Wild harbor seal (Phoca vitulina) population dynamics and survival in northern California

Pacific harbor seals (Phoca vitulina) within San Francisco Bay (SFB) have been described as stable compared with those in coastal northern California, like Tomales Bay (TB). Historical data (1970s - early 2000s) indicated an overall increase in adults and pup production. Recent data, however, revealed SFB and TB adult counts decreased while pup production increased. There is a paucity of life history data, such as survival rates, for northern California harbor seals. For 32 radio-tagged adult females, survival was 98.2% over 20 months 2011 through 2013, constant between bays and influenced by an individual’s axillary girth. Movement between bays supported this survival estimate. Lower resight probability in SFB compared with TB was likely due to multiple factors including sampling bias and haulout quality and area. These are the first adult/subadult survival and resight estimates for harbor seals in California, and suggest the stable SFB population is due to some reason other than poor adult female survival (e.g. emigration, poor pup survival).