96 



D. G. AINLEY AND G. A. SANGER 



merited with data on how much seabirds eat 

 and excrete, it is necessary for an understand- 

 ing of the energetic and ecological roles 

 played by the birds in the functioning of 

 marine ecosystems. 



Several studies that describe trophic rela- 

 tions within seabird communities have helped 

 to define the principals of community organi- 

 zation pertaining to the exploitation of avail- 

 able food resources and have given clues to 

 food-chain pathways. Trophic relations have 

 been described for breeding communities in 

 the Barents Sea (Uspenski 1958; Belopol'skii 

 1961), in the tropical Pacific Ocean (Ashmole 

 and Ashmole 1967; Ashmole 1968), in the 

 North Sea (Pearson 1968), and in the Chukchi 

 Sea (Swartz 1966). The last-named study per- 

 tained most directly to the geographic region 

 discussed in this paper, but several other 

 studies have provided sound information on 

 segments of communities in the northeastern 

 North Pacific and Bering Sea. These include 

 the work on three species of auklets (Aethia, 

 Cyclorrhynchus) in the Bering Sea (Bedard 

 1969a); investigations on cormorants and 

 other fish predators in British Columbia by 

 Munro (1941), Munro and Clemens (1931), and 

 Robertson (1974); studies of murres in Bristol 

 Bay by Ogi and Tsujita (1973); observations 

 on several species near the Pribilof Islands by 

 Preble and McAtee (1923); work on diving 

 species off Oregon by Scott (1973); and 

 studies of murrelets by Sealy (1975). 



A review of available reports reveals three 

 obvious gaps in the emphasis placed in sea- 

 bird food studies. First, few studies have ever 

 considered in detail the trophic relations of 

 seabird communities during the winter or non- 

 breeding season. Partial exceptions are the 

 works by Cottam (1939) and others on marine 

 diving ducks, species that are seabirds only 

 during the winter, and by several researchers 

 (Munro and Clemens 1931; Munro 1941; 

 Robertson 1974) on seabirds in British Colum- 

 bia. Divoky (1976) studied diets of pack-ice 

 gulls during the nonbreeding season, but 

 those species are not included in the present 

 analysis because they rarely are found south 

 of the Bering Strait. Second, no study has 

 considered the trophic relationships of an en- 

 tire seabird community, i.e., not just breeding 

 species but also nonbreeding species. In the 

 rather broad communities considered here, 



50-70% or more of the birds breed in another 

 part of the world. To say that these nonbreed- 

 ing species have no significant impact on re- 

 source exploitation or on organization and 

 evolution among breeding members would be 

 naive. Finally, few investigators have at- 

 tempted to fit birds into an entire ecosystem, 

 including lower trophic level origins as well as 

 fish, marine mammals, and man. 



The reasons for these gaps in study empha- 

 sis are readily apparent: the inconvenience of 

 marine research during the winter when 

 weather is stormy, the need for costly study 

 platforms (boats), and the difficulties in or- 

 ganizing the specialized community of biolo- 

 gists required for such tasks. A less obvious 

 but important reason is that oceanographers 

 and fishery biologists have overlooked sea- 

 birds as important members of marine 

 ecosystems. 



Diets of Seabirds in 

 Western North America 



Relatively good information exists for most 

 pelicaniformes of the region. A notable excep- 

 tion is the brown pelican (Pelecanus occiden- 

 talis), an endangered species. This is unfortu- 

 nate because dietary information is important 

 for understanding the species' ecology. Obser- 

 vations in eastern North America (Palmer 

 1962) and Peru (Murphy 1936) indicated that 

 their diet consisted of fish that occur at the 

 surface. The larger cormorants are pisci- 

 vorous, particularly on schooling fishes that 

 occur at moderate to great depths (Table 1). 

 The smaller cormorants feed more heavily on 

 benthic fish and decapod crustaceans. Cor- 

 morants apparently feed only during daylight 

 and then only for short periods because their 

 wettable plumage loses its buoyancy. Thus 

 they remain relatively close (50 km) to nesting 

 and loafing areas. 



Information on diets of marine ducks 

 (Table 2) is more nearly complete than for 

 most other seabirds. These birds fall into four 

 groups with some overlap: species feeding on 

 plants (Branta, Philacte, .Anas-type, and 

 Somateria fischeri); those feeding on benthic 

 crustaceans (Clangula hy emails, Histrionicus 

 histrionicus, Polysticta stelleri, S. mollis- 

 sima); those feeding on benthic molluscs 



