Point Reyes National SeashorePage 165 of 259
Marin harbor seals in 2021: fewer pups, record-low molt count, a record at Bolinas Lagoon
In 2021, monitors in Marin County, California, counted only 921 harbor seal pups and recorded the lowest molt-season total on record, yet one site, Bolinas Lagoon, set records. The National Park Service (NPS) shares preliminary results from its harbor seal monitoring at Golden Gate National Recreation Area and Point Reyes National Seashore.
Harbor seals on a sand bar at low tide. Though monitors recorded lower than usual molting numbers at Double Point and Drakes Estero, they counted a record number at Bolinas Lagoon in 2021. Photo: NPS.
Pupping season
Since monitoring began in 2000, monitors have counted an average of around 1,100 harbor seal pups each spring at the main Marin County sites. In 2021 they counted 921. Most of the decline came from Drakes Estero, usually one of Marin's largest pupping sites, with 350 pups in an average year. This year monitors saw more coyote activity there and counted only 230 pups, one of the lowest counts on record along with the previous year's. Harbor seals do not like coyotes, especially around vulnerable pups, and coyote activity is a likely cause of recent pup declines at Double Point too. The good news is more pups at Bolinas Lagoon in recent years: in 2021 it had the largest pup count ever recorded there, 270.
Molting season
Monitors also count seals in the summer molt, when all age groups come ashore to shed their fur, which gives biologists a good estimate of Marin's total harbor seal population. The 2021 season had the lowest total count on record: 2,400 seals at the main sites against an average of 3,673. The biggest declines were at Double Point and Drakes Estero, each used by approximately 1,000 seals in an average molt season. In 2021 monitors counted only 250 at Double Point and 600 at Drakes Estero. Coyote activity may explain part of this too. Counts at Double Point have been dropping since 2016, but this was the first year low counts were confirmed at Drakes Estero, since there were no molt counts the year before. Bolinas Lagoon was again the shining star: 620 molting seals against an average of 480, making it, for the first time, the top site for harbor seals in Marin County.
| Season | 2021 result | Average or comparison |
|---|---|---|
| Pups, all main sites | 921 | around 1,100 a year since 2000 |
| Pups, Drakes Estero | 230 | 350 in an average year |
| Pups, Bolinas Lagoon | 270, the largest ever there | not given |
| Molt, all main sites | 2,400, the lowest on record | 3,673 |
| Molt, Double Point | 250 | about 1,000 |
| Molt, Drakes Estero | 600 | about 1,000 |
| Molt, Bolinas Lagoon | 620 | 480 |
Preliminary data
Maximum harbor seal counts by site in 2021 during the breeding season (adults and pups) and molt season (all seals). Chart from NPS.
Maximum harbor seal pup counts at Marin County sites, 2000-2021. The solid line is the 2000-2015 baseline mean (1,101) and the dashed lines are one standard deviation from it. Not all sites were surveyed in 2020. Chart from NPS.
Maximum harbor seal molt-season counts at Marin County sites, 2000-2021. The solid line is the 2000-2015 baseline mean (3,673) and the dashed lines are one standard deviation from it. Not all sites were surveyed in 2020. Chart from NPS.
NPS notes that these are preliminary data, may change, and are not for use as official numbers, and asks users to wait for its published reports before publishing them. Weekly maximum counts by site, from late February through July, are charted in the original.
Sources
- National Park Service, 2021 Harbor Seal Monitoring Updates; rewritten in hubnx's own words.