Wetland ecotones as refugia for endangered fishes

Wetlands may serve as refugia for indigenous fishes from introduced predatory fishes because of their structural complexity, which may reduce hunting efficiency, or if the low oxygen conditions that prevail in many swamps limit exploitation by the predator. The vegetated ecotone between lake shores and open water may be extremely important, relative to the dense interior of swamps, because of the accessibility of ecotones for lacustrine fishes, and the less extreme physicochemical conditions generally encountered along lake margins. This paper examines the role of wetland ecotones as refugia for prey species from the introduced Nile perch Lates niloticus in Lake Nabugabo, Uganda, where increased numbers of Nile perch coincided with the disappearance or decline of many species from the open waters of the lake. Patterns of ecotone use were examined by comparing fish communities in wetland ecotones with fish communities in exposed ecotone areas with no wetland. Results suggest that wetland ecotones may protect some fishes from Nile perch predation. Nile perch were less abundant in wetland ecotones relative to exposed inshore areas, and were found less frequently in areas of low dissolved oxygen. A negative relationship was found between species richness among ecotones and dissolved oxygen, and a positive relationship between species richness among ecotones and structural complexity. It was difficult to separate the effects of structure and low oxygen on the habitat use of Nile perch. However, tolerance to low oxygen may permit prey species to exploit structured inshore habitats as refugia without adverse effects from the low oxygen conditions that can occur there.