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A steep decline in Puget Sound-area herring, a critical food source for larger fish, marine mammals and sea birds, has scientists mystified.

Not only are adult herring dying earlier than normal, but some fear that a stock that used to be one of the largest in Washington's inland marine waters could go extinct.

The 30-year decline in the small silvery fish has far-reaching implications, said Jim West, a Washington State Fish and Wildlife Department research scientist who found high concentrations of polychlorinated biphenyls and other toxic substances in herring in south Puget Sound.

“Virtually any predator worth its salt in Puget Sound is going to be eating herring,” Mr. West said.

Herring are a dietary stable for chinook salmon, cod and halibut, and also are eaten by porpoises, seals, sea lions and orcas. Freshly spawned herring eggs once drew swarms of marine birds, especially diving ducks called surf scoters that fly north to nest in the Canadian interior.

“It's thought to be important for them to feed on herring spawn while they're here, because there's not a lot of food up there for them,” said Joseph Evenson, a state Fish and Wildlife Department biologist. “They're basically working off their fat reserves.”

The decline in herring may explain the disappearance of thousands of scoters from parts of the inland waters, said Mr. Evenson and his boss, Dave Nysewander, the state's marine bird and mammal project leader.

One of the most severe herring declines is between Bellingham and the Canadian border, an area that once accounted for about one-third of the state's total stocks.

The Cherry Point stocks plummeted from about 9,000 tonnes in 1994 to 733 tonnes in 2000, then rebounded to 1,461 tonnes in 2003, but remain in danger of going extinct, said Duane Fagergren, a fish biologist with the state's Puget Sound Action Team.

In January, 2004, the Center for Biological Diversity and other activists sought protection for Cherry Point herring under the Endangered Species Act. A decision from the National Oceanic and Atmospheric Administration is expected in mid-May.

Much of the mystery to scientists is in the high percentage of herring that die before reaching the age of peak fertility.

Herring in the inland waters typically used to live as long as five to eight years, but now “it's rare to see one over four,” said Paul Hershberger, a U.S. Geological Survey fish pathologist who has been studying disease in herring since the mid-1990s.

In the 1970s, about 20 per cent of the sound's herring population died of natural causes annually, but now it's 67 per cent to 84 per cent, Hershberger said.

Environmental activists blame pollution and other habitat degradation, but state marine fish manager Greg Bargmann said disease, lack of food and predators also may be involved.

Mr. Hershberger cited the presence of a ichthyophonus, a parasite that was not noticed in the inland waters 30 years ago but is now common and infects the heart and other organs, and a virus that causes viral hemorrhagic septicemia, which infects mostly young herring.