PRODUCTION OF THE AFRICAN CATFISH (CLARIAS GARIEPINUS) I. GROWTH,MORTALITY AND YIELD SOUTH OF THE ZAMBEZI

SUMMARY

Clarias gariepinus were sampled from eight water bodies in southern Africa between 1973 and 1975. Aging results indicate a similar growth pattern for most areas with a near linear age-length relationship. Between year growth compensation was found to be present in Lakes Kyle and Mcllwaine in Zimbabwe, and to a lesser extent in Hardap Dam, South West Africa, but not present in Lake Kariba, Zimbabwe.

The fishable biomass of catfish from Lake Kyle is estimated between 0,61 to 1,14 kg ha^?1. Fishable biomass for all species is in the range 12,1 to 22,7 kg ha^?1.

The instantaneous growth rate was found to be between 0,20 and 0,25 for fully recruited age groups. The fishing mortality appears to be in the range of 0,4 for Lakes Kyle and Kariba and 0,5 for Lake McIlwaine. All of these levels are above the yield per recruit F₀, 1 levels of 0,27, 0,28 and 0,24 respectively which would normally indicate that each of these stocks is being overexploited. However, this does not appear to be the case in these tropical impoundments.     

Biology of the African clupeid Limnothrissa miodon with reference to its small size in artificial lakes

Summary Limnothrissa miodon is a clupeid from Lake Tanganyika which has been introduced to Lakes Kivu and Kariba and which invaded Lake Cahora Bassa. These lakes differ considerably from one another but the biology of Limnothrissa is similar in many respects in all of them. Similarities include its feeding and breeding biology, whilst its populations fluctuate, on both an annual and a seasonal basis, in relation to environmental changes. The major differences between the lakes concern the size to which Limnothrissa grows. Their average length in the two natural lakes, Lakes Tanganyika and Kivu, is about twice the average length in the two artificial lakes, Lakes Kariba and Cahora Bassa. This in turn affects their age of maturity and they breed during their first year in the reservoirs but during their second in the natural lakes. Their growth rates and performance are comparable up to about 6 months of age and the difference in their size seems to be due to high mortality in the artificial lakes where few Limnothrissa survive for more than a year. Possible explanations for their small size include the genetic composition of the original introduction and the unpredictability of the environment. Neither of these explanations is supported by evidence at present. Deficiencies in the available food and the effects of intense predation are probably the most likely causes. In both natural lakes, Limnothrissa are predatory or cannibalistic from 100 mm in length and 12 months in age, and fish in their diet may enable them to survive into their second year. The fish in Lakes Kariba and Cahora Bassa, in contrast, are neither predatory nor cannibalistic to any degree. There are no predatory fish species in Lake Kivu but there is a diverse community of pelagic piscivores in Lake Tanganyika which tends to contradict the predation hypothesis. However, the position of Limnothrissa in the reservoirs probably resembles that occupied in Lake Tanganyika by Stolothrissa which is also a small, annual species. The significance of this phenomenon is that Limnothrissa can maintain a high biomass and productivity in the face of intense predation. This trait may be of importance to fisheries management because it means that their yield can be greatly increased.     

The influence of river flow on pelagic sardine catches in Lake Kariba

The nutrient level of Lake Kariba is governed by the Zambezi river which provides about 70% of its water supply. Other tributaries are richer in nutrients and have an appreciable local effect. This is shown by the catches of Limnothrissa miodon which increase after strong flows in the Sanyati river. This is in contrast to natural lakes such as Lake Tanganyika and may be an important management factor in Lake Kariba and other man-made lakes. Possible periods of poor rainfall could thus reduce the Limnothrissa yield in Lake Kariba.     

Histological assessment of selected tissues in tigerfish (Hydrocynus vittatus Castelnau, 1861) from the Sanyati Basin,Lake Kariba,Zimbabwe: a DDT-sprayed area

A pilot investigation was conducted in 2014 in the Sanyati Basin of Lake Kariba to ascertain whether long-term DDT spraying in the Kariba catchment had a negative effect on fish health. The aim was to assess the health of tigerfish, Hydrocynus vittatus, by means of histological analysis and to analyse water, sediment and tissue samples for bioaccumulated levels of chlorinated pesticides, including DDT. Eighteen tigerfish were collected by seine netting along the north-eastern shoreline of the Sanyati Basin in April 2014 and samples of their gill, liver, kidney, muscle and brain tissue were processed for histology and assessed using light microscopy. No detectable levels of DDT and/or its metabolites were found in the water or sediment samples and only a low concentration of p,p′-DDE in fish tissues. No major histological alterations were observed in the fish tissues. Consequently, there seems to be no risk of DDT exposure following the consumption of tigerfish from the Sanyati Basin of Lake Kariba. These results were unexpected, given the historical use of DDT within the current study area. It is recommended that these findings be compared with those from other regions of Lake Kariba.     

Use of ecological indicators to assess the quality of Great Lakes coastal wetlands

Over 2000 coastal wetland complexes have been identified in the Laurentian Great Lakes watershed, each providing critical habitat for numerous aquatic and terrestrial species. Research has shown there is a direct link between anthropogenic activities (urbanization and agricultural development) and deterioration in wetland health in terms of water quality and biotic integrity. In this study, we evaluate coastal marshes throughout the Great Lakes basin using a suite of published ecological indices developed specifically for coastal wetlands of the Great Lakes (Water Quality Index (WQI), Wetland Macrophyte Index (WMI), and the Wetland Fish Index (WFI_Basin)). We surveyed 181 wetlands, including 19 in Lake Superior (11%), 11 in Lake Michigan (6%), 13 in Lake Huron (7%), 92 in Georgian Bay and the North Channel (51%), 18 in Lake Erie (10%), and 28 in Lake Ontario (15%), over a 13 year period (1995–2008). Water quality parameters were measured at every site, while paired fyke nets were used to assess the fish community (132 sites) and macrophytes were surveyed and identified to species (174 sites); all of this information was used to calculate the associated index scores. One-way ANOVA results showed that there were significant differences in wetland quality among lakes. According to the WQI, we found that over 50% of marshes in Lakes Michigan, Erie, and Ontario were in degraded condition, while over 70% of marshes in Lakes Superior, Huron, and Georgian Bay were minimally impacted. Georgian Bay had the highest proportion of wetlands in very good and excellent condition and least number of wetlands in a degraded state. The WMI and WFI showed similar results. This is the largest bi-national database of coastal wetlands and the first study to provide a snapshot of the quality of coastal habitats within the Great Lakes basin. We recommend this information be used to guide conservation and restoration efforts within the Laurentian Great Lakes.     

Synopsis of scientific results of the sarec sponsored project on the ecology of Lake Kariba

A summary of current research on Lake Kariba is given. Lake Kariba is now a phosphorus limited oligotrophic lake, dependent on annual nutrient input for the maintenance of production. Nitrogen fixation by cyanobacteria has become an important source of nitrogen in the dry season and that fish harvesting is an important phosphorus sink. An up to date plankton composition list has been produced and plankton biomass determined. Diving studies indicate large biomass of bivalves. The aquatic vegetation displays both longitudinal as well as depth gradients, related to light regimes. Only preliminary data are available on mud/water interactions, but the available data indicate considerable fluxes in the river mouth stations and shallow protected littoral areas. The role of predatory birds and crocodiles in the fishery economy of the lake is evaluated and indicates no major conflict between these predators and human fishing interests. The research also draws attention to pesticides in Lake Kariba as indicated by work on fish eating birds and crocodile research. Since the publication of ‘Lake Kariba’ by Balon & Coche (1975) the research project, THE ECOLOGY OF LAKE KARIBA, being undertaken by the University Lake Kariba Research Station constitutes the first multidisciplinary study of the lake, 30 years after the Zambezi river was impounded at Kariba Gorge. It could have offered a unique opportunity of comparing the lake now with what it was soon after impoundment.     

Comparison of fish yields from prediction models between Lakes Tanganyika and Kariba

Fish yield from catch data and from prediction models in Lake Kariba is compared to that in Lake Tanganyika. Fish yields from catch data in L. Tanganyika are above expectation from the rates of algal production, and this disparity is not experienced in L. Kariba. Of the two hypotheses that have been forwarded to explain the situation in L. Tanganyika — high trophic efficiency and high bacterial production, it is suggested that, with the experience from L. Kariba, only the hypothesis of high bacteria production could be important.This paper is published with the permission of the Director, National Parks and WildLife Management, Zimbabwe.     

Horizontal variations in associations of zooplankton in Lake Kariba

The horizontal distribution of the zooplankton in Lake Kariba was sampled by means of vertical hauls using 55 and 250 μm meshed nets. Crustacea were sparse in the main lake, but increased dramatically in the very shallow water at the end of Crocodile Creek. The composition of the zooplankton is influenced by the presence in the main lake of the Tanganyika sardine, Limnothrissa miodon , and by the large size of the dominant diatom in the phytoplankton. In the creeks the distribution can be explained by the zooplankton occurring in open spaces into which planktivorous fish do not enter because of the possibility of predation by Tiger fish ( Hydrucyori ) and birds.
The systematic composition of the zooplankton in L. Kariba is reviewed and compared with two natural lakes and another man-made lake. It is concluded that the zooplankton and the fishes in L. Kariba have reached approximately the specific diversity that one would expect from a similar sized natural lake in the same geographical area.     

THE BARRED MINNOWS (TELEOSTEI: CYPRINIDAE) OF ZIMBABWE: IS THERE CAUSE FOR CONCERN?

Summary

Two species of barred minnow, Opsaridium zambezense and O. peringueyi, occur in Zimbabwe. The former is widely distributed in the Zambezi basin and some other rivers but is now absent from Lake Kariba and possibly some of its tributaries. The latter, which was only recently recognised as a distinct species and admitted to the Zimbabwean list, is much scarcer. It occurs in the Save and Limpopo catchments and there are only four specimens in the Natural History Museum of Zimbabwe collection. These specimens are all more than 40 years old and the lack of more recent ones suggest that O. peringueyi may be extinct, or close to extinction, in Zimbabwe. This situation may have come about through habitat alteration and drought, and emphasises the need for further monitoring of the current occurrence/distribution of fish species in the country.     

Differential accumulation of Cd in durum wheat cultivars: uptake and retranslocation as sources of variation

Durum wheat (Triticum turgidum L. var. durum) accumulates Cd from the soil depending on various factors. When grown in hydroponic solution containing Cd (20 microg l(-1)), roots had higher tissue Cd concentrations than shoots or heads. Kyle (the higher grain-Cd accumulating cultivar) had lower root-Cd, and greater shoot-Cd and head-Cd concentrations than Arcola (the lower grain-Cd accumulating cultivar). These cultivar differences were greater at flowering and ripening than at tillering. Much of the root-Cd was lost between the flowering and ripening stages of development. Distribution of (106)Cd among plant parts, after a single 24 h feeding, demonstrated that root-to-shoot transfer of Cd in Arcola was similar to that of Kyle at tillering, but it had ceased at flowering in Arcola but not Kyle. None of the Cd in wheat heads at ripening originated from (106)Cd exposure in the previous 24 h, suggesting that grain-Cd is a function of total shoot accumulation. Both cultivars demonstrated basipetal translocation of Cd; Arcola at tillering translocated more Cd from shoots to roots than Kyle. The proportion of Cd(2+)/Cd(total) in the nutrient solution decreased with time, suggesting that plant activity altered the solution chemistry. The alteration probably resulted from either preferential depletion of solution Cd(2+) and/or addition of root exudates. Lower grain-Cd accumulation in Arcola possibly resulted from a combination of reduced root-to-shoot transfer of Cd at flowering, as well as enhanced shoot-to-root retranslocation of Cd, at least in younger plants. Plant-mediated changes in solution-Cd speciation did not play a role.     

The inshore fish yield potential of Lake Kariba, Zimbabwe

Four surplus yield models were used to estimate the yield potential of the Kariba inshore fishery. The classical methods gave maximum sustainable yield (MSY) values as high as 1333 t y-¹. These values are regarded as over-estimates in view of the present average annual catch of 619.4 t y-¹. The Schnute (1978) model gave an MSY of 719t y-¹ and a failure index close to 1 indicating that the data do not fit well into the model. Walter's (1986) graphical approach of fitting an approximate equilibrium yield curve to catch and effort predicts an MSY between 550 and 570t y-¹. It is suggested that the Kariba inshore fishery on the Zimbabwean side should be managed within this range, in Basins 4 and 5.

Résumé

Quatre modèles de rendement excédentaire ont été employés pour estimer le rendement potentiel de la pêcherie dans le lac Kariba. Les méthodes classiques donnaient un rendement maximal supportable (RMS) de 1333 ta-¹. On considere que ces valeurs sont surestimées au vu de la capture annuelle moyenne de 619 ta^-1. Le modèle de Schnute (1978) donnait un RMS de 719 ta^-1 et un risque d'erreur proche de 1, indiquant que les données ne s'intègrent pas bien dans le modèle. L'approche graphique de Walter (1986) qui tend à ajuster un niveau de rendement en équilibre approximatif entre I'effort et la prise prévoit un RMS compris entre 550 et 570 ta^-1. On suggère que la pêcherie dans le lac Kariba devrait être gérée dans cet ordre-là du côté zimbabwéken.     

The distribution of zooplankton in the Sanyati Bay,Lake Kariba; A multivariate analysis

A principal component and discriminant function analysis of zooplankton from the Sanyati Bay, Lake Kariba, indicated that the distribution of plankton was associated with the riverine in-flow into the lake.     

The ecology of vascular hydrophytes on Lake Kariba

Summary The main events in the development of the vascular hydrophyte flora of Lake Kariba are reviewed on the basis of the growth forms present in the Lake.The habitats occupied by free floating plants are still dominated by Salvinia auriculata and here the situation appears ho have reached a measure of stability.The colonies of sudd plants Salvinia mats have similarly reached an apparently stable state.The emergent vegetation of the shores of the lake is shown to be affected by a number of factors, the most important probably being the fluctuations in lake level.Attached hydrophytes with floating leaves are present in such small quantity as to be a relatively unimportant constituent of the present vascular hydrophyte flora of Lake Kariba.The significance to the productivity of Lake Kariba of the establishment of indigenous aquatic and semi-aquatic plants is stated. The detrimental effect of Salvinia mats in retarding the establishment of these plants is pointed out.The development of the submerged aquatic flora is briefly described.Finally these separate events are reviewed in the chronological order in which they occurred.

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Résumé Les principaux stages dans le développement de la flore vasculaire hydrophyte du Lac Kariba sont examinés selon l'échelle de croissance des plantes qui se recontrent actuellement dans le lac.Les habitats des plantes flottantes sans attache sont toujours dominés par la Salvinia auriculata; dans ces aires la situation semble avoir atteint, dans une certaine mesure, un état stable.Les colonies de plantes su sedd sur les radeaux de Salvinia paraissent aussi avoir atteint un état stable.Preuve est fournie que la végétation émergeant sur les rives du lac est influencée par un certain nombre de facteurs, dont les plus importantts. sont les variations dans le niveau du lac.Les hydrophytes á pédoncule et á feuilles rencontrent en quantité tellement réduite qu'elles ne constituent qu'un élément relativement sans importance de la flore vasculaire hydrophyte actuelle du Lac Kariba.En termes de productivité du Lac Kariba, les conséquences de l'établissement de plantes aquatiques ou semi-aquatiques est mise en lumiére. L'effet préjudiciable des radeaux de Salvinia en tant qu'ils retardent l'établissement de ces plantes est souligné.Le développement de la flore aquatique sumergée est esquissé. dans l'ordre chronologique de leur apparition.

     

Diets of large mammals in the woodlands around Lake Kariba,Rhodesia

Summary Large mammals that formerly occupied the Kariba basin of the Middle Zambezi Valley have, since it was flooded, been forced to live in a restricted range of deciduous woodland habitats. Feeding records in a study area on the shore of Lake Kariba showed that most herbivore species were browsers, and that only the tree components of their diets varied significantly between vegetation types. The common species varied a proportion of their diet seasonally. However, each depended upon a small range of food staples which differed from those of other species and acted as food refuges for part of the year. Diets overlapped during the wet season because of diversification, and in the late dry season because of common use of a restricted range of plants remaining green. There was a good correlation between the ability of a species to avoid dietary overlap and its biomass in the study area. The chance of interspecific competition occurring increased in the late dry season when most of the species would formerly have migrated from the study area to the flood plain. Despite the enforced occupation of only part of their former annual range the more common herbivores maintained a considerable degree of ecological separation through utilisation of different foods.     

The distribution of fish of riverine origin in relation to the limnological characteristics of the five basins of Lake Kariba

The limnological characteristics of Lake Kariba are briefly described. Any change in the physico-chemical nature of the lake waters naturally implies that the environment for fish is also changed. Dispersal of fish species according to conditions existing in the five basins of the lake is dictated by the marked graduation from fluviatile to partly lacustrine, to lacustrine conditions, when passing from Basin 1 through to Basin 5. The fish in Kariba are of Middle Zambezi River stock and as a consequence those showing strong ties with the riverine habitat, particularly in relation to spawning requirements, have orientated towards affluent rivers, or areas influenced by rivers. The Cichlids have adapted to the lacustrine conditions. Evidence of these trends in distribution have been extrapolated from commercial landings.     

Population dynamics of Bosmina longirostris (Crustacea: Cladocera) in Lake Kariba,Zimbabwe

The population dynamics of Bosmina longirostris in a large tropical reservoir, Lake Kariba, Zimbabwe, are described. Population density showed two peaks associated with periods of high nutrient fluxes. Fecundity was always low. Two eggs per female was the maximum ever recorded. Instantaneous rates of birth, death and population growth calculated from field data using the egg ratio method are discussed. The rate of biomass change fluctuated in a regular pattern, probably linked to food availability or predation by a clupeid, Limnothrissa miodon.     

The Zambezi River in Moçambique

SUMMARY. Surveys of the physico-chemical status of the River Zambezi in Moçambique were conducted between April 1973 and May 1974, to characterize the river prior to the closure of the Cabora Bassa Dam and to provide baseline data from which future changes in the river ecosystem can be followed. The temperature of the river water increases by about 3°C from Lake Kariba to the sea, and with an average pH of 7.8 the water is slightly more alkaline than Kariba Dam water. The river is well oxygenated with no sign of hydrogen sulphide. Transparency is generally very low and changes are mainly determined by the floods of local tributaries. The overall average total alkalinity is 55 mg 1^?1 CaCO₃, with no regular seasonal variation, average concentrations of chloride and sulphate are respectively, 5.4 mg 1^?1 Cl^? and 5.3 mg 1^?1 SO^2-₄. They increase under both the influence of the ionically rich Shire River and the sea; the latter due to vertical mixing. Total hardness and calcium hardness behave in a similar way, while conductivity and silica concentrations increase slightly until the Zambezi—Shire confluence, at which point, conductivity increases by 25% to an average peak value of 153 μS cm^?1, and silica decreases by 15% to a minimum average value of 15.5 mg 1^?1 SiO₂. Ammonia and nitrate concentrations show a clear seasonal variation, with respectively, minimum dry season and maximum wet season average values of about 0.05 mg 1^?1 and 0.3 mg 1^?1. The overall average concentration of nitrite is 0.004 mg 1^?1. Orthophosphate in the river is mostly affected by Lake Kariba, though other sources are also of some importance. Maximum average concentrations of about 0.4 mg 1^?1 PO^3-₄ were recorded during the dry season after the overturn of Lake Kariba. The concentration of dissolved metals is mainly determined by the outflow of Kariba and by the Shire River, with only K and Mg apparently affected by the outflow from Lake Kafue. It is concluded that the water quality of the Middle Zambezi is mainly determined by the Kariba Dam, with the Kafue Dam playing a minor role. At the time of the pre-impoundment survey, the main impact on water quality of the Lower Zambezi came from the Shire River. With the addition of yet another large reservoir on the Zambezi, in the form of Cabora Bassa, together with the appreciable effect of tributary rivers on the lake, and the likely increased impact of the Shire River on the now more regulated river, the quality of the Lower Zambezi is expected to alter considerably, with concomitant ecological changes.     

Book Reviews

Book reviewed in this article:
A Feast in the Wild by Russell Kyle.
In Defence of the Land Ethic: Essays in Environmental Philosophy by J. Baird Callicott.     

The seasonality of phytoplankton in the North American Great Lakes,a comparative synthesis

The phytoplankton and productivity of the North American Great Lakes has been studied extensively by Fisheries and Oceans Canada during the past 15 years to monitor the impact of nutrient and contaminant loading on the plankton of the ecosystem. Lakewide cruises were conducted at monthly intervals mainly during the spring to fall period. This provided extensive biomass, species, size, productivity and nutrient concentration data for the Great Lakes. These data were collected using the Utermöhl inverted microscope technique together with standardized taxonomic, productivity and data-handling procedures. These standardized methodologies were applied to all the Great Lakes which resulted in a comprehensive phycological and ecological data base for the first time. These data form the basis for the evaluation of the complex phenomenon of seasonality.The eutrophic/mesotrophic Lower Great Lakes exhibited well-developed seasonal peaks of high biomass, with inshore-offshore differentiation and spring maxima most pronounced in the inshore region. However, the oligotrophic Upper Great Lakes had low biomass and generally lacked well-developed seasonal patterns. No marked seasonal trends were observed in the ultra-oligotrophic Lake Superior. The seasonality of biomass and various taxonomic groups of phytoplankton showed differentiation between individual lakes and is discussed in detail. The seasonal succession of species provided interesting comparisons between the Lower Great Lakes, which harbour eutrophic and mesotrophic species, and the Upper Great Lakes, which harbour oligotrophic species.Due to the voluminous nature of our data, a general overview has been given for all the Great Lakes with Lake Ontario treated in detail as a case study. The Lake Ontario case study provides the state-of-the-art status ranging from the lakewide surveys of 1970 to the current research with minute organisms such as ultraplankton and picoplankton.