June 7, 2018 - The San Francisco Bay Area Network's Pinniped Monitoring Program recorded the first harbor seal pups of the 2018 breeding season at Drakes Estero and Tomales Bay on March 23rd, which was right on schedule. Pups were seen at all sites by the week of April 8th. The peak of pupping occurred during late April and early May, with a maximum number of approximately 1,000 pups recorded throughout Marin County locations. This peak is similar to the 16-year average of 1,080 pups. Drakes Estero and Bolinas Lagoon were the two top sites with approximately 360 and 260 pups, respectively. Bolinas Lagoon has had increased pup counts over the last six years and is now about 60% greater than its site average.

Two harbor seal mom and pup pairs are lying on an exposed mudflat.
Harbor seal moms and pups hauled out on a mudflat in Bolinas Lagoon (photographed through a spotting scope).

NPS / Sarah Codde

The number of surveys at Double Point has been reduced due to safety concerns at the observation spot. Surveys are now concentrated during the peak of pupping and, along with last year, we are seeing very low pup numbers at this site. This year’s peak of 150 pups is about 50% less than the site’s average. It is unknown why this site is declining, but could be due to an increase in coyotes and habitat effects from the large storms in 2017.

Preliminary Data

Maximum Harbor Seal Count By Site

Maximum harbor seal counts by site, where Drakes Estero has the highest number of breeding adults and pups.
Maximum harbor seal counts by survey site.

Maximum Harbor Seal Pup Counts, 2000-2018

Line graph showing maximum harbor seal pup counts for 2000-2018. Counts have mostly remained within one standard deviation around the mean. The lowest count was in 2010, the highest count was in 2013, and this year's count is right around the mean
Maximum harbor seal pup counts for 2000-2018 at Marin County sites. The black solid line on the graph represents the mean, or average, of the maximum pup counts from 2000-2017 (mean = 1,080), and the dashed lines represent one standard deviation from the mean.

The Details: Weekly Maximum Harbor Seal Counts By Site

Graph of maximum harbor seal counts by site (Tomales Point, Tomales Bay, Drakes Estero) and survey week from late February through May.
Graph of maximum harbor seal counts by site (Double Point, Bolinas Lagoon, Duxbury Reef, and Point Bonita) and survey week from late February through May.
Maximum weekly harbor seal counts by site and survey week

For More Information


Sarah Codde


San Francisco Bay Area Network - Pinniped Monitoring

Pacific Coast Science and Learning Center - Harbor Seals

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