Effects of Nile perch, Lates niloticus, on functional and specific fish diversity in Uganda's Lake Kyoga system

The introduction of Nile perch, Lates niloticus, to Lake Victoria, East Africa, interacted with eutrophication to cause a reorganization of the lake's food web and the extirpation of many endemic fishes. The Lake Kyoga satellite system lies downstream from Lake Victoria. It encompasses species-rich lakes where Nile perch are absent or very rare, and low diversity lakes where L. niloticus is abundant. In 1999 we surveyed seven lakes in the Kyoga system using experimental monofilament gill nets (1/4-1 inches variable mesh). At Boston University we assessed delta N-15 signatures of epaxial muscle from subsamples of the catch (n = 361). These signatures are often highly correlated with the near-term mean realized trophic position of an individual organism. A neural network analysis of fish length, species name, trophic level, and lake of origin fish explained 94% of the sample variance in delta N-15. We analysed statistical patterns in these signatures at a number of spatial scales. The relationship between trophic level and delta N-15 varied greatly among lakes. Higher diversity perch-free lakes had greater variance in delta N-15 values and fish lengths than lower diversity Nile perch lakes, suggesting an important relationship between species diversity and functional diversity. Against expectations, lake size was negatively correlated with delta N-15. Patterns in stable isotope signatures indicated that Nile perch lakes have shorter food chains than perch-free lakes. The results throw up two management problems for the Kyoga system. Impacted lakes need to be studied to understand and ameliorate the community-level effects of Nile perch introduction, whereas the species-rich nonperch lakes, which harbour a large proportion of the remaining diversity of regionally endemic taxa, are in need of conservation planning.