Fish population changes in the Sanyati Basin, Lake Kariba, Zimbabwe

Abstract

Large-fish population changes in the Sanyati Basin of Lake Kariba between 1960 and 1975 are evaluated and discussed. The early lake population following closure of the dam wall in 1958 was similar to the pre-impoundment riverine population with Labeo spp., Distichodus spp., Clarias gariepinus and two characid species dominating gill-net catches. Exceptions were mormyrids, scarce in the new lake although abundant in the river, and Oreochromis mortimeri, scarce in the river but expanding rapidly in the lake. Productivity in the new lake in terms of ichthyomass relative to later years was high. In later years following closure several of the early abundant mostly potamodromous species declined rapidly (C. gariepinus, Labeo spp., Distichodus spp., ) and by 1975 they were unimportant. Mormyrids, cichlids and two silurid species increased significantly in catches as did the characin, Hydrocynus vittatus; the latter as a result of the expansion of the freshwater sardine population, from 1970. While early populations were essentially herbivorous later populations were largely insectivorous/molluscivorous. Concomitant with these changes new species appeared, while a few disappeared. Possible sources of origin of the new arrivals are discussed. Salient features of the entire evolutionary process are summarized, while some of the lessons learnt from Kariba, and a few recommendations arising from this, are listed in the conclusion.