Study uncovers “smoking gun” in mass salmon die-offs

Coho salmon are an anadromous species, meaning they spend part of their lives in freshwater and most of it in saltwater. Born in freshwater streams, after a year or two they migrate to the ocean. Later, they’ll return, some to the very stream they hatched in, to spawn the next generation. Unfortunately, it’s been known for decades that after heavy rains, spawning coho salmon in urban rivers and streams die—in some cases as many as 90 percent of the fish in the waterway.

Working on the assumption that the fatalities were linked to something washing off of roadways into the water, a team of researchers has now discovered the culprit: an antioxidant chemical called 6PPD that is used to make tires last longer.

“We had this soup of 2,000 chemicals that took five years of a meticulous process of elimination [to identify 6PPD] … as the primary causal toxicant,” says Ed Kolodziej, an associate professor at the University of Washington. Kolodziej and his colleagues, including Zhenyu Tian, Jenifer McIntyre, and researchers in the chemistry department at the University of Toronto’s Scarborough campus, recently published their report in the journal Science.

With more than three billion tires produced annually around the world, all containing 6PPD, the researchers concluded that the chemical is “globally ubiquitous.” The chemical is intended to protect tires from the damaging effects of ozone. But when it reacts to ozone, it transforms into something called 6PPD-quinone, which the researchers then synthesized to determine that it was “the smoking gun.”

While their study focussed specifically on coho, the report does add that, “it is unlikely that coho salmon are uniquely sensitive, and the toxicology of 6PPD transformation products in other aquatic species should be assessed.” Birds and mammals who eat the dead fish—and consume the tainted water—could also be impacted.

Now that the cause is known, one option would be to treat runoff before it reaches waterways. But with coho spawning in more than 750 rivers and streams in B.C. and the Yukon alone, according to Fisheries and Oceans Canada, that’s not realistically cost effective.

The researchers feel a more plausible solution would be for tire manufacturers to develop a “salmon-safe” alternative. In response to the report, the U.S. Tire Manufacturers Association issued a press release that read, in part, “The tire industry uses 6PPD because it helps tires resist degradation and cracking, which is vital for passenger safety. 6PPD has been studied, but not enough is yet known about the newly discovered degradation product, 6PPD-quinone. We are committed to collaborating with researchers at the University of Washington and other scientists to better understand this product, fill knowledge gaps and determine next steps.”

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