Northern Gannets in the North Sea: foraging distribution and feeding techniques around the Bass Rock

Field observationsaround the largest NorthernGannet Morus bassanuscolony in the North Sea, theBass Rock, showed that 66%of all Gannets foraged inareas with very low densitiesof conspecifics, more than100 km from the colony.When one forager foundprey, even distant Gannetsresponded by joining thefinder to obtain a share ofthe bounty but, because of the low densities of Gannets far from the colony,feeding opportunities were typically exploited by small flocks, with relatively fewcompeting birds. Intraspecific competition was thus less intense than it would havebeen nearer the colony. Searching and feeding tactics of Gannets, as well asforaging associations with other top predators, were different between sea areas.Low numbers of Gannets per flock occurred within inshore multi-species feedingassociations, where Gannets hampered feeding opportunities for other seabirds(and themselves) by plunge-diving into compact schools of small prey fish. Largerflocks of competing Gannets formed in situations where an escape response inprey fish was absent (discards behind commercial trawlers) or weakened (fishschools herded by marine mammals). The association of Gannets with marinemammals was typically an offshore phenomenon, despite the abundance ofcetaceans in inshore waters. Behind trawlers, Gannets focused mostly onroundfish, between 22 and 30 cm in length. Discards were, however, a fairlyunimportant source of food during the breeding season and natural feedingopportunities were widespread