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Sea otters Enhydra lutris, well documented as 'keystone' predators in rocky marine communities, were found to exert a strong influence on infaunal prey communities in soft-sediment habitats. The effect of sea otter predation in subtidal soft-bottom prey communities in southeast Alaska was evaluated via a natural experiment comparing prey populations in similar habitats with and without sea otters. Results indicate that otters forage primarily on bivalve prey, especially the butter clam Saxidomus giganteus and dramatically reduce infaunal bivalve and sea urchin (Strongylocentrotus spp.) prey populations. Bivalve prey abundance, biomass and size were negatively correlated with sea otter occupancy. Otter-cracked shells of the deep-burrowing clams Tresus capax and Panope abrupta were rarely found, even at otter foraging sites where these clams accounted for the majority of available prey biomass, suggesting that these species have a partial depth refuge from otter predation. The differential impacts of otter predation on prey populations suggest that infaunal bivalves provide a more sustainable food base than do sea urchins.

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