Pacific harbor seal (Phoca vitulina richardii) monitoring at Point Reyes National Seashore and Golden Gate National Recreation Area: 2017 – 2018 monitoring seasons

Codde, Sarah; Allen, Sarah

Pacific harbor seals (Phoca vitulina richardii) are the most abundant and only year-round resident pinniped in the National Parks of the San Francisco Bay Area, California. Annual monitoring by National Park Service (NPS) staff, using peer-reviewed methods, began in 1996 with the current suite of monitoring sites adopted in 2000. The objectives of monitoring each site and the population as a whole are to: i) detect changes in population size, ii) detect changes in reproductive success as indicated by pup production, and iii) identify anthropogenic or environmental factors, including climate change, that may affect the condition of the population (Adams et al. 2009). Harbor seal surveys were conducted throughout the 2017 and 2018 breeding and molting seasons (March-May and June-July, respectively) once or twice per week at the largest Point Reyes National Seashore and Golden Gate National Recreation Area harbor seal colonies, collectively referred to as Marin County sites. Members of the Harbor Seal Monitoring Volunteer Program assisted NPS staff in completing 234 surveys in 2017 and 253 surveys in 2018 at eight Marin County sites, contributing approximately 425 hours of monitoring in 2017 and 430 hours in 2018. During the breeding season, the peak count of adult and immature seals recorded at all Marin County monitoring sites was 2,529 seals in 2017 and 2,536 seals in 2018. Both counts are similar to the 16-year average. The Drakes Estero Complex had the most adult and immature seals in both 2017 and 2018 (729 and 690, respectively), followed by Tomales Bay (588 and 453, respectively). During the 2017 breeding season, 950 seal pups were recorded at all Marin County sites, which is less than the 16-year average. During the 2018 breeding season, there was an increase from 2017 with 1,033 pups counted, which is similar to the 16-year average. Typically, the Drakes Estero Complex and Double Point account for the majority of pups at Marin County haul-outs, however, in 2017 and 2018, Bolinas Lagoon and the Drakes Estero Complex made up the majority of pups (58% and 60%, respectively). During the molting season, the peak count of seals recorded at Marin County sites was 3,124 in 2017 and 3,289 in 2018, both of which are lower than the 16-year average. During the monitoring surveys, 97 disturbances to seals were recorded in 2017 and 90 disturbances were recorded in 2018. The most frequent causes in both 2017 and 2018 were motorboats (30% and 29%, respectively), unknown sources (28% and 20%, respectively), and humans on foot (17% and 22%, respectively). Regional surveys were conducted throughout the breeding and molting seasons at locations in San Francisco Bay and Sonoma, Marin and San Mateo counties, with 13 surveys in 2017 and 12 surveys in 2018. Of these areas, Marin County sites accounted for 62% of breeding season adult and immature animals in 2017 (2,516 of 4,032) and 64% in 2018 (2,534 of 3,985), 72% of pups in 2017 (913 of 1,264) and 71% in 2018 (986 of 1,382), and 63% of seals during the molting season in 2017 (3,244 of 5,137) and 63% in 2018 (3,262 of 5,154), all of which are similar proportions to past years.

Pacific harbor seal (Phoca vitulina richardii) monitoring at Point Reyes National Seashore and Golden Gate National Recreation Area: 2017 – 2018 monitoring seasons

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