BIOLOGY OF THE AFRICAN CLUPEID LIMNOTHRISSA-MIODON WITH REFERENCE TO ITS SMALL SIZE IN ARTIFICIAL LAKES

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. © 1993 Chapman & Hall.