SEA BIRDS AS PROXIES OF MARINE HABITATS AND FOOD WEBS IN THE WESTERN ALEUTIAN ARC

We propose that ocean conditions of the Near Islands in the western Al eutian Are mimic those of the shallow continental shelf of the eastern Bering Sea to the extent that the marine community, including assembl ages of forage fishes and their avian predators, has distinctly coasta l characteristics. In contrast, marine avifauna and their prey at neig hbouring Buldir Island are distinctly oceanic. For example, at the Nea r Islands, the ratio of thick-billed to common murres, Uria lomvia and U. aalge, is low and black-legged kittiwakes, Rissa tridactyla, but n ot red-legged kittiwakes, R. brevirostris, nest there. Diets of murres and kittiwakes are dominated by sand lance, Ammodytes hexapterus, an abundant coastal species. At Buldir Island, thick-billed murres greatl y outnumber common murres, red-legged kittiwakes and black-legged kitt iwakes are both abundant, and diets of the birds consist primarily of oceanic squid and lantern-fish (Myctophidae). This mesoscale differenc e in food webs is apparently a consequence of the local physiography. A broad escarpment on the Near physiographic block creates a comparati vely expansive, shallow, shelf-like habitat around the Near Islands, w here a pelagic community typical of coastal regions flourishes. Buldir Island is the only emergent feature of the Buldir physiographic block , with little shallow water surrounding it and, apparently, little opp ortunity for other than oceanic species to exist. Patterns in the dist ribution of fishes, and thus of sea birds, throughout the Aleutian Isl ands might be largely explained by the presence or absence of shelf-li ke habitat and the relationship between physical environments and food webs. In the larger context of fisheries oceanography, this model for the Aleutian Islands improves our ability to interpret physical and b iological heterogeneity in the ocean and its relationship to regional community dynamics and trends in the abundance and productivity of ind ividual species at higher trophic levels.