PULLMAN, Wash. — Scientists at Washington State University’s (WSU) Puyallup Research & Extension Center have been working to find out why coho salmon in Puget Sound creeks seem to suffocate after rainstorms.
Coho, or silver salmon are born in freshwater streams in the Pacific Northwest, swimming hundreds of miles to the ocean, where they spend most of their lives. A tiny percentage make the arduous journey back upstream to spawn before dying .
In 2018, the die-offs were linked to bits of car tires shed by friction and washed into the stormwater runoff. In 2020, researchers zeroed in on one particular chemical culprit, a tire preservative known as 6PPD.
Now, new research led by Stephanie I. Blair, a Ph.D. student at WSU, outlines the biological mechanism for how that toxin kills the fish, laying the groundwork for tests to find an alternative to 6PPD.
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Blair, is the lead author of the report published in the journal Environmental Science & Technology, she began working in the lab in 2018, focusing on trying to understand the cardiovascular response behind the die-offs.
“Prior to publication of this study nobody really knew what the event was that drove what they call ‘coho urban runoff mortality syndrome,’” said Blair, “This is the first paper that gives a clear answer as to what’s happening.”
Understanding this makes it possible to design tests for potential alternatives to 6PPD, which is in virtually every automobile tire, scientists said. The need for an alternative is growing with concerns over the environmental impact of the chemical. Studies are increasingly showing that, while coho are one of the most sensitive to 6PPD-quinone, it is also toxic for other fish and mammals, with possible effects on human health.
“We need those tools to be available so we can start screening for alternatives to 6PPD,” Blair said. “This tells us how to evaluate a potential substitute.”
In lab experiments they used fluorescent markers to demonstrate there were certain points at the blood-brain and blood-gill barriers that were “leaky”—something was crossing through the cardiovascular firewall.
When 6PPD interacts with ozone, it becomes a toxic chemical known as 6PPD-quinone. The research showed that 6PPD-quinone breaches the cellular walls that protect the brain and vascular system, known as the blood-brain barrier and the blood-gill barrier, causing the coho to suffocate.
Researchers exposed the fish to runoff collected from a state highway near Tacoma and, separately, to concentrations of 6PDD-quinone typical for a runoff event. Fish exposed to both exhibited the behaviors associated with the die-offs, and subsequent examinations showed substantial disruption of the brain-blood and gill-blood barriers.
Several coho populations are listed as threatened or endangered, which has implications for the environment, economy, politics and treaty fishing rights of Northwest tribes.
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