The Bay

High Time For Harbor Seal Pupping

April 22, 2011

With their round, spotted bodies and gentle faces, Pacific harbor seals are universal crowd-pleasers. But unlike sea lions and elephant seals, these irresistibly cute creatures shun attention. A harbor seal may flee at the slightest disturbance, and mothers have been known to abandon pups when harassed.

Still, some harbor seals forgo the relative privacy of coastal rookeries when it comes time to birth their young. Instead, they choose locations right in San Francisco Bay, where pupping season is nearing its late-April peak.

“The Bay has conditions that for thousands of years have been good for seals,” explains Denise Greig, a research biologist at the Marine Mammal Center in Sausalito. She cites marshlands and protection from sharks as qualities that make the Bay attractive for pupping.

“The Bay is abundant with food,” adds Sarah Allen, a senior scientist with the National Park Service (NPS). “Anchovies, flounder, lamprey, and salmon are all important prey items in the Bay.”

Pupping takes place at nearly a dozen haul-out sites in the Bay, from Yerba Buena Island to Newark Slough. Most spots produce only a handful of pups each year, but two have consistently outnumbered the others: Mowry Slough, in the Don Edwards Wildlife Refuge, and Castro Rocks, next to the Richmond-San Rafael Bridge. Last year, NPS surveys found 68 pups between the two locations.

That’s nothing compared to certain coastal sites, like Marin County’s Drakes Estero and Double Point. The same NPS surveys tallied more than 200 pups at each location.

Coastal populations also received a boost from the 1972 Marine Mammal Protection Act, which outlawed disturbing marine mammals. The law improved Bay counts as well, but not to the same extent. Allen credits the difference partly to development along the Bay shoreline, as large-scale projects can distress seals into abandoning a haul out site. Such instances have occurred near the Bay Bridge, as well as in similar Bay environments. “There was a site that was abandoned in the Richardson Bay in the late 70’s and 80’s due to the development and disturbance there,” recalls Allen.

But with proper precautions, construction doesn’t have to cause site abandonment. Castro Rocks continued to draw harbor seals both during and after the 2001-2005 seismic retrofit of the Richmond-San Rafael Bridge. “Caltrans took extraordinary measures to protect that colony,” says Allen.

Of course, big construction work in the Bay is relatively rare compared to the smaller daily disturbances of people in boats, kayaks, and on foot along the shore. For that, education may be the best prevention. “Within SF Bay, kayaks are a common disturbance,” explains Allen, “because seals use remote areas to breed and these are areas where people are interested in kayaking. We try to get the word out with those communities that would be in areas where pupping occurs.”

The Marine Mammal Center performs similar functions, in addition to operating a veterinary research hospital. “We’re primarily trying to reach out to the public and explain what to do and what not to do,” says Bill Van Bonn, the center’s director of veterinary science. That’s particularly important in the case of harbor seals, because of their extreme sensitivity.

“You’re unlikely to disturb a sea lion by walking down Pier 39 and looking closely at them,” says Dr. Greig. “But that’s not the same case with harbor seals. They’re more prone to disturbances because of their tendencies and natural history.”

That natural history includes brushes with land predators, a danger other pinnipeds haven’t had to face. “Harbor seals came down from the arctic and had to deal with things on land like polar bears,” explains Greig. Adds Van Boon, “They learned the ‘stranger danger’ message a lot more quickly than the elephant seals because there were a lot more strangers around.”

But skittishness originally developed for survival can have devastating consequences if triggered accidentally. A mother harbor seal doesn’t spend every moment of the thirty-day nursing period with her pup. “She wanders a little bit; she goes into the water,” explains Dr. Van Bonn. “They’re out there in low tide, and somebody might walk by with their dog.” If that happens often enough, a mother might abandon her pup.

That fate accounts for a large number of the roughly 100 pups brought into the center each year. “Most of the pups we see, the primary problem is that they’re malnourished,” says Van Bonn, a condition that often indicates abandonment.

The Marine Mammal Protection Act’s guidelines say not to come within 300 feet of harbor seals, so few opportunities exist for the public to observe pupping season. Greig suggests that interested parties check out the Fitzgerald Marine Reserve, in Moss Beach, or the mouth of the Russian River, in Sonoma County, where a cliff provides safely remote viewing.

Perhaps the best way to celebrate pupping season is to give harbor seals the distance they need to produce a new generation.

About the Author

Erica Reder is a native San Franciscan. In addition to covering the environment beat at SF Public Press, she reports for Bay Nature Magazine and KPFA Radio. She holds a B.A. in history from Yale University. This story was originally published at