The Fish Fauna of Lake Victoria during a Centruy of Human Induced Perturbations

witte, f., kishe-machumu, m. a., mkumbo, o. c., wanink, J. h., goudswaard, P. c., van riJssel, J.c. & van oiJen, m. J.P. The fish fauna of Lake Victoria during a century of human induced perturbations. In J. Snoeks & A. Getahun (eds), Proceedings of the Fourth International Conference on African Fish and Fisheries, Addis Ababa, Ethiopia, 22-26 September 2008. Tervuren: Royal Museum for Central Africa, ‘Zoological Documentation Online Series’, pp. 49-66. Lake Victoria, by area the largest tropical lake of the world, is well-known for its diverse native fish fauna, which comprised about 500 endemic haplochromine cichlid species, two tilapiine species and 46 other species belonging to 12 families. During the past decades, the fish species diversity in the lake has declined dramatically due to human induced perturbations in the ecosystem. Based on literature and our own research findings we provide an overview of these changes and their most likely causes. During the first half of the last century, the increasing fishing pressure had a great impact on the native tilapiine cichlids and other large fish species. The shift in fish landings showed a classic example of fishing down the food web. Because of the dwindling catches, the Nile perch and four exotic tilapiine cichlids were introduced into the lake in the 1950s. Dramatic changes in the fish fauna occurred in the 1980s, with the upsurge of the introduced Nile perch and Nile tilapia, the decline of wetland zones, and increased eutrophication of the lake. The native tilapiines were replaced by the Nile tilapia, and several of the catfish species showed a dramatic decline, as did the lungfish. Most severely hit were the haplochromine cichlids, which disappeared from large parts of the lake, probably resulting in the extinction of many species. The introduced Nile perch and Nile tilapia, and the native cyprinid Rastrineobola argentea became the dominant fish species. Though water quality deteriorated and fish diversity decreased, fish landings rose from about 100,000 t y-1 in the 1970s to approximately 1 million t y-1 in the period 2005-2007. Since 1999 the biomass of Nile perch declined, whereas that of R. argentea increased. In the same period some of the sublittoral haplochromine species have recovered. Some of the surviving fish species, show remarkable changes in ecological and morphological features relative to the pre-Nile perch period, which seem to be adaptive responses to the changed environment. Although it may be possible to reconcile fisheries sustainability with biodiversity conservation in the lake basin, measures to reduce environmental stress in the lake are an urgent issue.