So watching the species die in agony is distressing: Adult coho have been seen thrashing in shallow fresh waters, males appear disoriented as they swim, and females are often rolled on their backs, their insides still plump with tiny red eggs that will never hatch.

“Coho have not done well where a lot of human activity impacts their habitat,” said Nat Scholz, a research zoologist for the National Oceanic and Atmospheric Administration. That’s to say the least.

A recent study traced a major coho salmon die-off to contaminants from roads and automobiles — brake dust, oil, fuel, chemical fluids — that hitch a ride on storm water and flow into watersheds. The contaminants are so deadly, they kill the salmon within 24 hours.

“Our findings are . . . that contaminants in stormwater runoff from the regional transportation grid likely caused these mortality events. Further, it will be difficult, if not impossible, to reverse historical coho declines without addressing the toxic pollution dimension of freshwater habitats,” said the study, published Wednesday in the journal Ecological Applications.

This sort of point-source pollution from antiquated sewer systems is a problem across the nation, including the Chesapeake Bay region. Rain overwhelms storm drains, commingles with human waste and surface road garbage, then flushes into ponds, creeks, streams and rivers.

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In Seattle and large cities across the Pacific Northwest, those waters are stocked with salmon. The finding could be a breakthrough in a mystery that has vexed scientists for years. But it fell short of explaining another mystery: Why are coho the only one of five salmon species to be affected? Chinook, sockeye, pink and chum don’t remotely experience the same mortality.

“This is the great mystery that we are working on,” Scholz said.

The future for a species that experiences up to 40 percent mortality before spawning in Puget Sound is no mystery. “The population will crash,” said Jay Davis, an environmental toxicologist for the U.S. Fish and Wildlife Service who coordinated field research for the study.

In separate interviews Thursday, Davis and Scholz estimated that it could happen in as few as six years. “We have to act now,” Davis said.

The study provides a road map to help politicians and ecologists help salmon. Over a decade starting in 2000, the U.S. Fish and Wildlife Service teamed with NOAA’s Northwest Fisheries Science Center in Seattle, the Wildlife Fish Conservancy and area First Nations tribes to survey more than 50 sites in Puget Sound.

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They used previous data to develop maps that pinpointed where coho die-offs occur and the rates of mortality to give agencies an idea of where to restore habitats. The methods they used to determine how coho die provided a window into how to improve their living space.

In one test, they stocked untreated storm-water runoff with coho salmon from a hatchery and witnessed 100 percent mortality. In another, they filtered the water through soil “to scrub out pollution,” Scholz said, and the coho survived.

The idea to take a closer look at storm runoff's effect on coho didn’t come out of nowhere. Thousands of miles east in the Potomac and Susquehanna rivers, scientists are studying the effects of contaminated stormwater on largemouth and smallmouth male bass whose testes developed into ovaries.

In addition to tar from rooftops, Freon and oil from cars, and human bile, storm water carries pharmaceuticals packed with hormones.

A dead salmon in the Pacific Northwest is nothing strange, Davis said; it’s what happens after spawning. “The surprising part was the dead fish that were still full of eggs.” The researchers cut females open to confirm their observations, but “sometimes you can look at a vent and see an egg or two coming out.”

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“Seattle’s growing every day. Amazon is here, Microsoft is here,” Davis said. “And the water quality was so poor that it’s killing [coho]."

“If we’re going to have people living in these watersheds, we’re going to have to deal with this problem,” Scholz said. “We’re experiencing major population growth pressures. We’re literally working around the clock.”