Diets and food consumption rates of pelagic fish in Lake Malawi, Africa

SUMMARY Studies of diel feeding periodicity, rates of food consumption and diet composition of pelagic fish were undertaken to resolve the dispute regarding the existence of a vacant niche for a pelagic zooplanktivore in Lake Malawi. Six species of zooplanktivore were abundant in the offshore pelagic zone, these were:Diplotaxodon limnothrissa, D.‘bigeye’,Rhamphochromis longicepsandCopadichromis quadrimaculatus(all haplochromine cichlids),Synodontis njassae(Mochokidae) andEngraulicypris sardella(Cyprinidae). The main piscivores were cichlids of the genusRhamphochromis. All cichlids were daytime feeders; some showed peaks in feeding activity at dawn and dusk that were related to vertical migration patterns of their prey.Synodontis njassaewas able to feed throughout the day, but fed most actively at night. Food consumption: biomass ratios (Q/Byr‐1) calculated from diel stomach contents data were 5.87–7.13 forD. limnothrissa, 12.79 forD.‘bigeye’, 4.20–24.7 forR. longicepsand 6.45 forS. njassae.AnnualQ/Bratios for other species, which ranged from 4.74 to 9.28, were obtained from an empirical model relating food consumption to fish morphology. Published estimates were used forE. sardellaandOpsaridium microcephalum(Cyprinidae). An estimate of total prey consumption by the pelagic fish population was obtained fromQ/Bratios and fish biomass estimates from acoustic surveys covering the entire offshore pelagic zone of the lake (24 000 km2). Diplotaxodon limnothrissa, R. longicepsandC. quadrimaculatushad a broad range of diets, with the ability to switch from crustacean zooplankton to larvae and pupae ofChaoborus edulis(Diptera) or larval and juvenileE. sardella. Diplotaxodon‘bigeye’ andS. njassaefed almost exclusively on fourth‐instar larvae and pupae ofC. edulis. Engraulicypris sardellafed mainly on crustacean zooplankton. The main prey of the large piscivorousRhamphochromisspecies wereDiplotaxodonspp. andE. sardella. The fish community consumed only 3% of estimated crustacean zooplankton production directly. Predation pressure was intense (> 80% of estimated production consumed by predators) on late instars of the zooplanktivorousC. edulislarvae and onE. sardellalarvae.Rhamphochomisspp. consumed 22–43% of estimated production of the zooplankton‐eating fishes. Although many members of the fish community do not feed directly on crustacean zooplankton, so that the food‐chain supporting fish production involves an extra trophic level, it is considered that the endemic fish community is well adapted to feeding on the low‐density prey in the pelagic zone, and that there is no vacant feeding niche.