DIETS AND FOOD-CONSUMPTION RATES OF PELAGIC FISH IN LAKE-MALAWI, AFRICA

1. Studies of diel feeding periodicity, rates of food consumption and diet composition of pelagic fish were undertaken to resolve the disput e regarding the existence of a vacant niche for a pelagic zooplanktivo re in Lake Malawi. 2. Six species of zooplanktivore were abundant in t he offshore pelagic zone, these were: Diplotaxodon limnothrissa, D. 'b igeye', Rhamphochromis longiceps and Copadichromis quadrimaculatus (al l haplochromine cichlids), Synodontis njassae (Mochokidae) and Engraul icypris sardella (Cyprinidae). The main piscivores were cichlids of th e genus Rhamphochromis. 3. All cichlids were daytime feeders; some sho wed peaks in feeding activity at dawn and dusk that were related to ve rtical migration patterns of their prey. Synodontis njassae was able t o feed throughout the day, but fed most actively at night. 4. Food con sumption: biomass ratios (Q/B yr(-1)) calculated from diel stomach con tents data were 5.87-7.13 for D. limnothrissa, 12.79 for D. 'bigeye', 4.20-24.7 for R. longiceps and 6.45 for S. njassae. Annual Q/B ratios for other species, which ranged from 4.74 to 9.28, were obtained from an empirical model relating food consumption to fish morphology. Publi shed estimates were used for E. sardella and Opsaridium microcephalum (Cyprinidae). An estimate of total prey consumption by the pelagic fis h population was obtained from Q/B ratios and fish biomass estimates f rom acoustic surveys covering the entire offshore pelagic zone of the lake (24 000 km(2)). 5. Diplotaxodon limnothrissa, R. longiceps and C. quadrimaculatus had a broad range of diets,with the ability to switch from crustacean zooplankton to larvae and pupae of Chaoborus edulis ( Diptera) or larval and juvenile E. sardella. Diplotaxodon 'bigeye' and S. njassae fed almost exclusively on fourth-instar larvae and pupae o f C. edulis. Engraulicypris sardella fed mainly on crustacean zooplank ton. The main prey of the large piscivorous Rhamphochromis species wer e Diplotaxodon spp. and E. sardella. 6. The fish community consumed on ly 3% of estimated crustacean zooplankton production directly. Predati on pressure was intense (> 80% of estimated production consumed by pre dators) on late instars of the zooplanktivorous C. edulis larvae and o n E. sardella larvae. Rhamphochomis spp. consumed 22-43% of estimated production of the zooplankton-eating fishes. 7. 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 tr ophic level, it is considered that the endemic fish community is well adapted to feeding on the low-density prey in the pelagic zone, and th at there is no vacant feeding niche.