Sea star and walrus predation on bivalves in Norton Sound, Bering Sea, Alaska

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Abstract
Pacific walruses (Odobenus rosmarus divergens) and sea stars (Asterias amurensis) are the primary predators on bivalve mollusks in Norton Sound and probably in most of the Bering and Chukchi Seas. Secondary predators in these areas include starry flounder (Platichthys stellatus) and king (Paralithodes spp.) and Tanner (Chionoecetes spp.) crabs. Walruses and sea stars consume the same species, but not the same sizes of bivalves. The primary walrus prey are large individuals of several bivalve species, which have no refuge from walrus predation. Sea stars feed on smaller species and on smaller individuals of the large bivalve species. Laboratory experiments and field observations indicate that large bivalves have refuges from sea stars. Large Serripes groenlandicus escape sea star predation by leaping, large Yoldia hyperborea by rapid burrowing into the sediment. Large Mya truncata and Macoma calcarea live too deep in the sediment (>15 cm) for capture by the shallow digging (Asterias amurensis. Since behavior and depth refuges are less effective for smaller bivalves, sea stars forage intensively on intermediate sizes and probably maintain the bimodal size distribution in some large bivalve species. Intense walrus predation on large clams probably produces the unimodal distribution of primarily small sizes in some local populations of M. calcarea and S. groenlandicus. Intense walrus or sea star predation may have important effects on the availability of bivalve prey for each other and for other predators.