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

Abstract

S U M M A R Y 1. 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. 2. Six species of zooplanktivore were abundant in the offshore pelagic zone, these were: Diplotaxodon limnothrissa, D . 'bigeye', Rhumphochromis longiceps and Copadichromis quadrirnaculatus (all haplochromine cichlids), Synodontis njussae (Mochokidae) and Engraulicypris sardella (Cyprinidae). The main piscivores were cichlids of the genus Rhamphochromis. 3. 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 njassae was able to feed throughout the day but fed most actively at night. 4. Food consumption: biomass ratios ( Q I B y r l ) calculated from diel stomach contents data were 5.87-7.13 for D. limnothrissa, 12.79 for 0. '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. Published estimates were used for E. sardella and Opsaridium microcephulum (Cyprinidae). An estimate of total prey consumption by the pelagic fish population was obtained from Q / B ratios and fish biomass estimates from acoustic surveys covering the entire offshore pelagic zone of the lake (24 000 km2). 5. Diplotaxodon limnothrissa, R. longiceps and C. quudrimaculatus 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. Diplotmodon 'bigeye' and S . njassae fed almost exclusively on fourth-instar larvae and pupae of C . edulis. Engraulicypris sardella fed mainly on crustacean zooplankton. The main prey of the large piscivorous Rhamphochromis species were Diplotaxodon spp. and E . surdella. 6. 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 zooplanktivorous C . edulis larvae and on E . sardella larvae. Rhamphochomis spp. consumed 2243% 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 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. 0 1996 Blackwell Science Ltd 489 490 E. H. Allison et al.

Parallel life history evolution in mouthbrooding cichlids from the African Great Lakes

“…If so, this would suggest that the evolution of large offspring size had different causes among rocky-shore and pelagic cichlids. The small fry of pelagic cyprinids and clupeids initially feed on unicellular algae (55)(56)(57), whereas the much larger fry of cichlid fish in all habitats feed mainly on crustacean zooplankton (55,58,59). Differential spatiotemporal patchiness of zooplankton and phytoplankton could lead to differing selection pressures among the fish families in the pelagic zones of the African lakes, simply from the initial divergence in offspring size and trophic level.…”

Stochastic modelling of Lake Malawi Engraulicypris sardella (Gunther, 1868) catch fluctuation

“…However, because fisheries scientists, ecologists and managers have not yet found a substantial evidence on this contentious biology and life span of E. sardella, it has been difficult to set management recommendations and strategies to sustain E. sardella stocks in Lake Malawi. Similar observation was made by Allison et al (1996). Unfortunately, the economic importance of Engraulicypris sardella in Malawi cannot be negated as it currently contributes over 70% of the total annual landings (Department of Fisheries, 2017).…”

Distribution and breeding biology of offshore cichlids in Lake Malawi/ Niassa

“…'bigeye' occurring in offshore samples belonged to a single species, however, individuals were occasionally found that could not be assigned to this or any described species (Allison et al 1995b). Specimens of D. 'bigeye' are held at the Monkey Bay Research Laboratory, Malawi.…”