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 1 



National Marine Fisheries Service 



Marine Mammal Division 



Seattle, Washington 



Abstract 



Literature on the diets of seabirds is reviewed for 70 species found in five sub- 

 arctic 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 dif- 

 ferent 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 catastrophies. 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 con- 

 ducted as to facilitate comparison with biological oceanographic data. 



The ecology, morphology, and much of the and Ashmole (1967), Ashmole (1971), Spring 



behavior of a seabird species are definable in (1971), and Sealy (1972). More concretely, in- 



terms of the food resources it exploits year- formation on trophic relations of seabirds is 



round and the spatial and temporal relations useful in several ways. In conjunction with 



between food and breeding sites. This general biological oceanographic data, it can provide 



point unifies such important reports as those insight into geographic location, marine habi- 



by Kuroda (1954), Bedard (1969a), Ashmole tat, depth, time of day, and general method of 



food capture by seabirds. Collected over 



'Present address: U. S. Fish and Wildlife Service, several years, it can provide a basis for under- 

 Office of Biological Services, 1011 East Tudor standing annual differences in seabird breed- 

 Road, Anchorage, Alaska 99503. ing phenology and success. Finally, supple- 



95 



