| In the marine environment, upper trophic-level predators integrate multiple environmental and population stressors, and as such represent the apex of a complex and integrated ecosystem. The Earth's coastal zones are under particularly intense pressure from human development and pollution, and marine predators such as marine mammals that depend on nearshore ecosystems are at risk from a number of anthropogenic impacts. The harbor seal, Phoca vitulina, has been identified as a prime candidate for studies of nearshore ecosystem condition. Understanding how harbor seal distribution and population fluctuations relate to various environmental and anthropogenic variables has important implications for the use of this species as an indicator of ecosystem condition. I modeled habitat selection by Pacific harbor seals in a large, highly urbanized estuary: San Francisco Bay, California (SFB). I assessed the influence of prey abundance, proximity of terrestrial haul-out sites, depth, bottom relief, and proximity to three potential sources of anthropogenic influence on the spatial distribution of seals. In addition, I assessed the spatial relationship between the distribution of harbor seals and the distribution of prey species known to occur in seal diets in SFB. Finally, I examined environmental correlates of the number of harbor seals using SFB, and developed and tested candidate models of environmental influences on seasonal and regional seal trends in SFB. In all habitat selection models, high prey abundance and proximity to the primary haul-out site were strongly linked to the spatial distribution of seals. Highest correlations between seals and their prey were found for seasonally-abundant benthic species, located within ∼10 km of the primary haul-out site. Most probable seal foraging habitat fell within central SFB, one of the most productive regions of the study area. I identified relationships between seal population trends and prey abundance indices, upwelling and X2, highlighting the influence of food resources on the abundance of seals using SFB. Rates of seal population change in SFB were density-dependent. These results reflect the importance of maintaining adequate local prey resources and of protecting haul-out sites, both of which play a central role in determining seal spatial distribution in nearshore waters. |
