1 1Institute of Biology Leiden, Leiden University, P.O. Box 9516, 2300 RA Leiden, The Netherlands, Koeman en Bijkerk B.V., Ecological Research and Consultancy, P.O. Box 14, 9750 AA Haren, The Netherlands
2 2Institute of Biology Leiden, Leiden University, P.O. Box 9516, 2300 RA Leiden, The Netherlands
3 3Tanzania Fisheries Research Institute (TAFIRI), P.O. Box 78850, Dar Es Salaam, Tanzania, Institute of Biology Leiden, Leiden University, P.O. Box 9516, 2300 RA Leiden, The Netherlands;, Email: MKishe@yahoo.com
Cichlid fishes of Lake Victoria are well known for their trophic specializations. In our study area, the sub-littoral waters of the Mwanza Gulf (Tanzania), at least 12 trophic groups coexisted in the 1970s. After the Nile perch upsurge and eutrophication in the 1980s, most cichlids disappeared from the area. During the 1990s, heavy exploitation of Nile perch reduced the abundance of this predator. Subsequently, some cichlid species, mainly zooplanktivores and small benthivores, recovered. To establish their ecological role in the changed environment, among other factors we studied their diet. Stomach contents of fish caught before and after the ecological changes were compared. The small benthivores shifted from a diet dominated by detritus and phytoplankton, supplemented with small quantities of midge larvae and zooplankton, to a diet of invertebrate preys of larger sizes. Currently, the diet includes zooplankton, midge larvae, shrimps and molluscs, and a very small amount of detritus and phytoplankton. Apparently, the recovering benthivores have a more carnivorous than a herbivorous diet. Concomitantly, the ratio of intestine length to standard length of the benthivores decreased with 30% and the average stomach fullness decreased as well. The observed dietary shifts may be due to (1) the changed availability of food types and a decline of the nutritious quality of detritus and phytoplankton due to the strong increase of cyanobacteria in the lake, (2) the decline of the former trophic specialists, and (3) the reduced water clarity. The diet of the former detritivores is currently very similar to that of the zooplanktivores. Consequently, if diet plays a role in the relatively slow recovery of the small benthivores compared to the zooplanktivores, this would only hold if they are relatively poorly adapted to their new diet.