Population fluctuations and their causes in the African Fish Eagle, (Haliaeetus vocifer (Daudin)) at Lake Naivasha,Kenya

Soft water lakes possess a highly characteristic vegetation adapted to limitation of carbon. Based upon hydrology, vegetation and geographic distribution, boreal and Atlantic lake types can be distinguished. Reducing the input of nutrients or liming, or both, the stream or its catchment is generally sufficient to restore typical soft water vegetation of boreal soft water lakes. The vegetation of Atlantic soft water lakes is subject to many anthropogenic degradation processes. Therefore, spontaneous recovery in the near future is not expected and restoration is urgently required. Removal of nutrient-rich, anoxic, organic sediments is a prerequisite for restoration of these lakes. In acidified or acid-sensitive lakes, additional measures against acidification are required. Controlled supply of calcareous, nutrient-poor water is much better than direct liming. The effects of these restoration measures strongly depend on the detrimental effects of processes such as atmospheric deposition, drainage, catchment acidification, eutrophication and reduced colonisation rates.     

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.     

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 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.     

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.     

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.     

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.     

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     

The diet of the Sea Eagle Haliaeetus albicilla and Golden Eagle Aquila chrysaetos in western Scotland

The diets of Sea Eagles Haliaeetus albicilla and Golden Eagles Aquila chrysaetos living sympatrically in western Scotland were studied using pellets and prey remains collected at roosts and nest sites throughout the year. Both species showed changes in diet between winter and summer, and there were differences between species in both seasons. As a rule, Sea Eagles took proportionately more seabirds, waterfowl (ducks and waders) and fish, whilst Golden Eagles took proportionately more rabbits and hares. Sea Eagles had a broader diet than Golden Eagles but a measure of dietary overlap based on taxonomic composition indicated an overlap of more than 90% between the two species.     

The influence of thermal stratification on pelagic fish yields in Lake Kariba, Zambia/Zimbabwe

Lake Kariba is thermally stratified for about 9 months of the year. The Tanganyika sardine, Limnothrissa miodon (Boulenger), thrives very well when the water is in circulation and when its food (zooplankton) is abundant due to increased nutrients in the water column. There is a fall in the sardine yields as the water column gets partitioned into a nutrient-poor epilimnion and a nutrient-rich hypolimnion during thermal stratification. A very high negative correlation ( r ²= 0.94, n = 32) was found between thermal stratification and the monthly yields of the sardines. This uneven annual distribution of sardine yields creates problems for the local fishery.     

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.     

Abundance and distribution of inshore fish in fished and protected areas in Lake Kariba,Zimbabwe

Abundance and size distribution of inshore fish populations was assessed by fishing with standard Lundgren survey gill-nets in areas open to fishing and reserved areas that are protected from fishing activities.The direct gradient analysis technique — Detrended Canonical Correspondence Analysis — revealed that fish abundance and their composition in Lake Kariba are strongly structured by lake morphometry.The comparison between fished and reserve areas showed that there was a significant reduction in mean length of commercially fished species in the fished areas and the size distribution curves were skewed, lacking small and large length classes. There was a higher abundance of non-commercial species in the fished areas. It was also revealed that the mochokid Synodontis zambezensis Peters 1852, was the most abundant species and was present at all depths sampled in all the stations.There were indications that non-commercial species were important in terms of abundance in the fished areas. These included S. zambezenzis, Schilbe intermedius (Ruppell, 1832) and other smaller species. This was attributed to them being selected against in the gill-nets.     

Some notes on the population dynamics of Hydrocynus vittatus (Castelnau) in Lake Kariba

Summary The data obtained from a routine gillnetting research programme carried out on Lake Kariba from October 1963 to September 1965 have been presented.Length-frequency ofH. vittatus was correlated with gonad activity.It was found that the gillnetting technique used provided sufficiently adequate data to trace the monthly growth of four separate year-classes from 14 cm up to 34 cm in length. This growth rate was found to be less when compared to data collected in 1959, just after the lake had filled. H. vittatus of 31 cm possessed the most active gonads during January 1965 and comprised 93% males, which were two years old. The catchability ofH. vittatus in the research gillnets decreased considerably for fish over 34 cm in length, which might account for the fact that females in breeding condition were scarce in the catches.The results obtained have been discussed with reference to previous literature.It was concluded that mostH. vittatus in Lake Kariba started to breed at the end of their second year during the rainy season.

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Zusammenfassung Die vorliegende Mitteilung stützt auf Daten, die während eines Forschungsprogramms mittels Kiemennetzen in der Kariba-Talsperre, in der Zeit vom Oktober 1963 bis September 1965, gesammelt wurden.Die Längengruppen beim Tigerfish,Hydrocynus vittatus (Castelnau), werden mit den Gonadenentwicklungsstadien korreliert.Die Kiemennetzmethode hat sich zum Verfolgen des monatlichen Wachstums von vier unterschiedlichen Jahresklassen, zwischen 14 cm und 34 cm als zulänglich erwiesen.Das Wachstum war langsamer in den Jahren 1964–65 als in den Jahren 1959–60, den ersten zwei Jahren nach dem Vollaufen der Talsperre.Die Gonaden vonH. vittatus, von einer durchschnittlichen Länge von 31 cm, waren im Januar 1965 am stärksten entwickelt. Diese Gruppe enthielt 93% Männchen, deren Alter sich als zweijährig feststellen liess.Die Fangbarkeit vonH. vittatus mit den benutzten Kiemennetzen war bedeutend geringer für Fische mit einer Körperlänge von mehr als 34 cm. Vielleicht wurden aus diesem Grund so wenig reife Weibchen gefangen, da deren Durchschnittskörperlänge 35 cm beträgt.Die erhaltenen Resultate werden mit Angaben aus dem einschlägigen Schrifttum verglichen.Es wird gefolgert, dass die meisten in der Kariba-Talsperre gefangenenH. vittarus and Ende ihres zweiten Lebensjahres, während der Regenzeit, anfangen sich fortzupflanzen.

     

Prosobranch mollusc species- and size-preferences of Sargochromis codringtonii (Cichlidae) in Lake Kariba,Zimbabwe

From the stomach contents of 28 wild-caught Sargochromis codringtonii from Lake Kariba the size and species of snails consumed were estimated from opercula found. An energetic cost/benefit ratio of the various sizes of the most frequent prey, Bellamya capillata, was calculated. The size of snails consumed increased significantly with fish size, with large fish showing individual size preferences. Ingested snails were of sizes with relatively low energetic cost/benefit ratios.     

Population density and intra- and interspecific competition of the African Fish Eagle Haliæeetus vocifer in Kyambura Game Reserve, southwest Uganda

Population density and intra- and interspecific competition of the African Fish Eagle Haliæeetus vocifer were studied in Kyambura Game Reserve, southwest Uganda. Density varied significantly between 1.75 and 3.25 individuals per km of shore for the breeding biotopes along the Kazinga Channel and crater lakes and between 0.45 and 0.58 for unsuitable biotopes such as the Kyambura Gorge or the bush grassland. The age-class distribution showed extremely high percentages of immatures in these unsuitable biotopes (50–95%) as a consequence of intraspecific competition for space and food. Some immatures survived in the bush grassland, feeding on lion and leopard kills and very possibly on birds. Each territory was up to five times larger than breeding pair territories along the Kazinga Channel (5.1 km² in comparison with 0.9 km²). Interspecific competition for space was likely between immature African Fish Eagles and other species of raptors. Ecological segregation with regard to space was assessed between these species in the bush grassland.     

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 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.