Fish production in Lake Kariba,reconsidered

Synopsis Revised total and available production, yield and mean biomass per ha were calculated for each species in Lake Kariba, and for the whole lake. The revision was undertaken because (1) the original value 1 g for W₀ for each species was too high, (2) B_i+1 was occasionally used instead of B_i in the calculation of mean biomass for an interval i to i + 1, and (3) species' contributions to mean parameter values for the whole lake were not weighted according to their mean standing crop. Revised values are, A = 1224, P = 720, Y_A = 400, Y_{Pp, = 202 and B} = 827 kg ha^–1y–1. These correspond to 38.2%, 66.7%, 107.1% and 25.8% respectively of the values calculated initially.     

Growth and mortality of the introduced Lake Tanganyika clupeid, Limnothrissa miodon, in Lake Kariba

The growth and mortality of the small clupeid fish Limnothrissa miodon were studied in Lake Kariba, a large, man-made lake in central Africa. Growth could be described by the von Bertalanffy equation l_t = 74.2 (1—e^{−0.254( t +0.07)}) where length ( l ) is expressed in millimetres and time ( t ) in months. The monthly rates of mortality were high and increasing under intensive fishing; in 1983 the total mortality rate Z =1.15 and natural mortality M =0.7. Limnothrissa appears to be stunted in the man-made lakes where it occurs (Kariba and Cahora Bassa) where it grows at a faster rate but to a smaller size than it does in natural lakes (Kivu and Tanganyika). Various hypotheses to account for this are discussed and it was concluded that this is an adaptation to the relatively unstable conditions found in the man-made lakes.     

The feeding habits of Sargochromis Codringtonii in Lake Kariba, Zimbabwe

The food of adult Sargochromis codringtonii in Lake Kariba was dominated by prosobranch snails with pulmonate snails forming an insignificant dietary component. S. codringtonii does not show particular preference for any snail species, feeding on those species that are readily available. Morphometrical analysis of the feeding structures of S. codringtonii showed that the fish is well adapted to handling a molluscivorous diet. The pharyngeal jaws with their flat crowned teeth are well adapted to crushing molluscs. The daily food consumption rate of S. codringtonii was estimated over different seasons. Estimates of food consumption indicate that the fish consumed 13.96% of their dry body weight per day in summer falling to 4.46% in winter. The low consumption of pulmonate snails suggests that S. codringtonii can not be a successful biological control agent against the vectors of schistosomiasis.     

Crustacean parasites of fishes in Lake Kariba,Zimbabwe, preliminary results

During a survey of fish ectoparasites in Lake Kariba (Zimbabwe), two crustaceans were often encountered. Dolops ranarum (Branchiura), parasite of the body, mouth and gill chamber, infected mostly cichlids, the catfish Clarias gariepinus, Synodontis zambezensis and two Mormyrids. Among cichlids clear host preferences were shown. No seasonal variations could be detected in the infections. D. ranarum parasitized predominantly fish above 15 cm in standard length. Lamproglena monodi (Copepoda) parasitized only the gills of cichlids with a preference for Serranochromis codringtonii, S. macrocephalus and Tilapia rendalli. Small fish were seldom infected. Seasonal variations of the prevalence occurred, the hot season being the less favourable. Few other crustaceans were found during the survey: a single specimen of Lamproglena hemprichii on the gills of the tigerfish, Hydrocynus vittatus; three specimens of Afrolernaea longicollis, one found on the gills of Hippopotamyrus discorhynchus and two on the gills of Mormyrops deliciosus; a hundred of Ergasilus mirabilis in a specimen of H. discorhynchus.     

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.     

Aspects of the feeding ecology of Sargochromis codringtonii in Lake Kariba, Zimbabwe

Sargochromis codringtonii has always been considered a good candidate for the biological control of snails. However, there is lack of data on the feeding ecology of this species. The diet of S. codringtonii was examined by an analysis of stomach contents. The food of adult S. codringtonii in Lake Kariba is dominated by prosobranch snails. Pulmonate snails form a small insignificant component of the diet of S. codringtonii , and the reasons for this are discussed. Electivity indices were used to investigate prey selection. S. codringtonii does not show a particular preference for any snail species. It feeds on any snail species that is readily available. A morphometrical analysis of the feeding structures of S. codringtonii showed that the fish is well adapted to handling a molluscivorous diet. The daily food consumption of S. codringtonii was estimated over different seasons using the Elliot & Persson (1978 ) model [ J. Anim. Ecol. 47. 977–991]. Estimates of food consumption indicate that the fish consumed 14.0% of their dry body weight per day in summer and this falls to 4.5% in winter. The low consumption of pulmonates means that S. codringtonii cannot be a successful biological control agent against the vectors of schistosomiasis.     

Growth,mortality and reproduction of Sargochromis codringtonii in Lake Kariba,Zimbabwe

Age determination of Sargochromis codringtonii was undertaken using scalimetry. Marginal scale analysis, used for age validation, indicated that annulus formation occurred in October and November. The possible reasons for this are discussed. S. codringtonii is relatively long-lived, surviving up to 8 years. It is sexually dimorphic with males growing at a faster rate and attaining a larger size than females. It is suggested that the mouth brooding habits of the female fish is one possible reason for sexual dimorphism. A higher total mortality rate (Z) was recorded for females as compared to males. S. codringtonii reproduces throughout the year, but with a peak during the hot wet months. The right ovary is atrophied and the possible significance of this is discussed.     

Diatom associations and succession in Lake Kariba,South Central Africa

Diatom Associations formed by 1% or more of species, found by the Thomasson analysis method are discussed in relation to the morphology and limnology of the four basins of Lake Kariba. A brief comparison is made with associations in two stretches of the Zambesi river. A dominant association formed by certain species of Melosira demonstrated a pattern of succession, in relation to the thermal cycle in the lower, middle and upper lake by variations of the percentage occurrence of species forming it: Melosira granulata dominated during homothermy and remained so throughout stratification in the lacustrine conditions at the barrier or wall; M. granulata v. angustissima dominated during overturn and became the dominant species in the riverine conditions of the two uppermost basins. Results from the Central basin illustrated the intermediate position of this area of the lake. The pattern was disrupted in the far N.E sub-basin of the Sanyati basin, which in general showed individuality and where accidental and ambivalent plankters became co-dominant with the Melosira association during homothermy.A second association, formed by species of Cyclotella, Fragilaria and Synedra was a regular feature of the diatom population, it was sub-dominant to the melosira association at the barrier but gradually decreased up-lake and was superseded by accidental and ambivalent planktonic species. Except for the N/E Sanyati sub-basin, the diatom associations showed a gradual change from upper to lower lake, thus basin individuality was not seen.     

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 decline of Salvinia molesta on Lake Kariba

The floating fern,Salvinia molesta Mitch., underwent explosive growth following the creation of Lake Kariba in 1958. By 1962 this aquatic weed covered 22% of the Lake's surface but later declined to an apparently stable level of 10–15%. Coverage declined again in 1973 to 5% and by 1980 only about 1% of the lake was covered bySalvinia. This paper discusses the ecological context in which this decline took place and the role of the introduced neotropical grasshopper,Paulinia acuminata De Geer. This insect appears to have made a major contribution towards the decline ofSalvinia, but it is suggested that its effect was greatly enhanced by nutrient stress onSalvinia because of competition for nutrients between the floating weed and other ecosystem components. Nutrient supply to Lake Kariba is limited and the increasing requirements of submerged macrophyte, mussel and fish populations, which are all expanding probably reduced nutrient availability toSalvinia. Mussels and fish, with standing crops of 167 000t and 30 000t, respectively, are seen as particularly important nutrient reservoirs. It is concluded that the decline ofSalvinia in Kariba was part of the lake's maturation process and could possibly have occurred without the introduction ofPaulinia, but at a much slower rate.     

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.

     

The density and biomass of large mammals in Lake Nakuru National Park

Over the past 70 years, human activity has severely limited the diversity and movements of large mammals in the region of Lake Nakuru National Park, Kenya. Game counts were begun in April 1970 to assess the status of those species remaining. The density of large mammals per km² was found to be 64.20±8.86 (mean ±S.D.) and the biomass density of the six major species in kg/km² was 6298±801. The biomass was most heavily concentrated in the grassland-woodland area south of the lake. Defassa water-buck was the dominant species accounting for 76% of the biomass density. The probable reasons for their dominance are: (1) favourable habitat; (2) decreased competition; (3) reduced predation; (4) control of poaching; and (5) non-migratory habits. Future management may ultimately depend on the relationship of waterbuck to the food resources available.     

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.     

Zonation of submerged macrophyte vegetation in Lake Kariba,Zimbabwe and its ecological interpretation

The submerged vegetation of Lake Kariba is described in relation to degree of slope (lake morphometry), depth and light transparency. The direct gradient analysis technique — canonical correspondence analysis and the TWINSPAN classification programs were used to analyse the data set. The western end of the lake with low transparency has a low species diversity (with Vallisneria aethiopica dominating). Species diversity increases with increased transparency in the other parts of the lake. The classification revealed monospecific communities for all species as well as mixed communities with Lagarosiphon as the associate species with the broadest distribution. The ordination revealed a first axis strongly related to depth and transparency gradients and the second axis related to slope. Vallisneria aethiopica has a growth form adapted to grow in shallow areas subjected to wave action. Potamogeton octandrus also growing in shallow areas is restricted to sheltered areas. Lagarosiphon ilicifolius precluded from the shallow exposed areas grows at medium depth but has a growth form adapted to tap light at the surface. Najas pectinata and Ceratophyllum demersum are adapted to grow in deep water.Abbreviations CCA Canonical correspondence analysis     

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 eels of Lake Kariba: distribution, taxonomic status, age, growth and density

It had been predicted that eels would disappear from Lake Kariba because juveniles would be unable to surmount the dam which was closed in 1958. This prediction has been proved wrong. In 1970–71 an abundant population of eels was discovered in the lake, mostly at depths from 25–40 m. The eels have intermingled characters of both the mottled Anguilla nebulosa labiata and the plain-colored A. mossambica . A clear separation of these two taxa was impossible and therefore conspeciflc status of these two species is suspected; the name A. n. labiata is retained for the population.
The age structure of the eel cohorts suggests that juveniles surmount the dam in their second year of life and then spend approximately 7 years in streams of the lake drainage. The fastest maturing individuals which are in best condition then emigrate to the ocean, whereas the slower maturing eels remain in the lake longer. The oldest eel in the samples was 18 years old.
The catch per unit of effort for hoopnets was 2.35 kg in the upper part of the lake and 0.41 kg for the lower part of the lake. The density at Namazambwe was assessed at 46 eels per 1 ha. The unexploited eel population of Lake Kariba could form a valuable resource.