Cascading Effects of the Introduced Nile Perch on the Detritivorous/Phytoplanktivorous Species in the Sublittoral Areas of Lake Victoria

a In the 1980s an explosive increase o f the introduced Nile perch (Iates sp.; Harrison 1991) in Lake Victoria caused the destruction o f approximately 65% of the endemic haplochromine cichlid~ The eradication o f approximately 200 vertebrate species in less than a decade may well represent the largest extinction event among vertebrates during this century. The introduction o f the Nile perch can be considered as a large.scal¢ albeit unintendea~ experiment Farreaching changes in the f ood web are taking place. We present data on the importance o f haplochromines in the ecosystem prior to the Nile perch boor~ An analysis o f the pelagic community in the sublittoral area o f the Mwanza Gulf revealed that the phytoplanktivores formed 18% o f the biomass o f the total haplochromine community. In the benthic community, the detritivores that frequently included phytoplankton in their diet comprised 31% of the biomas~ We examine the hypothesis that algal grazing was reduced by the disappearance o f haplochromine phytoplanktivores and detritivore~ The disappearance o f these groups may have contributed to the recent algal b l o o m To investigate this hypothesis we consider a second major change in the systerrt Coinciding with the decrease o f the haplochromineg the number o f aty/d prawn~ Caridina nilotica, has strongly increased We present pretiminary evidence that the stock o f the detritivorous haplochromines that formerly comprised most o f the demersal ichthyomass has (partially?) been replaced by the praw~ We discuss the possible mechanisms underlying this major change in the food web. With respect