Marine & Environmental Sciences Faculty Proceedings, Presentations, Speeches, Lectures

A Perspective on Harbor Seal Trophic Interactions in Prince William Sound and the Gulf of Alaska

Event Name/Location

Legacy of an Oil Spill: Ten Years After Exxon Valdez, Anchorage, Alaska, March 1999

Presentation Date

3-1999

Document Type

Conference Proceeding

Description

The number of harbor seals (Phoca vitulina) in Prince William Sound (PWS) has declined to approximately one-fourth of the 1975 population, and food web dynamics have been investigated as a possible factor. Stable carbon and nitrogen isotope ratios (delta13C and delta15N) are established in the primary producers of the food chain and provide information of prey consumed at different locations and trophic levels within PWS and the adjacent Gulf of Alaska. Offshore prey have more depleted isotope values than the same species within PWS. Prey from pelagic and benthic environments have similar delta15N values but the benthos is more enriched in 13C than pelagic species. Satellite tracking data of harbor seals tagged by Alaska Department of Fish and Game personnel reveal some seals leave the sound to feed in the gulf for extended periods of time. The nutritional values of these prey relative to pelagic prey, such as herring and capelin, are unknown but likely lower. Stable isotope values from archived and modern harbor seal tissues from 1950-1996 showed that no significant difference in the delta15N values occurred during the past 47 years and this indicated no trophic shift occurred during that time. However, during that same period, a decrease in the delta13C became evident in the bone collagen of three species of phocids, including harbor seals, and otariids throughout the Bering Sea and Gulf of Alaska. That decline tracked the decline in zooplankton biomass estimates and primary productivity rates as exhibited by the stable isotope records in bowhead whale baleen. All these data in combination may indicate that the carrying capacity of the northeastern Pacific Ocean and Bering Sea has declined since the 1960s and implies serious consequences for top trophic level organisms which require high primary productivity to support the prey density necessary for successful recruitment.

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