• Highly adaptable, Coho can be found in rivers and streams across North America. They generally weigh from 8 to 12 lbs and run from 18 to 24 inches in length. (Photo: Wild Salmon Center)

“It rained and they all died.”

A rubber preservative commonly used in car tires is washing off roads and into Washington streams, killing Coho salmon. There is indication that the same is happening north of the border, in B.C.

In January, scientists at Washington State University announced they’d identified the chemical responsible for the mass deaths of Coho salmon returning to streams to spawn as 6PPD. This was a mystery 20 years in the making, and Jennifer McIntyre, assistant professor of aquatic toxicology at Washington State, has been there for most of it. 

“About 20 years ago a lot of effort was being put into stream restoration in our more developed areas in Puget Sound,” says McIntyre. 

In the fall, the scientists went back to see how their restored site was doing. Sure enough, there were adult Coho using the system. It was a success and they were excited. Then it rained.

“These gorgeous, full-size adult salmon were acting strangely, rolling over and flowing downstream … within a few hours, they found those fish dead,” says McIntyre.

Dead salmon in the fall is nothing unusual. After spawning, adult salmon die. But the manner of death was so strange that it prompted scientists to look closer. What they found was worrying. Many of the fish hadn’t spawned. In an already declining population, fish dying before they can reproduce is a potential death knoll.

McIntyre and her colleagues spent the following years searching for further evidence. They focused their search in the streams and waterways of the complex estuarine system that makes up Puget Sound — an inlet of the Salish Sea that straddles the border of Washington and B.C. They found more instances of the acute mortality phenomenon, often occurring in alarmingly high percentages. 

After ruling out factors like disease or temperature, the evidence began to point towards stormwater runoff. But they still didn’t know exactly what in the water was doing the damage.

“We tried to make our own stormwater mixture. How hard could it be? We knew there are metals and hydrocarbons in there,” says McIntyre. “We made up a mixture and tested it on the fish and nothing. Couldn't kill them, couldn't make them sick, even at 10 times higher concentration of metals. We were missing something.”

It was important McIntyre and her team found the answer — and not just to researchers in the U.S. Coho salmon also spawn north of the border in B.C., which shares the Salish Sea with the maze-like waterways of Washington’s Puget Sound. Problems in Coho salmon are seen by conservationists in Canada as a potential indicator of wider issues.

“Salmon really are the canary in the coal mine here in this part of the world,” says Michael Meneer, CEO and president of the Pacific Salmon Foundation. “They're facing the impacts of climate change most directly, constantly and acutely.”

Could Coho salmon in Canadian streams be facing the same gruesome, untimely deaths as their American relatives?

McIntyre is Canadian and is in touch with researchers in her homeland, like Paul Cipywnyk,  chair of the Byrne Creek Streamkeeper Society in Burnaby. Cipywnyk has been finding dead Coho salmon in similar circumstances to that seen in B.C. — and not just the adults.

“When we were releasing Coho smolts, they were also dying,” says Cipywnyk. “There would be hundreds of these fish floating belly up on the surface a day or two after they'd been released.”

More than half the dead salmon Cipywnyk found last spawning season had not spawned. 

“In a little creek like ours where we only get 40 or 50 Coho salmon back each year, that’s really depressing,” says Cipywnyk.

McIntyre’s team decided to try a new strategy. They’d observed that it wasn’t just any urban stormwater killing the fish, but stormwater in areas with a high road density. The team collected samples of roadway runoff. This time, they got it right. When exposed to the samples, the Coho salmon reacted the same way they’d done in streams after a rainfall. The scientists were close to finding the culprit, but still hadn’t identified the killer chemical. They decided to bring in Edward Kolodziej, environmental chemist and associate professor at the University of Washington, to try and nail this chemical down.

Kolodziej and his team began analyzing the chemicals and were surprised to find just how chemically complex road runoff is. One of Kolodziej’s postdoctoral students, Zhenyu Tian, made the breakthrough. He identified the killer chemical as 6PPD, a common tire rubber preservative. Tires need antioxidant chemicals to protect them from ground-level ozone; without them, the rubber cracks and breaks down.

“6PPD is designed to react with ozone,” says Kolodziej. “We figured out that the compound that's formed after it reacts with ozone is toxic to Coho salmon.”

The teams on both sides of the border are exploring two options: finding environmentally benign alternatives to 6PPD and other chemicals; and building green infrastructure that can treat road runoff before it gets into streams. 

“We’ve got to reorient our industries to help maintain and recover many of these salmon stocks,” says Meneer. “We want to figure out how we in Canada can be a part of the research and also to strongly encourage the industry to get to a solution as soon as possible.”

McIntyre recognizes that more research is needed in both these areas, and is continuing her work. The journey to save the Coho isn’t over yet.

“We're starting to craft studies to look for those safer alternatives and make recommendations,” says McIntyre. “It’s been a 20-year road. In a way, we're at the end of that road. But we're at the beginning of another.”