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Lake Victoria's fish fauna included a large endemic flock of 300+ haplochromine cichlid species. About two-thirds of these species have disappeared or are threatened with extinction. The main cause of this large extinction event is predation by Nile perch, an introduced predator. We describe the decline of the haplochromine species and demonstrate that the rate and sequence of their decline was determined by their relative abundance, their adult size and their habitat overlap with Nile perch. Many non-haplochromine species declined as well, but in contrast, stocks of the native pelagic cyprinid Rastrineobola argentea and the introduced Oreochromis niloticus increased. There are also indications of an increase in phytoplankton, macrophytes, prawns and benthic organisms. Many of these rapid changes in the ecosystem were probably effects of the increase of the Nile perch and the disappearance of the haplochromines. The original fish fauna included many primary and secondary consumers. Currently secondary and tertiary consumers dominate. The food web in the sub-littoral and offshore areas of the lake changed considerably due to the stock replacements.

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