Document: LAU-3-84-10

Biodiversity lost and found: The resurgence of indigenous fishes in the Lake Victoria region.

CHAPMAN, L.J.* ¹, C.A.CHAPMAN ¹, J.P.OLOWO ¹, P.J.SCHOFIELD ² and L.S.KAUFMAN ³

University of Florida, Gainesville, FL 32611 USA ¹
University of Southern Mississippi, Hattiesburg, MS 39406 USA ²
Boston University, Boston, MA 02215 USA ³

Abstract:
In Lake Nabugabo, Uganda, a small satellite of Lake Victoria, 50% of the indigenous fishes disappeared from the open waters subsequent to the establishment of the introduced predatory Nile perch (Lates niloticus). However, several of these species persisted in wetland refugia (e.g., ecotonal wetlands, swamp lagoons). Over the past few years, overfishing of large Nile perch in Lake Nabugabo has coincided with a resurgence in the open waters of some indigenous species. In a 5-year study, we documented the pattern of resurgence in the lake using annual transects in the inshore and offshore waters of exposed (no wetland) and wetland habitats, and repeat sampling of swamp lagoons. In 1995, haplochromine cichlids were largely confined to inshore areas of wetland transects and were very rare in Nile perch stomachs, as were most other indigenous species with the exception of two open-water indigenous fishes and introduced tilapia. Comparison of fish distributions in Lake Nabugabo to two nearby satellite lakes without Nile perch suggested that some fishes had shifted their distribution from open waters to ecotonal wetlands in response to Nile perch predator pressure. By 1999, haplochromine cichlids were abundant in inshore and offshore areas of exposed and wetland transects, and the size distribution had shifted to include a higher proportion of larger individuals. Several non-cichlids also reappeared in the main lake including three of the four original mormyrid species. Resurgence of some haplochromine cichlids has also occurred in the larger Lakes Kyoga and Victoria. This demonstrates a quick response to a decrease in predator pressure induced by overfishing of Nile perch and highlights the importance of faunal refugia in the Lake Victoria basin. Nile perch show a strong ontogentic dietary shift from invertebrate-feeding juveniles to piscivorus adults. Comparison of stomach contents of Nile perch between 1995 and 1999 in Lake Nabugabo, and between lakes where the predator is native and lakes where it has been introduced, showed that the size at which Nile perch shift to piscivory is larger when haplochromines are rare. Haplochromines are now reappearing in the stomachs of small Nile perch in Lake Nabugabo; and over time, we predict a fluctuating equilibrium between predators (fishermen and Nile perch) and prey.

Keywords: non-indigenous species, Nile perch, aquatic conservation, East Africa, haplochromine cichlids

This abstract is being presented at: 1:30 PM in session:
Oral Session #62: Freshwater Fish Ecology.