First they circle. Then they gasp at the surface of the water. Soon they can’t swim. Then they die.

For decades now, scientists have known something was killing beautiful, adult coho salmon as soon as they hit Seattle’s urban waters, ready to spawn. They had escaped the orcas, the fishermen, traveled thousands of miles, only to be mysteriously killed as soon as they finally reached home.

In a breakthrough paper published in the Dec. 3 issue of Science, a team of researchers revealed the culprit behind the deaths of coho in an estimated 40% of the Puget Sound area — a killer so lethal it takes out 40 to 90% of returning coho to some urban streams before they spawn. It is a killer hidden in plain sight.


More specifically, a single chemical, 6PPD-quinone, derived from a preservative that helps tires last longer.

Through painstaking analysis and building on years of prior research, the team, including researchers from the Center for Urban Waters in Tacoma, the University of Washington and Washington State University, isolated the killer from a witch’s brew of some 2,000 chemicals in roadway runoff.

The chemical is a globally common tire rubber antioxidant. But when it does its job, interacting with ozone in the atmosphere, the chemical transforms to a substance that is highly toxic to coho.


Bound up in the rubber, this chemical taints tire-wear particles shed by tires onto roads. The tire dust is in roadway runoff that seeps, trickles and pours into water bodies, including urban streams, every time it rains. The more traffic on the road, the higher the dose.

Coho salmon, returning with the first fall rains, take the hit. They usually die within hours.

The pollutant is particularly problematic for waters near busy roads. Translation: most of central Puget Sound and its sprawl. Ironically, the millions of dollars spent to make these areas more salmon friendly and boost fish populations have created ecological traps for coho coming back to toxic waters.

Some of the scientists who published the paper were both exhilarated at the breakthrough and concerned by the findings.

“I find it incredibly sad to watch the adults when they are sick,” said co-senior author on the paper, Edward Kolodziej, an associate professor in both the UW Tacoma Division of Sciences & Mathematics and the UW Department of Civil & Environmental Engineering.

Every fall he’d make a visit to Longfellow Creek in West Seattle and mourn what he saw.


“When you see a fish in the field, and know something is happening that is not understood, you just have to take that very seriously. … It was just so evident to everyone we were working on a real problem.”

It was the cross-disciplinary approach of bringing together experts in the biology and chemistry of the problem that finally cracked the mystery. Dogged determination helped, too.

Lead author Zhenyu Tian, a research scientist at the Center for Urban Waters at UW Tacoma said there were times when he wondered if they would ever figure out just what chemical was the killer.

He finally had the idea it might be not the tire itself but something related to it — and hit on the preservative. They figured out that the preservative, 6PPD, goes through an environmental transformation that turns it into 6PPD-quinone — a coho killer.

“This is the smoking gun. You go through all the lines of evidence and it lines up.”

Tian modestly makes it sound easier than it was, Kolodziej said. “It looks so nice and tidy in the paper. They went above and beyond,” he said of Tian and Jenifer McIntyre, an assistant professor at WSU’s School of the Environment.


“If you are a scientist, you are among the people most familiar with failure; you have to be so comfortable with not succeeding,” Kolodziej said. After all, most research is a product of figuring out what doesn’t work, what doesn’t answer the question pursued, Kolodziej said. “It is slow and difficult and positive reinforcement is rare,” he said of scientific research.  “You have to trust the scientific process. You put in the work, like so many things in life, it is about putting in the work. “

It has taken decades to solve this problem, noted McIntyre, who is based in Puyallup and is among scientists who have been sleuthing out the coho killer for years.

Nat Scholz of the National Oceanic and Atmospheric Administration’s Northwest Fisheries Science Center in Seattle was the lead author in a 2011 paper with McIntyre and other authors that showed that coho pre-spawn mortality was routine in urban streams. They joined with other scientists who documented that bio-filtration through soil solves the problem, purifying the water.

Good to know, but tough at scale — given how big the problem turned out to be. Scholz and another team of co-authors led by Blake Feist at NOAA’s Northwest Science Center documented in their 2017 paper the link to traffic volume on roads and even mapped the scope of the problem across an estimated 40% of Puget Sound country. In 2018, McInyre, Scholz, Kolodziej and other scientists linked the coho deaths to tire wear particles in roadway runoff.

With 3.1 billion tires globally produced annually for more than 1.4 billion vehicles, tire-wear particles are a daunting pollution problem. The preservative 6PPD appears to be used in all tires, the scientists noted in the most recent paper, and tire-wear particles are likely present in creeks near busy roads across the world.

There also is not today a salmon-safe tire on the market, or treatment for tires to make tires salmon-safe. Any vehicle is implicated, whether gasoline-powered or electric, privately owned or transit. If it’s got a tire, and it moves on a road, it’s part of the problem.

Dan Kent is executive director of Salmon Safe, a nonprofit that certifies products made in a manner that is better for salmon. He sees a ripe target and ready market for salmon-safe tires. “We’re definitely interested, we have just been waiting for these findings.”