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

摘要

Field observations around the largest Northern Gannet Morus bassanus colony in the North Sea, the Bass Rock, showed that 66% of all Gannets foraged in areas with very low densities of conspecifics, more than 100 km from the colony. When one forager found prey, even distant Gannets responded by joining the finder to obtain a share of the bounty but, because of the low densities of Gannets far from the colony, feeding opportunities were typically exploited by small flocks, with relatively few competing birds. Intraspecific competition was thus less intense than it would have been nearer the colony. Searching and feeding tactics of Gannets, as well as foraging associations with other top predators, were different between sea areas. Low numbers of Gannets per flock occurred within inshore multi-species feeding associations, where Gannets hampered feeding opportunities for other seabirds (and themselves) by plunge-diving into compact schools of small prey fish. Larger flocks of competing Gannets formed in situations where an escape response in prey fish was absent (discards behind commercial trawlers) or weakened (fish schools herded by marine mammals). The association of Gannets with marine mammals was typically an offshore phenomenon, despite the abundance of cetaceans in inshore waters. Behind trawlers, Gannets focused mostly on roundfish, between 22 and 30 cm in length. Discards were, however, a fairly unimportant source of food during the breeding season and natural feeding opportunities were widespread.