When officials in Seattle spent millions of dollars restoring the creeks along Puget Sound — tending to the vegetation, making the stream beds less muddy, building better homes for fish — they were thrilled to see coho salmon reappear.

But when it rained, more than half, sometimes all, of the coho in a creek would suffer a sudden death.

These die-offs — reported from Northern California to British Columbia — have stumped biologists and toxicologists for decades. Numerous tests ruled out pesticides, disease and other possible causes, such as hot temperatures and low dissolved oxygen.

After 20 years of investigation, researchers in Washington, San Francisco and Los Angeles say they have found the culprit: a very poisonous yet little-known chemical related to a preservative used in car tires.

"We pretty much figured out that anywhere there's a road and people are driving their car, little bits of tire end up coming off your tire and end up in the stormwater that flows off that road," said Ed Kolodziej, an environmental engineer and chemist at the University of Washington, Tacoma/Seattle, whose lab led a study that was published in the journal Science. "We were able to get all the way down to this one highly toxic chemical — something that kills large fish quickly and we think is probably found on every single busy road in the world."

Coho salmon, also known as silver salmon, are prized among fishermen and an important indicator species. Their range has historically stretched from the creeks nestled in the redwood forests near Santa Cruz all the way north to the waters of Alaska. The few coho populations that still exist in California are either endangered or threatened.

Peter Moyle, a longtime salmon expert, said, "The challenge when you talk about declines of really sensitive fish like coho salmon, is that there are so many things that are affecting them simultaneously. … That's why it's so interesting that in these Puget Sound streams, they found this one chemical that seems to be the smoking gun."

The smoking gun turned out to be related to a chemical called 6PPD, which is essentially a preservative to keep tires from breaking down too quickly. When 6PPD hits the road and reacts with ozone gas, the chemical transforms into multiple new chemicals, including a compound known as 6PPD-quinone.

It's unclear how exactly this chemical kills coho salmon, but it may be doing something to the lining of the salmon's vascular system, said Jen McIntyre, an aquatic ecotoxicologist.

Once coho are exposed to 6PPD-quinone, they begin to breathe erratically, almost like they're gasping for air. They lose equilibrium and start spiraling in circles, unable to stay upright in the water. Eventually they drop to the bottom, where they stop moving and die.