- Pacific harbor seals (Phoca vitulina richardii) are one of Oregon’s most common coastal predators, numbering between 10,000 and 12,000 individuals (Brown et al. 2005b). They consume more than 149 species or types of marine prey within the Pacific Northwest, which include a large variety of commercially important fisheries species. Despite their potential economic impacts and ecological role, little quantitative data are available regarding individual harbor seal spatial foraging behaviors and dietary habits along the Oregon coast, particularly outside of their estuarine habitat.
In order to examine the movement and dietary ecology of Pacific harbor seals in Oregon, I used satellite telemetry to track 24 adult harbor seals captured in two locations on the Oregon coast from September 2014 to September 2015. I also collected a whisker from each animal for dietary estimation via stable isotope analysis, namely the quantification of δ13C and δ15N enrichment as proxies for trophic level and spatial habitat use. These data were examined from three separate perspectives to highlight the ecological role of harbor seals along the Oregon coast.
Chapter 1 is a quantitative assessment of spatial habitat utilization of Pacific harbor seals. I quantified individual and population-level home range area, core area, foraging trip distance and duration, percent presence within eleven inland waters including bays and rivers, proximity to two wave energy test sites, use of Oregon’s five marine reserves as well as marine protected areas and comparison areas, and percent use of inshore vs. at-sea habitats for all study animals.
Chapter 2 is a multi-level examination of variability and assessment of behavioral repeatability for harbor seals. It included an examination of differences and predictability in spatial behavior and diet for seals at the levels of individual, capture site, and whole sample population. This was accomplished by measuring ‘repeatability’ of specific behaviors. Repeatability and spatial use were compared to stable isotopes in generalized linear and linear mixed effects models to highlight strategies in foraging.
Chapter 3 investigates how local oceanography, dietary composition, and spatial movement were related for seals. I utilized generalized linear mixed models and linear mixed effects models to examine which environmental and site-related variables were most associated with movement and dietary patterns of the study animals. This was examined from a series of models of individually summarized parameters (n = 24), and from three models examining the point-by-point parameters of haulout status, in bay vs. at sea, and distance from shore (n = 57,220).
Overall, this dissertation demonstrates associations between stable isotope-derived diet and patterns in spatial habitat use, suggesting that stable isotope analysis of δ13C and δ15N can inform two-dimensional satellite telemetry, but also may provide post-hoc information regarding drivers of spatial movement of harbor seals. Results revealed a high degree of between-individual variability in diet and spatial behavior, a limited use of Oregon’s newly established marine reserves and wave energy sites, and extensive utilization of Oregon’s continental shelf. This dissertation represents the first in-depth description of spatial habitat use for Pacific harbor seals, a common marine mammal and upper trophic-level predator, along the Oregon coast.