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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

The ecology, morphology, and much of the behavior of a seabird species are definable in terms of the food resources it exploits year-round and the spatial and temporal relations between food and breeding sites. This general point unifies such important reports as those by Kuroda (1954), Bédard (1969a), Ashmole and Ashmole (1967), Ashmole (1971), Spring (1971), and Sealy (1972). More concretely, information on trophic relations of seabirds is useful in several ways. In conjunction with biological oceanographic data, it can provide insight into geographic location, marine habitat, depth, time of day, and general method of food capture by seabirds. Collected over several years, it can provide a basis for understanding annual differences in seabird breeding phenology and success. Finally, supplemented with data on how much seabirds eat and excrete, it is necessary for an understanding of the energetic and ecological roles played by the birds in the functioning of marine ecosystems.

Several studies that describe trophic relations within seabird communities have helped to define the principals of community organization pertaining to the exploitation of available 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 pertained 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 (Bédard 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 seabird food studies. First, few studies have ever considered in detail the trophic relations of seabird communities during the winter or nonbreeding 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 Columbia. 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 entire 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 nonbreeding species have no significant impact on resource exploitation or on organization and evolution among breeding members would be naive. Finally, few investigators have attempted 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 emphasis 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 organizing the specialized community of biologists required for such tasks. A less obvious but important reason is that oceanographers and fishery biologists have overlooked seabirds as important members of marine ecosystems.

Diets of Seabirds in Western North America

Relatively good information exists for most pelecaniformes of the region. A notable exception is the brown pelican (Pelecanus occidentalis), an endangered species. This is unfortunate because dietary information is important for understanding the species' ecology. Observations 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 piscivorous, 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. Cormorants 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 hyemalis, Histrionicus histrionicus, Polysticta stelleri, S. mollissima); those feeding on benthic molluscs (Somateria spp. and Melanitta spp.); and those feeding on fish (Mergus serrator, Clangula hyemalis, and Melanitta deglandi). A study by Perthon (1968), one of the few on a seabird's diet during most of a year, showed a seasonal change in diet for S. mollissima in Norway. In general, waterfowl seem to specialize in their diets much more than other seabirds and, for that reason, are perhaps more restricted in their distributions. Some marine ducks are known to dive to considerable depths (reviewed by Kooyman 1974), but usually they occur in shallow waters where plants and sessile invertebrates are readily available.

The summer diet of the pigeon guillemot (Cepphus columba) is the best known among seabirds in the region being considered here (Table 3). Only in the extreme southern part of its range (i.e., the California Channel Islands) is there no information available on its diet. The species feeds on organisms, mostly fish, from rocky habitat and apparently can dive to considerable depths (Follett and Ainley 1976). Because so much is known about guillemot diets during summer, a study of the winter diet would be valuable.

The diets of other alcids are known well enough to at least characterize them broadly. The larger species, murres, tufted and horned puffins (Lunda cirrhata, Fratercula corniculata), and the rhinoceros auklet (Cerorhinca monocerata), feed heavily on fish, mainly species that school in midwater (Table 4). To a great degree, these birds are opportunistic, feeding rather heavily at times on cephalopods and crustaceans, particularly nektonic forms. Morphological differences between the two murre species suggest that thick-billed murres (Uria lomvia) feed on benthic organisms much more than do common murres (U. aalge), and that the latter species is more piscivorous (Spring 1971); however, field data on diets are barely adequate to confirm this. Ogi and Tsujita (1973) analyzed the stomach contents of murres drowned in salmon gill nets but did not separate the two species. For the present paper we considered them to be mostly U. aalge, since this species predominates in the region of the food study (Bartonek and Gibson 1972). Adult murres sometimes eat different items than they feed to their chicks (Spring 1971; Scott 1973). The smaller alcids, ancient and marbled murrelets—Synthliboramphus antiquus and Brachyramphus marmoratus—(Table 5) and auklets (Table 6), feed on macrozooplankton: crustaceans, and fish and squid larvae. Little is known about the food or feeding ecology of Kittlitz's murrelet (B. brevirostris). Its diet is probably similar to that of the other murrelets, especially the marbled murrelet, its allopatric congener, but the diets of the other murrelets differ somewhat (Bédard 1969b; Sealy 1975). The Kittlitz's murrelet's shorter bill suggests that it feeds more on invertebrates. Alcids feed in deep or shallow water, depending on food distribution. Some alcid species can be found at great distances from land, particularly in winter (Hamilton 1958; Scott et al. 1971).

Information on the diets of other seabirds in the region is fragmentary and sometimes rather anecdotal. A little is known about the feeding habits of loons (Gavia spp.) and grebes (Podiceps spp. and Aechmophorus occidentalis), especially off British Columbia (Table 7). The larger of these birds feed mainly on inshore fish, but as species become progressively smaller, there is a tendency toward eating crustaceans. Work by Madsen (1957) in Denmark, indicated that loons and grebes tend to take prey near or on the bottom. Much more information is available on these birds' diets at their freshwater breeding sites but this provides only partial insight into what they might eat in marine habitats.

Information is especially poor for albatrosses and petrels (order Procellariiformes) (Table 8). Yet, based on sheer numbers alone, members of this diverse group are easily among the most ecologically dominant of the region (Sanger 1972; Ainley 1977). The Laysan albatross (Diomedea immutabilis) seems to be a squid specialist; the black-footed albatross (D. nigripes), northern fulmar (Fulmarus glacialis), scaled petrel (Pterodroma inexpectata), and the fork-tailed and Leach's storm-petrels (Oceanodroma furcata and O. leucorhoa) appear to be large, medium, small, and tiny versions, respectively, of surface-feeding generalists that eat whatever they can find, including live and dead fish, squid, coelenterates, crustaceans, and other organisms. The shearwaters (Puffinus spp.) feed to an unknown degree on schooling fish, squid, and crustaceans that occur near the surface. For these very abundant shearwaters, that, unfortunately, is close to the extent of our knowledge both for the North Pacific, where they winter, and the South Pacific, where they breed. Most petrels remain in oceanic habitats, but shearwaters, particularly the sooty shearwater (Puffinus griseus), and sometimes fulmars feed close to, if not within, the inshore neritic habitat. A much better understanding of the diets of this group is sorely needed.

Knowledge on the food of gulls, shorebirds, and related species is surprisingly scanty in view of all that is known about their breeding biology and social behavior. Little is known about the marine food of phalaropes, but by inference from their association with storm-petrels, plankton-feeding whales, and convergence lines (Martin and Myers 1969), their tiny size, and their method of feeding (picking at minuscule items on the water surface), one can guess that they feed on zooplankton and detritus. Skuas (Catharacta skua) and jaegers (Stercorarius spp.) apparently eat what they can find at the surface, as well as whatever they can steal from gulls and terns. Almost all the literature on their feeding (Bent 1946) dwells on accounts of their stealing from other birds. That spectacular behavior would seem to be so energetically costly, though, that it is probably less important than we have been led to believe. Rather surprisingly, the question of what foods the gulls and terns eat in the eastern North Pacific is difficult to answer from the literature (Tables 9 and 10). Some information exists for five of the larger larids at isolated places, but little is known about food elsewhere in their respective ranges, and the diets of the seven smaller gulls and the terns are practically unknown. Studies on gull diets in the Atlantic region (e.g., Spaans 1971; Harris 1965) provide information on what to expect from the same species in the Pacific, but that information must be considered only in general terms because, the birds being somewhat opportunistic, their diets differ greatly from one locality to another (Ingolfsson 1967). A few observations are available for arctic terns (Sterna paradisaea) in Alaska, but little information exists for other terns (Table 10). Bent (1921) noted that Aleutian terns (S. aleutica) sometimes associate with arctic terns during feeding.

Finally, we must include raptors, particularly the peregrine (Falco peregrinus) and bald eagle (Haliaeetus leucocephalus), because they are important predators on the smaller seabirds (White et al. 1971, 1973). Peregrines have, in fact, been observed feeding on storm-petrels far at sea (Craddock and Carlson 1970).

Trophic Relations Within Seabird Communities

We have compared and summarized in general terms the food partitioning by species in five rather broad oceanographic regions and their subdivisions in the northeastern North Pacific and Bering Sea, based on the specific details on diets presented in Tables 1 through 10. The five broad regions, defined oceanographically by Dodimead et al. (1963) and Favorite et al. (1976) and modified by Sanger (1972), are shown in Fig. 1. The five oceanographic regions (domains) were divided further into inshore neritic, offshore neritic, and oceanic habitats (Sanger and King, this volume). We did not include estuarine habitats or sheltered bays in the analysis.