Fish introductions and their impact on the biodiversity and the fisheries of Lake Victoria

Abstract

Recent fish introductions in Lake Victoria have had both positive and adverse implications on its biodiversity and fisheries. This paper reviews the impact of six introductions in Lake Victoria basin with emphasis on areas where the introduced species have put more risks or benefits to the host environments. There have been six introductions of fish species (Lates niloticus, Oreochromis niloticus, Cyprinus carpio, O. melanopleura, O. leucostictus and Tilapia zillii) in Lake Victoria in a span of 20years. A significant ecological imbalance has been created through the depression of haplochromiine cichlids, changes in food webs and food chains, and alterations of water quality. There is evidence for example, that the introduced O. melanopleura has crossbred with T. zillii while O. niloticus has crossbred with O. variabilis with apparent dominance of the introduced species in these hybrids. The native species (Labeo victorianus, Barbus spp, Haplochromis spp., Protopterus aethiopicus, Bagrus docmac, Clarias spp, Alestes spp, Synodontis spp and the native tilapia species) have declined considerably in the commercial catches in the last few decades. On the other hand, it has been a great boost to the riparian communities, and Kenya as a whole from the increased commercial fish landings from less than 20,000 m.tons in the 1970’s to over 193,000 m. tons in the 1990’s. Presently, the introduced L. niloticus and O. niloticus compose over 70% of the total landings by weight. Fish protein food supply has increased significantly; number of fishermen now is over 36,000 (up from 11,000 in the 1971) with the export sector directly employing over 500,000. Given the dependants as well as those engaged in the auxiliary activities in this sector, the fishing industry currently supports over 1 million people. On the other hand, a significant nutritional loss has also been noted as a result in the decline in the local’s favourite fish species (endemic species) in the catch. With the market expansion, the local community have been reduced to low-grade fish, which do not meet the export requirement and fish skeletons. The only native species landed in large quantity is the Rastrineobola argentea, which has gained popularity for its use as animal feed. There is urgent need to document all fish introductions in this country with appropriate evaluation of each case from time to time. This evaluation report will contribute to the development of guidelines to introduction programs in future. The need for research in order to better anticipate with confidence the cost of any introductions before such programs are implemented cannot be overemphasised.