The behaviour of seabirds foraging at fishing boats around Shetland

Abstract

Among the different types of fishing vessels around Shetland, whitefish trawlers attract the largest numbers of scavenging seabirds and provide the most food. Offal was almost all consumed by seabirds, predominantly by Fulmars Fulmarus glacialis, which excluded other species by their aggression. Fulmars generally ignored discarded whole fish, which were mainly taken by Great Black‐backed Gulls Larus marinus, Gannets Sula bassana and Great Skuas Catharacta skua. Although flatfish were usually ignored because seabirds found them difficult to swallow and they sank faster, most discarded roundfish were consumed. Herring Gulls L. argentatus, Lesser Black‐backed Gulls L. fuscus and Kittiwakes Rissa tridactyla were rarely able to obtain offal or discards. Herring Gulls and Lesser Black‐backed Gulls spent much time on the periphery of feeding flocks while Kittiwakes rarely attempted even to join these. Most of the birds at trawlers were in adult plumage, and it is suggested that the low proportion of immature birds present was a further reflection of the highly competitive feeding conditions at trawlers. We suggest that likely changes in fishing practice and seabird population sizes in the immediate future may result in Herring Gulls, Lesser Black‐backed Gulls and Great Skuas finding feeding on waste around trawlers increasingly difficult, so they may be further displaced by Fulmars, Gannets and Great Black‐backed Gulls.