Effects of winter fishery activities on resting numbers , food and body condition of large gulls Larus argentatus and L . marinus in the south - eastern North Sea

C Influences of fishery activities on herring gulls and great black-backed gulls (Larus argentatus, L. mannus) wintering on the island of Helgoland, south-eastern North Sea, were studied When cod flshery discards were available close to the island. 83 to 87 % of the gulls' pellets contained fishery discard remains and 70 to 73 % of all pellets consisted exclusively of these. At times when no nearby trawling took place (Christmas/New Year), numbers of herring gulls dropped by up to 86 % and numbers of great black-backed gulls by up to 80%. Similar declines were observed during a trawling moratorium in February 1998. Adults of both gull species had a significantly lower body mass during periods of no fishing around the island (13 and 24 % less, respectively). Retraps of single gulls support these findings. The results show for the first time a strong influence of discard availability on numbers and body condition of large gulls wintering in the south-eastern North Sea. Effects on gull winter mortality and thus on population dynamics can be assumed.