BALIRWA, J.S

Fisheries Resources Research Institute, P.O. Box 343 Jinja, Uganda.

Evolution and importance of the Tilapia fishery in Lake Victoria with suggestions for its sustainability

In many Great Lakes ecosystems of the world, natural evolutionary processes are masked by climatic changes and anthropogenic impacts. Anthropogenic impacts are often characterised by over fishing, and exotic species introductions as management tools. However, the inherent factors of ecosystem diversity, potential resilience of some species, and endemic species survival strategies were not integrated in ecosystem management even though introduced species sometimes produce socio-economic benefits in apparent conflict with biodiversity concerns. The immense Lake Victoria (c. 69,000 km2) in East Africa provides an example of how understanding diverse biophysical features of the lake (climate, shoreline topography, habitats, species richness) and socio-economic impacts (fish production) is beneficial to the integrated management of the Great Lakes. In studies carried out between 1988 and 2000, the spread of an introduced fish, the Nile tilapia (Oreochromis niloticus) in Lake Victoria and the species' occupancy of various habitats in the lake were investigated. Of all the gill-net sampled habitats, the shallow (< 3 m deep) vegetation-dominated habitats contained a higher Nile tilapia biomass (46.6±4.0 kg.ha-1) and fish density than deeper (3-5 m) habitats only 70 m away from the shoreline where fish biomass was 6.2± 0.1 kg.ha-1). Analysis of trawl data for 1999 and 2000 showed that in the 4-10 m depth habitats, the Nile tilapia contributed 13% and 32% respectively, to the biomass of all the fish caught. This illustrates the ecological importance of shallower habitats for the species. Species richness was higher in the shallower habitats despite the dominance of the introduced fishes. Compared to the native habitats of the exotic fishes, lake morphommetry and climatic patterns have promoted the spread of the Nile tilapia in Lake Victoria to a more dominant position than the species exhibits in its native habitats. These findings are of direct application in the management of fish stocks, biodiversity and environment of Great Lakes.