Co-Occurrence of Cetaceans and Seabirds in the Northeast Atlantic

Data on the distribution of cetaceans and seabirds in the Northeast Atlantic sampled in late summer of 1987 and 1989 were analysed. The distribution of ten species of cataceans and eleven species of seabirds could be summarized into nine groups, of which four embraced the majority of birds and cetaceans. Three local seabird species, which had not left the breeding colonies during the survey periods, concentrated in near-colony areas. No cetaceans appeared to show similar use of shelf habitats close to land. The harbour porpoise Phocoena phocoena, however, showed affinity for shelf waters in general and formed a well-defined second group with sooty shearwater Puffinus griseus and common murre Uria aalge. A third group of several small- to medium-sized cetaceans and two local petrels were concentrated mainly along the shelf edges, and over more remote banks and ridges. A fourth group was formed by two species of large cetaceans, two migrating seabird species and the white-sided dolphin Lagenorhynchus acutus, which were seen mainly in ocean basins. Despite the fact that more cetaceans than seabirds were recorded in areas distant from land, we show that densities of the groups peaked outside the shelf areas over oceanic banks and ridges. The patterns found should not be regarded as reflections of habitat entities, but rather as a first insight into common features to the distribution of cetaceans and birds in the study area. A clearer distribution overlap between seabirds and cetaceans may be expected over the more remote shelf areas such as oceanic banks and ridges outside the breeding season, when these environments become available to large populations of non-breeding seabirds.