INTERACTIONS BETWEEN WHITE-BREASTED CORMORANTS PHALACROCORAX-CARBO (AVES, PHALACROCORACIDAE) AND THE FISHERIES OF LAKE MALAWI

1. The status of white-breasted cormorants Phalacrocorax carbo lucidus as possible competitors with the fisheries of Lake Malawi was assessed. 2. Numbers of P. carbo on the lake were estimated, from visual counts and photographs of resting sites taken at last light, to be 10 000 individuals in 1979. 3. Observations were concentrated on a colony of 5000+ birds on Boadzulu Island, in the south-east arm of the lake, the main area of commercial fishing. The birds fished in a single large flock, in shallow water, at or near the sandy or muddy bottom. 4. The birds' diet was assessed qualitatively and quantitatively from examination of regurgitates and cast pellets recovered at the nests, of fish dropped by the birds during their feeding activities, of the stomach contents of shot birds, and from the amounts consumed by captive cormorants. The mean daily weight of fresh fish caught by an adult, non-breeding cormorant was 250 g, rising to 293 g during the breeding season. 5. Annual fish consumption of the Boadzulu Island colony, including nestlings, was 491 t, of which 392.5 t came from the south-east arm of Lake Malawi. The entire white-breasted cormorant population of Lakes Malawi and Malombe consumed 950 t year-1. 6. The standing stock of available prey in the areas fished by the Boadzulu birds was 126.3 kg ha-1. This stock should be able to withstand indefinitely the cormorant exploitation of 21.6 kg ha-1 year-1. 7. Available prey in the areas fished by the cormorants was estimated from sample trawls. The birds took fish in rough proportion to their abundance. 8. The average catch by man over the years 1979-87, in the south-east arm of Lake Malawi, of the fish groups which formed the main prey of the cormorants, was 10417 t. The harvest by the birds in the same area in 1979 represents 3.8% of this figure, or 14.9% of the 2634 t of small demersal cichlids, the cormorants' preferred prey, in the catch. 9. But the cormorants forage in areas which are not exploited by either commercial or traditional fishing, and most of the demersal fish which are the cormorants' main prey have very restricted distributions within the lake, so that only a few species with wide distributions are common in both commercial fishing and cormorant feeding areas. The cormorants' harvest of fish species common in commercial trawling is 70 t year-1. This is only 2.7% of the human harvest of demersal cichlids. 10. It is concluded that the white-breasted cormorant has no significant impact on the commercial or traditional fisheries at resent carried out on Lake Malawi.