We studied the diet of Brycinus sadleri in Lake Victoria after extensive environmental changes during the 1980s. To check for diet expansion following these changes, as observed in some other fish species, we compared our results with data from the 1950s. Stomach contents were analysed in relation to fish size and time of the day to investigate ontogenetic and diel shifts in the diet.
Comparison of diets before and after the environmental changes showed a shift to more generalist feeding. The main food sources in the 1950s were plants during daytime and surface insects at night. Hardly any plant material was eaten during the late 1980s, but chironomid larvae, chaoborid larvae and pupae, shrimps, Odonata nymphs and fish contributed to a broader daytime diet. These prey species were also commonly taken at night, though, like in the 1950s, surface insects accounted for the major part of the diet. We found dietary shifts during ontogeny. Juveniles fed exclusively on zooplankton. Chironomid larvae, relatively large food items in comparison with zooplankton, were the key prey of sub-adult fish. Their importance decreased in the diet of adults, which included even larger prey like shrimps, Odonata nymphs and fish. At night, surface insects replaced chironomid larvae in the diet of adult fish.
Diet expansion may result from reduced competition after the decline in fish mass following the Nile perch boom. An alternative explanation, increasing the prey spectrum in response to deteriorated light conditions, is not likely to hold for B. sadleri. The ontogenetic shift to increasingly larger prey and the nocturnal shift to surface insects probably reflect optimal foraging behaviour and diel vertical migration patterns of the fish respectively. These dietary shifts emphasize the feeding flexibility of B. sadleri, which may have been essential in surviving the environmental changes in Lake Victoria.