Stable isotope analyses of food web structure and fish diet in Napoleon and Winam Gulfs, Lake Victoria, east Africa

The food web structures in Napoleon and Winam gulfs, Lake Victoria, were characterized using stable nitrogen and carbon isotope analyses. Similar biota in Napoleon Gulf had significantly lighter delta(15)N values and heavier delta(13)C values than similar biota in Winam Gulf indicating different basal isotopic values. In both gulfs, Nile perch (Lates niloticus) was the top trophic predator while Nile tilapia (Oreochromis niloticus) was littoral and feeding at lower trophic levels. Rastrineobola argentea and Yssichromis laparograma had surprisingly high delta(15)N values, close to those of Nile perch, which were not consistent with the high isotopic values of their assumed zooplankton prey. Caridina nilotica, a freshwater shrimp, had a wide range of delta(13)C values but low delta(15)N values, consistent with their appearance in nearly all habitants in the lake, and their presence in the stomaches of most fish species. Nile perch showed an increase in delta(15)N and delta(13)C values with size, signifying that piscivory increases and their dietary reliance on invertebrates decreases as they mature. Stable isotope values for Napoleon Gulf biota which were adjusted for different basal values were not statistically different from those of Winam Gulf biota, suggesting that stable carbon and nitrogen isotopes fractionate consistently through trophic transfers in Lake Victoria. The stable isotope data illustrate a short food web, with the top predator Nile perch feeding on a restricted set of fish and macroinvertebrate species, including its own young.