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Abstract
Nile tilapia, Oreochromis niloticus and Nile perch, Lates niloticus were introduced into Lake Victoria in the 1950s and 1960s. The former was to boost the then declining tilapiine fishery, while the latter was to convert the abundant bony haplochromines to fish flesh. Fish samples were collected by bottom trawling from 1998 to 2000 and in 2004 to 2005 from the Kenya waters of Lake Victoria. The goal was to evaluate the current population characteristics and ecology of the species. Length frequency distribution and size at maturity of Nile tilapia depicted a stable population with high proportions (30%) of mature fish, unlike Nile perch where more than 98% of the individuals were immature. Nile tilapia previously distributed in < 10 m, were caught in areas up to 20 m depth. Catches of Nile tilapia have increased from < 1% in 1970-80s to > 25% in 2005; while Nile perch declined from 95% in 1990s to 58% in 2005.
The increase in Nile tilapia catches and its spread into deeper waters is attributed to declining stocks of the predatory Nile perch, availability of suitable food and the species occupying vacant niches left by declining stocks of indigenous species, especially the haplochromines. Increase of anoxic conditions in the lake is also precluding Nile perch thereby affording tilapia refuge. The previously herbivorous tilapia has diversified its diet to include insects and fish. All these factors could be contributing to an increase in Nile tilapia biomass in the lake. If management measures to reduce Nile perch exploitation and its rapid decline are not instituted, the next important demersal commercial fishery in Lake Victoria will probably be Nile tilapia. The establishment of Nile tilapia will be replacing a multimillion-dollar export industry of Nile perch, which will have several economic consequences. However, biodiversity in the lake is expected to increase and thus so will the availability of protein to local communities.
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