2024 Harbor Seal Monitoring Updates

San Francisco Bay Area Inventory & Monitoring Network

A harbor seal mom and pup rest on the shoreline along Tomales Point in Point Reyes National Seashore. National Park Service and Marin County Parks staff and volunteers counted 682 pups in 2024, well below the long-term average of 1,052 pups.

NPS / Dan Russell

Breeding Season Summary

Over the 24 years that the National Park Service has been monitoring harbor seals in Marin County, pup counts have been fairly stable. We’ve sometimes recorded low counts during El Niño years. However, this year marks the lowest pup count we’ve ever recorded. National Park Service and Marin County Parks staff and volunteers counted 682 pups in 2024, well below the long-term average of 1,052 pups. This follows a few years of lower pup counts indicating a decline was starting. Drakes Estero, Double Point, and Tomales Point experienced the largest declines compared to their long-term averages. Tomales Bay, Bolinas Lagoon, Duxbury Reef, and Point Bonita were either similar to their respective long-term averages or only slightly below average.

There are many factors that can affect successful pup production, so it is difficult to know why this year was especially low. The strong El Niño in 2023 and 2024 is one possible contributing factor. The warm water during El Niño years can have cascading effects such as reducing harbor seal prey and driving them farther offshore, making it harder for seals to forage. An increase in coyotes at Drakes Estero and Double Point over the past few years could be another contributing factor. Coyote presence could be causing seals to move to other areas that we don't survey. In addition, harbor seal monitors have witnessed coyote predation of pups.

Molt Season Summary

Every year harbor seals of all age classes (except pups) haul out in the summer months to molt (shed) their fur and grow a new layer. We’ve recorded a decline in seals in Marin County during the molt season since at least 2021, and this year was no different. Just like during the breeding season, the molt season count of all seals was the lowest on record. Only 1,788 seals hauled out to molt during the summer of 2024. That's about a 50 percent decline from the long-term average of 3,366 molting seals. Our counts at Tomales Bay, Point Bonita, and Bolinas Lagoon were about average or only slightly below average. But at Drakes Estero, Double Point, Tomales Point, and Duxbury Reef we saw large declines compared to their respective long-term averages. At Double Point and Drakes Estero, once the largest National Park Service harbor seal sites in Marin, we found 80 percent and 60 percent declines from their long-term averages, respectively.

It is still a mystery why the number of harbor seals hauled out during the molt season has seen a much larger decline compared to the breeding season. Why this decline has been continuing for so many years also remains a mystery.

Preliminary Data

2024 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, 2000-2024

Maximum harbor seal pup counts for 2000-2024 across all Marin County sites. The black solid line on the graph represents the mean, or average, of the maximum pup counts from 2000-2023 (mean = 1,052). The dashed lines represent one standard deviation from the mean, or the normal range of variation. Not all sites were surveyed in 2020.

Maximum Harbor Seal Pup Counts by Site, 2000-2024

Maximum harbor seal pup counts for 2000-2024 at Point Bonita, Bolinas Lagoon, Duxbury Reef, Double Point, Drakes Estero, Tomales Bay, and Tomales Point. Drakes Estero and Double Point 2024 counts are much lower than where they used to be, though Double Point counts have been lower for longer. The 2024 Bolinas Lagoon count is the lowest at that site since 2009. Not all sites were surveyed in 2020.

Maximum Harbor Seal Molt Counts, 2000-2024

Maximum harbor seal molt season counts for 2000-2024 across all Marin County sites. The black solid line on the graph represents the mean, or average, of the maximum seal counts from 2000-2023 (mean = 3,366). Dashed lines represent one standard deviation from the mean, or the normal range of variation. Not all sites were surveyed in 2020.

Maximum Harbor Seal Molt Counts by Site, 2000-2024

Maximum harbor seal molt season counts for 2000-2024 at Point Bonita, Bolinas Lagoon, Duxbury Reef, Double Point, Drakes Estero, Tomales Bay, and Tomales Point. The most apparent changes are at Double Point, where counts plunged starting in 2016, and at Drakes Estero, where the last three years have had record low counts. Not all sites were surveyed in 2020.

The Details: 2024 Weekly Maximum Harbor Seal Counts By Site

Maximum weekly harbor seal counts by site and survey week.

For More Information

Webpages

San Francisco Bay Area Network - Pinniped Monitoring

Pacific Coast Science and Learning Center - Harbor Seals

Data

Pinniped Monitoring for the San Francisco Bay Area Network: 1997-2023 - Data Package

Contact

Sarah Codde

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