The Behavioral Response of Harbor Seals to Seasonal Prey Pulses of Spawning Pacific Herring

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Thomas, Austen C.
Western Washington University
Harbor seal--Behavior--Washington (State)--Puget Sound; Harbor seal--Food--Washington (State)--Puget Sound; Pacific herring--Predators of--Washington (State)--Puget Sound; Marine Biology; Puget Sound (Wash.); masters theses
thesis / dissertation description
The Pacific harbor seal (Phoca vitulina) is a noteworthy omission from the list of predators that aggregate at herring spawning sites, despite strong suggestive evidence that they are likely to respond to herring pulses. Working with the hypothesis that spawning herring aggregations are seasonally important prey for harbor seals, we tested several predictions using an analysis of harbor seal prey remains, GPS telemetry, and satellite-linked time/depth recorder data. Contrary to predictions, herring in harbor seal diet was comprised of 77% juveniles and 23% adults in the spawn season, versus 33% juveniles and 67% adults in the post-spawn season. Seal diving focus peaked at night during the post-spawn season, and seals exhibited less diving effort during the spawn season. Harbor seals did not however appear to alter their foraging behaviors to take advantage of spawning herring aggregations. The lack of response by harbor seals to spawning herring pulses is likely explained by seasonal differences in adult herring profitability, the availability of alternative prey, or a decline in local herring biomass.