1Institute of Evolutionary and Ecological Sciences, University of Leiden, P.O. Box 9516, 2300 RA Leiden, The Netherlands
2Haplochromis Ecology Survey Team (HEST), P.O. Box 1866, Mwanza, Tanzania
3Tanzania Fisheries Research Institute (TAFIRI), P.O. Box 475, Mwanza, Tanzania
4Fish Culture and Fisheries Group, Wageningen Institute of Animal Sciences, Wageningen Agricultural University, P.O. Box 338, 6700 AH Wageningen, The Netherlands.

Diet expansion retains food web complexity after species reduction in the sublittoral fish community of Lake Victoria

The species-rich fish community of Lake Victoria was characterized by a high number of specialist feeders. During the 1980s, the upsurge of introduced Nile perch (Lates niloticus) and concomitant environmental perturbations had dramatic consequences. About 200 species of haplochromine cichlids disappeared from the lake, while many others declined strongly. In accordance with the theory that generalists survive perturbations better than specialists, wide food spectra are common among the surviving cichlid and non-cichlid species. However, several survivors used to be specialists. They expanded their diet, particularly by including macrobenthos, after the ecological changes. This may be attributed to increased macrobenthos and/or decreased fish biomass, resulting in reduced competition.

In the sublittoral waters of the Mwanza Gulf (Tanzania) more than 110 species of haplochromine cichlids belonging to 12 trophic groups coexisted with various other taxa. They formed a food web that was complex by the large number of trophic links. Shortly after the Nile perch boom, the key taxa in this area reduced to Nile perch, the cyprinid dagaa (Rastrineobola argentea), the cichlid Nile tilapia (Oreochromis niloticus) and the characid Brycinus sadleri. During the last decade, the cichlid zooplanktivores Haplochromis pyrrhocephalus, H. laparogramma and H. tanaos recovered and they now occur in even higher densities than before the ecological changes. The new food web seemed rather simplified, comprising phytoplanktivorous Nile tilapia, herbivorous (with some insect prey) B. sadleri, zooplanktivorous dagaa and haplochromines, and piscivorous Nile perch. Only the variable diet of juvenile Nile perch seemed to increase food-web complexity again. However, the other species have all expanded their diet, now including insect larvae and shrimps (Caridina nilotica), and some even molluscs and fish. Consequently, the new food web has retained complexity through a high frequency of intraguild predation.