2021 Harbor Seal Monitoring Updates

Breeding Season Summary

Since harbor seal monitoring started in 2000, monitors have counted an average of around 1,100 harbor seal pups each spring at the main harbor seal locations in Marin County. This year, they counted only 921. Most of the decline comes from Drakes Estero. Typically, Drakes Estero is one of Marin’s largest pupping sites with monitors recording 350 pups there in an average year. But this year, monitors saw an increase in coyote activity and only 230 pups⁠—one of the lowest counts on record along with last year's pup count. Harbor seals don’t like coyotes, especially not around their vulnerable young pups. Coyote activity is a likely culprit for recent pup declines at Double Point as well. The good news is that monitors have seen more pups at Bolinas Lagoon these last few years. That continued this year with the largest pup count ever recorded at this site⁠—270 pups!

Molting Season Summary

Though monitors recorded lower than usual molting harbor seal numbers at Double Point and Drakes Estero, they counted a record number at Bolinas Lagoon in 2021.

NPS

Monitors also count harbor seals during the summer molt season. This is the time of the year when all age groups come onshore to shed their fur. Biologists are able to get a good estimate of the total harbor seal population in Marin County during this time. Unfortunately, the 2021 molt season had the lowest total seal count on record. The 2,400 seals recorded at the main Marin County sites was far below the average of 3,673 seals. The largest individual site declines were recorded at Double Point and Drakes Estero. Approximately 1,000 seals use each of those sites during an average molt season. But in 2021, monitors counted only 250 seals at Double Point and 600 seals at Drakes Estero. As during the pupping season, the declines may be due, in part, to the increased coyote activity that monitors have noticed at both sites. Counts at Double Point have been decreasing since 2016, but this is the first year monitors confirmed low counts at Drakes Estero (there weren’t any molt season counts last year). On the bright side, Bolinas Lagoon was the shining star once again! Monitors recorded 620 molting seals there this year compared to an average of 480 seals. Bolinas Lagoon, for the first time, was the top site for harbor seals in Marin County.

Preliminary Data

2021 Maximum Harbor Seal Count By Site

Maximum harbor seal counts by site during the breeding season (adults and pups) and molt season (all seals).

Maximum Harbor Seal Pup Counts, 2000-2021

Maximum harbor seal pup counts for 2000-2021 at Marin County sites. The black solid line represents the baseline mean, or average, of the maximum pup counts from 2000-2015 (mean = 1,101). The dashed lines represent one standard deviation from the mean. Not all sites were surveyed in 2020.

Maximum Harbor Seal Molt Counts, 2000-2021

Maximum harbor seal molt season counts for 2000-2021 at Marin County sites. The black solid line represents the baseline mean, or average, of the maximum seal counts from 2000-2015 (mean = 3,673). The dashed lines represent one standard deviation from the mean. Not all sites were surveyed in 2020.

The Details: 2021 Weekly Maximum Harbor Seal Counts By Site

Maximum 2021 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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