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Bay Seals on the Ebb

The number of seals inhabiting coastal waters grew steadily in the 1980s and in areas like Point Reyes, it more than doubled. But the seal population that has long lived, fished and tended its pups in San Francisco Bay didn't grow at all - puzzling Dianne Kopec and Jim Harvey of Moss Landing Marine Laboratories.

The sustained low - 425 seals Baywide in 1989 - inspired Kopec and Harvey to begin the first comprehensive research study on these marine mammals conducted in the Bay in over 15 years. Their study, funded in part by the S.F. Estuary Project, is also the first to explore the top of the estuarine food chain for impacts from human activities.

The study began with the capture of 70 seals at haul-out spots in the North, Central and South bays. Researchers approached in a fast boat and ringed offshore waters with a large seine net. "The seals' natural defense is to go into the water but not swim away," says Kopec. "That gave us time to encircle them with the net."

After hauling the net full of seals ashore, they weighed, measured and tagged the marine mammals, attached radios equipped to track the creatures from air and land and took blood samples and fecal swabs. Most seals found themselves back in the Bay within 15 minutes.

Laboratories checking for PCBs, DDT and other pollutants in the seals' blood discovered especially high levels of PCBs - 51.8 parts per billion total wet weight. Levels one fourth as high caused serious reproductive problems in a study of captive seals in Europe's North Sea. Mercury and selenium also turned up at elevated levels (O.9 ppm selenium), and Kopec thinks the latter may have something to do with the scarlet hue of many seal coats. Up to 40 percent of the Bay's seals have "red coat" - compared to only 5 percent in the overall population of the Northern Hemisphere.

The seals could be encountering these pollutants in their food. Scat analysis revealed that they mostly eat yellowfin gobi, staghorn sculpin, plainfin midshipmen and white croaker. There's little data on local contaminant levels in these fish because none are commercially important, according to Harvey. But a Los Angeles white croaker study revealed high DDT concentrations. And studies on contaminants in fish caught by Bay Area subsistence fishers - who favor croaker - are planned for this year. Since the seals spend virtually all their time in the Bay, Harvey says they're more likely to be exposed to pollutants from urban sources. Radio tracking showed 87 percent never ventured out of the Golden Gate.

During the 1991 breeding season, Kopec and Harvey documented an alarming dive in the South Bay seal population. The number of seals hauling out at Mowry and Newark Sloughs dropped by over 50 percent, from a maximum of 340 to a minimum of 150. "[The drop] was not compensated for by increases in other areas," says Kopec. Low seal counts have continued through 1993.

For their final report, due out this January, Kopec and Harvey are now busy analyzing their results and considering possible reasons for the seals' demise. Clearly, the PCBs and other pollutants in their blood could be a major factor. And Kopec thinks declines in Bay fish stocks after the long years of drought might be another.

"The fact that trace elements have accumulated to toxic levels, levels we know are harmful to harbor seals and mammals in general, is cause for grave concern," she says.

Contact: Dianne Kopec (415)728-5816

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