Hypoxia tolerance of introduced Nile perch: implications for survival of indigenous fishes in the Lake Victoria basin

The introduction of predatory Nile perch (Lates niloticus) into the Lake Vi ctoria basin coincided with a dramatic decline in fish diversity. However, remnant populations of indigenous fishes persist in lagoons and satellite l akes separated from the main lakes by extensive areas of swamp, while other indigenous species find refuge in ecotonal areas at edges of marginal swam ps in the main lakes. Low-oxygen conditions in these wetlands may physiolog ically stress Nile perch and therefore minimize its interaction with prey s pecies. This study examined the low-oxygen tolerance of Nile perch collecte d from Lake Nabugabo, Uganda, by documenting behavioural and physiological strategies that relate to oxygen uptake. In response to hypoxia, Nile perch used aquatic surface respiration (ASR) at the air-water interface, ventila ting their gills with water from the surface. However, several lines of evi dence suggest that Nile perch in Lake Nabugabo are inefficient at ASR and r elatively intolerant of low oxygen conditions. These include high threshold s for ASR relative to other indigenous fishes of the Lake Victoria basin, n o decrease in gill ventilation rate with the onset of ASR, a faster time to loss of equilibrium in hypoxic conditions than other species from the regi on, and a high critical oxygen tension (24 mm Hg).