MEPS 312:265-275 (2006)  -  doi:10.3354/meps312265

ABSTRACT: Harbor seals Phoca vitulina and other pinnipeds in the Gulf of Alaska have declined since the 1980s. The search for causation has considered top-down and bottom-up influences as independent factors. Research on other systems, however, has revealed that resource availability and predator densities synergistically determine the predation rates experienced by intermediate consumers. From this premise we developed a dynamic state variable model of behavior for the declining harbor seal population of Prince William Sound, Alaska. We modeled separate scenarios in which seals were prey to (1) transient killer whales Orcinus orca at and near the surface and Pacific sleeper sharks Somniosus pacificus throughout the water column, or (2) killer whales only. In both scenarios, resource decrements reduced the time spent by seals at the haulout (a refuge lacking food), increased the time spent at foraging areas, and lengthened surface intervals and dive durations. Because of this behavioral compensation, per capita fish consumption remained relatively constant, but predation rates increased as resources declined, despite fixed predator densities. Foraging effort and predation rates increased further when energy stores were lower at the onset of simulation periods, but in all scenarios seals not killed by predators had achieved a high level of energy stores by the reproductive season. These behavioral mechanisms proposed by the model potentially explain—at least partially—why the population has been declining while seals have maintained good energy stores throughout temporal shifts in resource availability. More generally, simulations suggest that overfishing and other factors that reduce fish populations indirectly increase predation rates on seals, but data are needed to test this hypothesis. Our model also encompasses a broader ecosystem perspective by predicting how resource level determines the relative strength of trait- and density-mediated interactions, whereby predators of seals indirectly affect fish populations by influencing the foraging behavior and density of seals. The behavioral modeling approach presented here is an additional tool for resource managers attempting to optimize fisheries exploitation and pinniped conservation.

KEY WORDS: Predation risk · Resource availability · Harbor seal · Phoca vitulina · Dynamic state variable model · Pinniped declines · Optimal diving · Prince William Sound · Indirect interactions