Limnothrissa Research Articles

A preliminary assessment of the biomass of the pelagic sardine Limnothrissa miodon in Lake Kariba

The sardine Limnothrissa miodon was very patchily distributed in Lake Kariba in 1981–1983 and its mean biomass was 59 23 kg ha−1, declining from 90–91 kg ha−1 in 1981 to 38–66 kg ha−1 in 1983. In addition, the mean size of the fish fell from 55–2 mm in 1981 to 49–9 mm in 1983. These changes are attributed to the effects of drought and reduced river flows as well as to the high level of commercial fishing for this species. There was a correlation between commercial catches and biomass estimates which allowed estimation of biomass in previous years from fishing statistics. Fishing effort appeared to be a major influence on sardine abundance: total biomass in 1985 was about 10% of what it was in 1974 when the fishery began. When expressed in terms of unit area, as is usual for African lakes, the pelagic biomass appeared to be greater in Lake Tanganyika, where Limnothrissa is endemic, than in Lake Kariba, although there is little difference in their primary productivity, but consideration in terms of volume rather than area shows that this was not the

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 lt= 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 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 (r2= 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.

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.

On the possible introduction of non-indigenous zooplankton-feeding fishes into lake Malawi, Africa

It has been proposed that two zooplanktivorous sardines from Lake Tanganyika, Stolothrissa tanganicae and Limnothrissa miodon, be considered for possible introduction into Lake Malawi. Our data indicate that these clupeids, if introduced, would compete directly with the indigenous cichlid fishes, including the commercially important ones, all of which feed preferentially upon zooplankton. We conclude that such an introduction has a high probability of bringing about a collapse of the cichlid fisheries in Lake Malawi without the compensating increase in sardine catch. Such a prediction, of course, cannot be tested without the experiment being conducted. The data suggest that the risk to reward ratio is too large to consider such a drastic biological manipulation of Lake Malawi.

The influence of food availability, breeding seasons and growth rate on commercial catches of Limnothrissa miodon (Boulenger) in Lake Kariba

Commercial catches of Limnothrissa miodon in Lake Kariba show clear seasonal fluctuations. Recruitment to the pelagic fishing areas was investigated. A von Bertalanffy growth curve for the species was calculated and the breeding season determined from the calculated hatching dates of observed alevins and juveniles. Distribution of the different size classes was determined by a horizontal transect. Analysis of body composition showed a decrease in total lipids at the time when catches were dropping. This was shown to be due to a decline in triglyceride fat reserves apparently resulting from food stress and possibly resulting in increased natural mortality. Recruitment to the fishing areas and the total mortality rate are believed to cause the seasonal fluctuations in catch. The annual cycle of production may be caused by the summer inflow of nutrients from the rivers. Growth is apparently stunted in comparison to the species’ growth rate in Lake Tanganyika, probably due to the lower nutrient status of Lake Kariba.

Feeding habits of the clupeid Limnothrissa miodon (Boulenger), in Lake Kivu

A limnological sampling programme implemented during 1980 in Lake Kivu, Rwanda, confirmed a seasonal increase of the pelagic plankton biomass at the end of the dry season during the windy period of August/September. From plankton samples taken in the littoral and pelagic zones of the Lake it was concluded that a successful introduction of the freshwater clupeid Limnothrissa miodon Boulenger, during 1958–1960, had not obviously influenced the species composition of the Copepoda in the Lake.

Success of the Introduction of the Fresh Water Clupeid Limnothrissa miodon (Boulenger) in Lake Kivu

The freshwater clupeid Limnothrissa miodon was transplanted with success from Lake Tanganyika to Lake Kivu during 1958–1960. An artisanal fishery to exploit the clupeid stock started in the beginning of 1980. Based on the results of this artisanal fishery a minimum sustained yield of 4,000 tons/year was estimated for the Rwandese territorial waters of Lake Kivu. Biological research revealed information about growth rate, condition factor K, fat content, migration, distribution and reproduction of Limnothrissa. An obvious relation was found between fat content and sexual activity with the seasonal fluctuations of plankton production. The main spawning peak coincided with a plankton bloom caused by the turnover of the lake at the end of the dry season in September. At that time a small percentage of the clupeids had fat in their body cavity. It is stated that horizontal migration is mainly induced by reproduction, while the vertical migration is determined by feeding habits. Socio-economic benefits of the artisanal fishery using trimarans and lift nets were compared with those of the semi-industrial purse seining.

Seasonal changes in fat and fatty acid composition of Limnothrissa miodon Boulenger (Pisces: Clupeiformes) from Lake Tanganyika, with reference to their nutritive value

The fat content and fatty acid composition of whole sun-dried Limnothrissa miodon Boulenger caught during 1973 have been measured. The fat content seems to increase during the dry seasons. The fatty acids however showed no defined seasonal variations.

Approaches to estimating fish biomass and potential yield in Lake Tanganyika

The annual fish yield of 73 000 tonnes (22.5 kg/ha) from Lake Tanganyika consists primarily of two planktivorous clupeids Stolothrissa tanganicae (Stdr.) and Limnothrissa miodon (Blgr.) and their centropomid predators, three Lates species (Nile perch) and Luciolates stappersii (Blgr.). At the north and south ends of the lake, the Lates spp. have been fished‐up by purse‐seines, and the clupeids and young L. stappersii predominate there. Estimates of pelagic fish biomass may employ (i) catch and fishing effort data (ii) abiotic variables (iii) echo‐sounding surveys (iv) tropho‐dynamic relationships.

The relationship between the diurnal movements of some of the zooplankton and the sardine Limnothrissa miodon in Lake Kariba, Rhodesia1

The numbers of zooplankton at the surface in the open waters of Lake Kariba increased at night. Of the dominant species, the most extensive migrations were shown by adults of Mesocyclops leuckartii; their nauplii did not show any distinct diurnal movements. Bosmina longirostris exhibited similar, but lesser, movements. Keratella tropica, Keratella cochlearis, and Trichocerca chattoni remained more or less static in the epilimnion throughout the study.For some months after turnover, certain zooplankters move from the floor of the lake to the surface (60 m) each day. When temperature stratification in the monomictic lake intensifies and the hypolimnion becomes devoid of oxygen, these migrations are limited to the depth of the thermocline.These movements are similar to those of the Tanganyika sardine Limnothrissa miodon.

THE FEEDING HABITS OF THE WHITEWINGED BLACK TERN ON LAKE KARIBA

Summary Bego, G. W. 1973. The feeding habits of the Whitewinged Black Tern on Lake Kariba. Ostrich 44: 149–153. The stomach contents of 33 Whitewinged Black Terns were examined over the period 26 August 1971 to 6 January 1972. The most important single food item was the sardine, Limnothrissa miodon. This sheds new light on the habits of this fish. The terns fed primarily during calm weather. Sardines are caught by hovering and diving. Fishes of the genera Haplochromis and Synodonlis, also appeared in the stomach contents. Terns taken from marginal localities appear not to feed on fish. Of the insects eaten Odonata are the most important aquatic source of food, while many terrestrial insects are eaten (Orthoptera, Hemiptera, Coleoptera, Isoptera). Such insects could have been found drifting on mats of Salvinia on which the terns rest, otherwise insects are either hawked or skimmed from the surface of the water.

Population changes within a group of fish species in Lake Tanganyika following their exploitation

Pelagic fish populations in Lake Tanganyika consist mainly of two small clupeid species and four centroponiid species which prey on them. Exploitation with purse‐seines began in the southeast arm in 1962 and catches were sampled up to early 1969. Inshore clupeid populations were also sampled with a scoop‐net (lusenga of the kind used in the traditional inshore fishery.Clupeid biomass reached annual maxima around September. One clupeid species, Stolothrissa tanganicae Regan, dominated pelagic catches except in 2 years when about equal quantities of the other clupeid, Limnothrissa miodon Blgr. were caught. Numbers of each predator (three species of Lates and Luciolates stappersii Blgr.) decreased from 1963 to 1966 and remained low thereafter. The clupeid catch rose from 1964 to 1967 and remained high in 1968. The average nightly catch weight per year of all species together altered relatively little. Changes in population size distributions occurred.Sampling methods were concluded to be valid for clupeid populations, but probably indi‐cated only general changes in predator populations. Clupeid life‐cycles are mostly accomplished within a year, and appear closely related to the periodic and spatial variations in plankton production. The two species are competitive and can replace one another in the pelagic zone. Certain distribution patterns of the clupeids and of the predator young are believed to be adaptations to severe predation. Low replacement rates under fishing pressure account for the decline of predator species, and the clupeid increase resulted from reduction in predation. A fairly stable exploited phase has apparently been reached in which clupeid biomass is much greater and predator biomass much less than in the natural state. The data encourage certain predictions. Similar major trends occurred in the much larger fishery in the Burundi sector of the lake.