Evidence of bottom-up control of diet driven by top-down processes in a declining harbor seal Phoca vitulina richardsi population

Abstract

Two mechanisms of population control dominate most biological systems: bottom-up and top-down regulation. It is possible, however, that top-down mediation may lead to bottom-up control of a population if predators simultaneously compete for the same prey. Harbor seal Phoca vitulina hchardsi populations in Glacier Bay (GB) and Prince William Sound (PWS), Alaska, have declined drastically since the 1970s, with PWS recently stabilizing and GB continuing to decline. Hypotheses for the declines include both bottom-up and top-down processes. We hypothesized that increased competition and predation risk are causing harbor seals in GB to forage on lower quality prey. We combined analyses of prey remains in scat and stable carbon and nitrogen isotope ratios in blood and hair to compare seal diets in these areas. Seal diets in GB and PWS varied spatially and temporally due to changes in resource availability and sexual segregation. Adults showed clear divergence in diet during specific times of the year in both areas. Sexual segregation of diet in GB was most prevalent during spring and fall, while in PWS segregation was greatest during late summer. Diet of seals in PWS showed annual variation not found in GB, likely following prey cycles. In GB during summer, all seals switched to a diet with a lower fat content, including more intertidal/demersal species such as rockfish and sculpin. This switch coincided with an increase in competitors and predators entering GB. The change in diet, combined with higher emigration of harbor seals out of GB, suggest that increased competition and risk of predation may contribute to overall population declines. © Inter-Research 2009.