Are Bay Harbor Seals Facing a New Chemical Health Threat?

Between globs of oil, six-pack rings, used condoms, and discarded sippy cups, harbor seals have plenty of hazards to dodge in San Francisco Bay. But some potential threats to their health may be more insidious. An “emerging contaminant” found circulating in blood samples from harbor seals is perfluorooctane sulfanate (PFOS), a persistent compound used in Scotchgard, fire extinguisher foam, and other stain-resistant and water-repellent coatings. (In 2002, Scotchgard’s manufacturer 3M voluntarily withdrew its PFOS products, at the same time saying there was no health risk.)

PFOS has been detected in the marine environment worldwide, but preliminary work done by the Marine Mammal Center’s marine biologist Denise Greig, in collaboration with the San Francisco Estuary Institute, suggests that the chemical’s levels in seals from the San Francisco Bay may be two to three times higher than levels reported in seals from the Baltic Sea or Norwegian Arctic.

Greig plans to follow up on these results with another year of sampling that will focus on the health of the youngest seals. “I’m trying to investigate whether first-year survival is affected by disease or contaminant levels, but I don’t have a big enough sample size yet,” she says. “My big push right now is to monitor the tagged animals we’ve already sampled. We have 35 juvenile harbor seals out there with tags, including some of our rehabilitated animals, and we really want to know what kind of body condition they are in.”

Seals have lived in the bay for thousands of years, says Greig. They “haul out” (rest), give birth to live young, and feed in the bay. “This is their place,” says Greig. But that also means that they are exposed to whatever runs off into the storm drain system—or doesn’t get treated at sewage treatment plans—and ends up in the bay.

“Over the years, we’ve seen animals strand and come into rehabilitation with a variety of illnesses and injuries,” says Greig. “So we are trying to understand whether some of these diseases that we see in stranded animals, or have been detected in harbor seals in other locations, are likely to be having an impact on the wild population. We’re looking at contaminants and immunity and exposure to a variety of pathogens—Giardia, Leptospira, Toxoplasma, influenza—and assembling a health profile. Then we’re investigating whether these factors have an impact on survival.”

Greig says other environmental stresses that are not part of her current study may also play a part in the overall health and reproduction of harbor seals: habitat degradation, human-caused disturbances, and changes in prey availability. “The increasing coastal population puts pressure on the marine environment: everything from plastics and refuse to sewage and boat traffic and noise,” she explains. Correlating seals’ health with the combined effects of so many stresses is tricky. “It’s a little like the studies that are trying to understand why there are high rates of breast cancer in women in the Bay Area. There are all these factors to tease out,” she says.

Harbor seals haul out at about a dozen sites around the bay; their favorite spots include Mowry Slough in the south bay, Yerba Buena Island, and Castro Rocks by the Richmond-San Rafael Bridge. Mowry Slough is most popular for pupping, followed by Castro Rocks.

Fortunately, the harbor seals did not fare as badly as birds during the Cosco Busan spill. Observers noticed seals with oil and tar blotches on their bodies, says Greig, but they seemed healthy, and no oil-covered harbor seals stranded. Greig explains that oil on a harbor seal does not impact its thermoregulatory capability the way it does with fur seals or sea otters, which depend on their fur for warmth. Since harbor seals do not groom themselves the way fur seals and sea otters do, they are not as likely to ingest oil. However, she says that oil in the bay still puts them at risk of inhaling fumes or ingesting oil-soaked prey.

People who spot a tagged or stranded seal should report it to the Mammal Center, says Greig, adding that reports from the public are very helpful to her study. However, she asks that people not pick up stranded pups or adults and instead call (415) 289-7350 or report sightings to

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