Unraveling the Mystery of Coho Salmon Die-Offs: The Role of Tire Chemicals in Puget Sound Ecosystems

Title: Groundbreaking Research Unveils Cause of Coho Salmon Die-Offs in Puget Sound

By Shawn Vestal, WSU News & Media Relations

For years, scientists at Washington State University’s Puyallup Research & Extension Center have been unraveling a perplexing mystery: why do coho salmon in Puget Sound creeks appear to suffocate after rainstorms, often surfacing, gasping, and swimming in circles before succumbing?

The answer, it turns out, lies in the very tires that keep our vehicles rolling. In 2018, researchers linked these tragic die-offs to tiny particles shed from car tires, washed into stormwater runoff. By 2020, they pinpointed a specific chemical culprit: a tire preservative known as 6PPD.

Now, a new study led by WSU PhD student Stephanie I. Blair has established the biological mechanism behind how this toxin kills the fish, paving the way for potential alternatives to 6PPD.

When 6PPD interacts with ozone, it transforms into a toxic compound called 6PPD-quinone. Blair, collaborating with a team from WSU and the University of Washington, demonstrated that this compound breaches the protective cellular walls of the brain and vascular system—known as the blood-brain barrier and the blood-gill barrier—leading to oxygen deprivation in the fish.

“Prior to the publication of this study, nobody really knew what drove what they call ‘coho urban runoff mortality syndrome,’” said Blair, the lead author of the paper published in Environmental Science & Technology. “This is the first paper that gives a clear answer as to what’s happening.”

Understanding this mechanism opens the door to designing tests for alternatives to 6PPD, which is present in nearly every automobile tire. The urgency for a substitute is growing, as studies increasingly show that while coho salmon are particularly sensitive to 6PPD-quinone, it is also toxic to other fish and mammals, raising concerns about potential human health impacts.

“We need those tools to be available so we can start screening for alternatives to 6PPD,” Blair emphasized. “This tells us how to evaluate a potential substitute.”

Blair is nearing the completion of her PhD program at WSU while also working for the Confederated Tribes of the Umatilla Reservation. An enrolled member of the Sault Ste. Marie Tribe of Chippewa Indians, she also goes by her Ojibwe name, Negonnekodoqua.

Co-authors of the paper include Jenifer McIntyre, an associate professor of aquatic toxicology whose lab has been at the forefront of this issue. McIntyre collaborates closely with researchers at UW and the U.S. Geological Survey Western Fisheries Research Center to understand the harmful impacts of 6PPD-quinone and work towards a replacement.

Coho salmon, or silver salmon, are an iconic species of the Northwest. Born in freshwater streams, they embark on a long journey to the ocean, returning upstream to spawn before dying. However, several coho populations are now listed as threatened or endangered, raising significant implications for the environment, economy, and treaty fishing rights of Northwest tribes.

Blair, who joined the lab in 2018, has focused on understanding the cardiovascular responses behind the die-offs. Through lab experiments on fish exposed to stormwater runoff, she and McIntyre used fluorescent markers to reveal “leaky” points at the blood-brain and blood-gill barriers, indicating that something was crossing through this vital protective barrier.

They suspected 6PPD-quinone was the cause, and their recent findings confirm it. Fish exposed to runoff collected from a state highway near Tacoma, as well as to concentrations of 6PPD-quinone typical for runoff events, exhibited the same distressing behaviors associated with the die-offs. Subsequent examinations revealed significant disruptions in the brain-blood and gill-blood barriers.

“Every single time the coho show the surfacing symptoms and the loss of equilibrium, it always has blood-gill and blood-brain barrier disruption,” Blair noted. “You will always find that. Every single time you have a sick fish from exposure to 6PPD-quinone, this is very causally linked.”

As researchers continue their work, the hope is that understanding the impact of 6PPD-quinone will lead to safer alternatives, ensuring the survival of coho salmon and the health of the ecosystems they inhabit.