The impacts of introduced species on lake ecosystems: A case of Lakes Victoria and Naivasha, Kenya.

Abstract

Fish and other aquatic organisms have been introduced into new environments for various reasons. Provision of sports fishery, supplemental to stocks, to fill empty niches, control weeds, and disease vectors and even create a commercial fishery are some of the reasons why fish species are introduced into new ecosystems. Lake Victoria and Lake Naivasha in Kenya have seen several introductions as discussed in this paper. In Lake Victoria for example, some of the species introduced include Nile perch, Lates niloticus, Nile tilapia, Oreochromis niloticus, Tilapia zilii, and Oreochromis esclentus among others. The most successful introductions have been that of the Nile perch, Nile tilapia and water hyacinth. Lake Naivasha too has witnessed a number of introductions into its fisheries too. Some of the successful introductions into the lake are those of the Common carp, Cyprinus carpio, the largemouth bass, Micropterus salmoides, the Nile tilapia, O. niloticus and Louisiana red swamp crayfish, Procambarus clarkia. Aside from the fish species mentioned, some invasive plant species have also been introduced into these two lake ecosystems. Of notable success and impacts have been the water hyacinth, Eichhornia crassipes and Nile cabbage, Pistia stratiotes. These introductions have had both negative and positive impacts on the fisheries and the ecology of the two lakes. Some of the impacts have been predation on the native species by the introduced species like in the case of Nile perch and largemouth bass in Lake Victoria and Naivasha respectively. Competition for resources and hybridization with the native species has also been witnessed in the case of the tilapiines in the two lakes with undesirable results. Blockage of navigation routes caused by the invasive plants has been a major problem in the two lakes sometimes leading to massive postharvest losses by the fishermen. On the other hand, some introduced species have led to an increase in fish landings as well as utilization of the niches that were initially vacant within these ecosystems. It can therefore be concluded that introduced species often have more negative effects on the native species and therefore such introductions should be based on sound scientific research in order to minimize their effects within the new environments.