Distribution of Nile perch Lates niloticus in southern Lake Victoria is determined by depth and dissolved oxygen concentrations

Although Nile perch Lates niloticus is assumed to be sensitive to low oxygen concentrations, it was found in deep water in Lake Victoria, where oxygen depletion is common during the rainy season. Since factors determining Nile perch distribution are not well understood its spatial distribution in the Mwanza Gulf of Lake Victoria was analysed in relation to depth, temperature and dissolved oxygen (DO) concentrations. Of these factors, DO concentration and the interaction of depth and DO concentration explained the distribution of Nile perch, whilst temperature had no significant effect. In periods of normoxia, Nile perch preferred to stay near the bottom at depths of 12-35 m, where densities of shrimps, their main prey, were high. However, Nile perch were apparently driven away from these areas by hypoxic conditions in the rainy season. They apparently escaped the seasonally hypoxic layers in deep water by horizontal inshore migration and by vertical movement in upward direction. Sudden upwelling of these deep layers is a threat to the Nile perch stock and the Nile perch fishery.