FLATFISH SELECTION BY HERRING-GULLS LARUS-ARGENTATUS AND LESSER BLACK-BACKED GULLS LARUS-FUSCUS SCAVENGING AT COMMERCIAL BEAMTRAWLERS IN THE SOUTHERN NORTH-SEA

Flatfish selection by scavenging herring gulls and lesser black-backed gulls was studied onboard a commercial beamtrawler in the southern No rth Sea, June-August 1993. Dab (median total length 18 cm), plaice (23 cm), sole (22 cm) and solenette (10 cm) dominated the flatfish discar d fraction of the catch. The overall consumption amounted to 30.5% of the discarded flatfish. Flatfish were selected on the basis of fish wi dth rather than length or species, with very low consumption rates (pe rcentage consumed of number offered) of flatfish >8 cm width. All disc arded solenette, 98% of all sole and 92% of all dab were of suitable s ize for these gulls, whereas only 12% of all discarded plaice were sma ll enough to be consumed. Consumption rates of discarded flatfish of ' suitable size' for herring gulls and lesser black-backed gulls (<8 cm width) ranged from 25.9% (plaice) to 40.5% (dab). Success indices and selected size classes of flatfish in scavenging herring gulls and less er black-backed gulls feeding on flatfish were similar, but the first were more efficient than the latter and lost fewer flatfish as a conse quence of kleptoparasitism. In contrast to earlier suggestions (based on dietary studies in colonies), there was no evidence that herring gu lls were outcompeted by lesser black-backed gulls when feeding on disc arded fish.