[Trophic Relations of Seabirds in the Northeastern Pacific Ocean and Bering Sea]
by
David G. Ainley
Point Reyes Bird Observatory
Stinson Beach, California 94970
and
Gerald A. Sanger[15]
National Marine Fisheries Service
Marine Mammal Division
Seattle, Washington
Abstract
Literature on the diets of seabirds is reviewed for 70 species found in five subarctic oceanographic regions of the northeastern North Pacific Ocean and Bering Sea. Species inhabiting estuaries and sheltered bays are not included. The diets of cormorants, marine ducks, alcids, and marine raptors are best known; less information is available for loons, grebes, petrels, and gulls. Enough is known, however, to broadly characterize the diet of each species. Less than 7% of all species feed on one type of prey, about 60% feed on two or three types, and the rest feed on four or more types. Only 12% of all species feed on eight or more types of prey. Most seabirds (77%) feed as secondary and tertiary carnivores. Where overlap in diet exists, seabirds partition resources through use of different feeding methods, selection of different-sized prey, and zonation of habitat. Species that have specialized diets are probably more susceptible than others to local environmental catastrophes. Species whose feeding methods are highly adapted for exploitation of resources in polar and subpolar habitats are not adapted for coping with oil pollution. Competition between birds and man for marine resources can sometimes benefit seabirds and at other times harm them. More research is needed on seabird feeding relations so that the ecological roles played by marine birds can be defined and placed in perspective. Such work should be conducted at the community level, year-round, and should be so conducted as to facilitate comparison with biological oceanographic data.