The ecology of bivalve communities in Prince William Sound, Alaska: Influence of the Exxon Valdez oil spill and predation by sea otters

Ecology of intertidal and subtidal bivalves was studied in Prince William Sound, Alaska following the Exxon Valdez oil spill and after population reductions in sea otters (Enhydra lutris ). Improved understanding of population structure, residual oil effects, and otter predation effects were study objectives.;Residual oil effects on intertidal Protothaca staminea were studied using reciprocal experimental transplants of tagged clams between unoiled and oiled sites. Residual oil affected survival and growth rates. Clams depurated hydrocarbons from tissues when moved to an unoiled site and accumulated hydrocarbons when moved from an unoiled to oiled site.;Juvenile/adult subtidal clam densities were greater at unoiled sites while size-frequency distributions indicated larger clams at oiled sites. Macoma spp. was found in rare, dense patches at unoiled sites. At oiled sites, small numbers of several species were collected; no dense aggregations were encountered. Sea otters consumed mainly venerid clams at both areas and Macoma at unoiled sites. Otters ate larger clams at oiled sites. Otter predation and differential clam settlement were probably important factors structuring subtidal bivalve communities, though habitat differences, especially for Macoma probably had some influence.;Bivalve responses to significant reductions in otter density were examined to test predictions based on sea otter trophic paradigms. Density and size patterns were examined. A concept termed "symmetry" was evaluated where demographic shifts following otter reductions are opposite, but occur over comparable time frames, as shifts by otter appearances at high densities to areas previously without otters. Prey responses to reduced otter density were asymmetrical for bivalve prey evaluated. Shifts toward higher density and greater preponderance of larger prey following otter reductions required more time than shifts observed when otters occupy locations previously lacking otters.;Possible explanations for asymmetry include residual oil effects, interannual prey recruitment variation, and unpredictable natural disturbances. Possible relationships of otter threshold densities to various states of prey population dynamics could result in incorrect perceptions of asymmetry. Results weaken deterministic trophic cascade models involving sea otters and suggest that in protected coastal marine habitats, trophic linkages of sea otters, benthic prey, and associated communities may be less certain than in exposed coastal areas.