For more than three decades, scientists have been rounding up harbor seals from Puget Sound, drawing their blood, sampling bits of blubber and running other tests to check for signs of disease.

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TACOMA, Wash. – For more than three decades, scientists have been rounding up harbor seals from Puget Sound, drawing their blood, sampling bits of blubber and running other tests to check for signs of disease.

Their goal: to track how harmful concentrations of toxic chemicals are affecting the animals’ health.

For researchers trying to pin down the scope and effects of toxic contamination, harbor seals perform a canary-in-a-coal-mine service, said Peter Ross, a Canadian marine mammal toxicologist, who collaborates with state fish and wildlife scientists in Washington.

“They are the laboratory animal of the ocean around here,” Ross said.

The sleek, spotted critters — the state’s largest population of marine mammals — have helped biologists create Puget Sound’s longest scientific record of pollution in aquatic animals.

Over the years, scientists have found harmful concentrations of toxins like PCB, or polychlorinated biphenyl, an industrial compound that can cause cancer, in the blubber of seals. PCBs have been shown to affect seals’ metabolism and make them vulnerable to illness, scientists say.

Some researchers recently found a link between South Sound harbor seal contaminants and impaired immune system response.

In addition to making harbor seals more vulnerable to disease, chemical contaminants appear to be affecting the animals’ thyroid hormones, Ross said.

The pollution problem isn’t limited to chemicals. Disease-causing bacteria — likely waste runoff from people and livestock — also affect the seals. “The combination of chemical and biological pollution means South Sound seals are hit with a double whammy,” Ross said.

Harbor seals don’t migrate, and they are reclusive. They shy away from people and can be fierce, making it difficult to examine them up-close. So every fall, biologists go out in fast boats and use strong nets to catch them. Like cowboys rounding up cattle, researchers haul the seals to shore, hold them still, then brand them.

“By branding animals we get to follow individuals throughout their lifetime. It’s a mark they don’t lose,” said Harriet Huber, a biologist who works for the National Marine Mammal Laboratory in Seattle and has studied area harbor seals since 1990.

Huber’s research centers on about 600 permanently marked seals whose sides are branded with 3-inch digits, large enough for a biologist to identify with a long-distance spotting scope.

“The more information you have about a wild population, the better able you are to protect it,” Huber said.

The federal Marine Mammal Protection Act has safeguarded seals since 1972. It requires federal officials to manage marine mammals based on population work such as Huber’s. Scientists in Canada and Alaska used to base their analyses on animals that were killed, then dissected.

From June through November, state wildlife biologist Dyanna Lambourn records each sighting of a branded seal in Huber’s database, noting births, deaths and other encounters.

Lambourn ran a recent roundup earlier this month, taking the lead on blood draws, blubber punches and mucous-membrane swabs. Researchers worked in teams, weighing, measuring, checking teeth and tagging seals before releasing them.

While Lambourn and others marked and examined the seals, one person crouched on top to hold each still. Some yelped or growled. The big ones tried to get free.

Steve Jeffries, who coordinates a state Fish and Wildlife Department team of marine mammal biologists, has used similar capture methods since 1978. He cautioned volunteers not to put too much pressure on the animals because they might stop breathing.

Huber noted that eight out of roughly 4,000 animals captured have died.

Statewide, harbor seals number about 32,000. They haven’t always been that numerous. Because fishermen complained that seals took too many fish, the state once paid a bounty to reduce their numbers.

The statewide seal population numbered about 5,000 in 1970, Jeffries said. It began to recover after passage of the Marine Mammal Protection Act, which banned killing.

Concentrations of contaminants in South Sound seals have declined since the 1970s, but scientists say the levels remain high enough to cause harm.

Also, newer chemicals, like flame retardants, have begun to show up in harbor seals.

“The big concern is that not only are there still legacy contaminants, but new ones are being created and used that are essentially unrestricted, but similar to ones that have been banned that we know cause problems,” Jeffries said.