Refugia for endangered fishes from an introduced predator in Lake Nabugabo, Uganda

Wetlands may protect fishes from introduced predatory fishes by providing both structural and low-oxygen refugia for prey species tolerant of the conditions that prevail in these habitats. We examined the potential of wetlands as refugia for fishes in Lake Nabugabo, Uganda, where increased numbers of an introduced predator, the Nile perch (lates niloticus), coincided with the decline or disappearance of many indigenous species in the main lake. In 1993 and 1994 we sampled fishes in three wetland habitats: the marginal wetland ecotones of the lake, wetland lagoons separated from the lake by densely vegetated marsh habitats, and a stream separated from the lake by dense papyrus swamp. Fish distributions in these wetlands were then compared to results from two earlier fish surveys: the 1962 Cambridge expedition to Lake Nabugabo, which was conducted prior to the increase in the Nile perch population; and a 1991-1992 survey of the open lake, which reported the disappearance of 16 indigenous species. In our 1993-1994 surveys 9 of the 16 species not recovered in the 1991-1992 open-lake survey were found in the wetland ecotones or beyond the margins of the lake in wetland lagoons and tributaries. Three of these species were found only beyond the margins of the lake int he tributaries and lagoons within extensive wetlands. Three endemic haplochromine cichlids were abundant offshore in 1962. Of these species one has disappeared and the others are now largely confined to inshore areas. Other species that were abundant in the open waters of the lake in 1962 (e.g., the lungfish Protopterus aethiopicus) are now found primarily in wetlands. Two species, the characid Brycinsu jacksonii and the cyprinid Rastrineobola argentea, are still abundant in the open waters. This study highlights the need for quantitative survey work to identify wetland refugia in the Lake Victoria Basin and suggest that some species thought to have disappeared in the mass extinction of fishes in Lakes Victoria and Kyoga may still survive in refugia. Some fish populations could recover under effective ecosystem management.