Graduate Thesis Or Dissertation

Foraging behavior of the Pacific harbor seal (Phoca vitulina richardii) in the Pacific Northwest and potential impacts of coastal hypoxia on foraging efficiency

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  • In the last decade, the California Current Large Marine Ecosystem has experienced a trend of increased severity of upwelling-driven coastal hypoxia. This thesis strove to examine the potential upper trophic level impacts of moderate and severe hypoxia in the CCLME. Initially I conducted a literature review of Pacific harbor seal (Phoca vitulina richardii) diet in the regions of Oregon and Washington. Thirteen papers were reviewed which detailed harbor seal foraging studies in Washington, Oregon and the Columbia River from 1931 until 2012. 148 prey species or genera were described in harbor seal diet in Oregon, Washington and the Columbia River. Accordingly, I created energetic individual-based models of harbor seal spatial foraging behavior in response to hypoxia-related habitat compression for three predominant prey species, Pacific herring (Clupea pallasii), English sole (Parophyrs vetulus) and Pacific sandlance (Ammodytes hexapterus). The model was composed of three submodels which simulated hypoxia and spatial foraging, foraging energetics, and dive mechanics. Hypoxia was scaled from 0 to a level of 1, which represented a severe inner-shelf hypoxic or anoxic event in which up to 80% of the inner shelf water column was hypoxic, and species lost 50% of their horizontal habitat. Response variables were also evaluated at the intermediate hypoxia levels of 0.25 and 0.75. A sensitivity analysis was conducted to illuminate which predictor variables drove a variety of response variables. Hypoxia strongly drove energetic balance, travel cost to foraging and time spent foraging at depth for all three prey species. Model output was not highly sensitive to caloric content of prey. For smaller harbor seals, energetic gains were maximized by foraging on Pacific herring or sandlance during normoxia, and sandlance during severe hypoxia. Larger adult harbor seals benefitted most from a herring- or sole-based diet during normoxia, herring and sandlance during moderate to severe hypoxia. The results suggest that shifts in spatial foraging behaviors during hypoxia may be readily apparent upon spatial analysis of TDR data. The information gathered in this thesis will be used in the experimental design of a field-based study of the effects of coastal hypoxia on harbor seal foraging behavior.
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