Last updated: August 4, 2021
Article
Monitoring Team Counts Fewer Harbor Seals than Usual Across Marin County in 2021
NPS
July 2021 - Last year, COVID-19 meant that biologists weren’t able to do their usual harbor seal surveys at park sites throughout Marin County. The few surveys they were able to do left them with more questions than answers. Namely, where did a large portion of Drakes Estero’s seal population disappear to? This year, the monitoring team hoped to find clues. But with the 2021 season now wrapping up and countywide harbor seal counts below average, some of the mystery remains.
Since monitoring started in 2000, monitors have counted an average of 1,150 harbor seal pups each spring in Marin. This year, they counted only 970. 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!
This summer’s harbor seal molt season also yielded lower than average counts at most sites. At both Double Point and Drakes Estero, monitors’ counts were far below average. 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.
It’s possible that the growth of the Bolinas Lagoon harbor seal colony is from seals leaving Drakes Estero and Double Point. But with overall numbers below average, have other seals left Marin altogether? Will seal numbers bounce back? Stay tuned for our next clues in 2022!
For more information
- San Francisco Bay Area Network Pinniped Monitoring webpage
- Pacific Coast Science & Learning Center Harbor Seals webpage
- Contact Marine Ecologist Sarah Codde