2018 Harbor Seal Monitoring Updates

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Molting Season Summary

August 6, 2018 - Following the breeding season, harbor seals of all age classes and sexes haul out on a daily basis to molt. The molt process consists of seals shedding their old fur and growing a new layer. Seals need to molt on land to reduce their heat loss and because sunlight stimulates hair follicle growth. It takes an individual seal about three weeks to complete the molting process.

Approximately 60 harbor seals lying on large rocks surrounded by water. The seals vary in color from beige to white or black.
Harbor seals, in varying stages of molting, hauled out on rocks in Point Bonita Cove in Golden Gate National Recreation Area.

NPS / David Donnenfield

During the 2018 molt season, a total count of 3,022 seals was recorded at Marin County locations. This maximum count is lower than the 18-year average of approximately 3,600 seals and is similar to the low count in 2017 (2,800). The peak counts at each site vary in time, and we recorded site-specific peaks between the end of June and mid-July.

Drakes Estero had the largest number of seals with almost 1,100, followed by Bolinas Lagoon with 650 seals. Bolinas Lagoon has had a large increase in seals since 2014 and has an 18-year average of 500 seals. Double Point, which for many years had been a top site and has an 18-year average of 930 seals, continues the decline that started in 2015 with only 340 seals recorded during the 2018 molt season. The Tomales Bay site was also lower than its average of 400 seals with 300 seals recorded in 2018. During molt season surveys in 2018, there were many disturbances caused by clammers on the mudflats, which may have led to lower counts. The remaining sites, Duxbury Reef, Tomales Point, and Point Bonita, all had average harbor seal counts.

Breeding Season Summary

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 seals during the molt season. Breeding season counts are there for comparison and show that Drakes Estero also had the largest count of seals during the breeding season.
Maximum harbor seal counts by site during the breeding season (adults and pups) and molt season (all seals).

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.

Maximum Harbor Seal Molt Counts, 2000-2018

Line graph showing maximum harbor seal molt counts for 2000-2018. Counts have mostly remained within one standard deviation (SD) of the mean. The highest count was in 2004, the lowest were in 2010 and 2011, and this year's is about one SD below the mean.
Maximum harbor seal molt season counts for 2000-2018 at Marin County sites. The black solid line on the graph represents the mean, or average, of the maximum seal counts from 2000-2017 (mean = 3,613), 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

Contact

Sarah Codde

Webpages

San Francisco Bay Area Network - Pinniped Monitoring

Pacific Coast Science and Learning Center - Harbor Seals


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Last updated: August 8, 2018