of the number and weight of fish consumed by piscivorous birds would seem to be an obvious consideration. Accordingly special investigation was undertaken into the food requirements of fish eating birds which live in the lake area. The method employed, on this occasion, to obtain the required information has differed radically from that more generally used by investigators working with similar objects in view. The common practise has been to examine the stomach contents of a considerable number of birds of the same species, which have been destroyed in order to make information available.
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