Foraging behavior of the Pacific harbor seal (Phoca vitulina richardsi) in Humboldt Bay, California

Coastal ecosystems are under intense pressure from anthropogenic and environmental factors in California. Human development, pollution and climate change continue to degrade coastal habitats. Marine predators such as marine mammals that depend on nearshore ecosystems are at risk from these anthropogenic and environmental factors. The Pacific harbor seal (Phoca vitulina richardsi) is one of the most common marine mammals along the coast of northern California and likely to be heavily impacted by changes in the coastal environment, yet little is known about their foraging behavior or habitat use. To accurately predict the response of local marine predators to changes in the coastal environment an understanding of their foraging behavior is required. This study aimed to develop baseline data on the foraging habitat and foraging behavior of the Pacific harbor seal in northern California. I radio tagged 12 seals from haul-out sites in Humboldt Bay, California and recorded their at-sea distribution between September 2005 and June 2006. I used fixed kernel analysis to measure the size and identify the locations of foraging areas. I characterized foraging behavior by measuring the distance from haul-out sites to foraging areas, the spatial overlap of foraging areas, the water depth at foraging areas and foraging trip duration. Foraging area size ranged from 10.2 to 37.4 km2 (mean= 26.7 ± 3.5 km2, ±SE). Seals used foraging areas both within and outside of Humboldt Bay. Seventy-seven percent of the seals foraged within 13 km of the capture site (mean= 21.1 ± 13.4 km) and 88% of the seals foraged within 13 km of their primary haul-out site (mean= 11.2 ± 2.0 km). There was a broad overlap between the foraging areas used by all animals (mean= 57.2 ± 4.3%). Foraging occurred at shallow to moderate depths (mean= 23.9 ± 14.5 m). Foraging trips generally lasted fewer than 24 hours with mean trip duration of 16 hours ± 59 minutes. The baseline data collected in this study, coupled with previous analysis of diet and haul-out site use will help to predict the impact of changes in the coastal environment on harbor seals in northern California.