The feeding ecology of tilapia and the fishery of Lake Naivasha, Kenya.

The feeding ecology of tilapia and the fishery of Lake Naivasha, Kenya. By Mucai Muchiri A thesis submitted for the degree of Doctor of Philosophy of the University of Leicester, 1990. Lake Naivasha is a shallow freshwater lake lying 100 Km north of Nairobi, Kenya. The lake is an important tourist attraction owing to its rich birdlife. The lake also supports a canoe-based commercial fishery that exploits three introduced species, Oreochromis leucostictus, Tilapia zillii and Micropterus salmoides. Two other fish species occur in the lake; a cyprinid, Barbus amphigramma (a riverine invader) and Lebistes reticulatus (also introduced). The only endemic species, Aplocheilichthyes antinorii was last recorded in 1962. Lake Naivasha experiences wide and frequent water level fluctuations sometimes with severe ecological consequences which are described. This study sought to examine the feeding and food resource sharing of O. leucostictus (the mainstay of the fishery) and T. zillii in view of the apparent limited food resources and the implications of recent ecological changes to the lake fishery. Results showed that although O. leucostictus and T. zillii occupy the same habitats, there is sufficient food resource partitioning for competition not to occur. The diet of O. leucostictus in Naivasha consists of detritus (48.9%), phytoplankton (22.0%) and benthic macroinvertebrates (16.8%). T. zillii feeds on detritus (43.3%), adult insects (28.4%), macrophytes (9.7%) and benthic macro-invertebrates (7.5%). In adjacent Oloidien Lake similar food items are consumed by the two species except that T. zillii feeds more on macrophytes (23.6%) and benthic macro-invertebrates (14.6%). Detritus in Naivasha is not limiting and overlap in the other food items shared is < 45%. There was a marked progressive improvement in the well-being of both tilapia species between 1987 and 1989 (except for O. leucostictus population in Oloidien) as shown by computed relative condition factors. O. leucostictus was found to host a parasitic nematode, Contracaecum sp., but was not significantly affected by it. T. zillii was not infected at all. Maximum sustainable yield computed by use of catch statistics is 418.8 metric tonnes and an optimum effort of 54 canoes per year. That Lake Naivasha has higher fishery potential is discussed.