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Researchers Reveal Why Seattle Salmon Bite the Dust After Rainstorms

A chemical found in car tire debris washes off roads into waterways, killing coho salmon returning to spawn

Coho salmon returning from its years at sea to spawn, seen near the Suquamish Tribe's Grovers Creek Hatchery. (K. King / USFWS)
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The return of coho salmon to the rivers and creeks around Seattle was a conservation triumph—the hard-won result of millions of dollars of habitat restoration work. But around 20 years ago, when the muscular, determined bodies of cohos started returning to these urban waterways to spawn, a mysterious phenomenon cast a dark pall over their homecoming.

After it rained, the iconic fish started dying in droves, but nobody knew why. In the streams hit by the unknown scourge, 40 to 90 percent of the salmon went belly up.

“To be running into these sick fish was fairly astonishing,” Jenifer McIntyre, a toxicologist at Washington State University, tells Catrin Einhorn of the New York Times. “In those early years, we debated intensely, what could be the cause of this?”

Now, new research published in the journal Science has finally nailed the culprit: tires.

The researchers identified a chemical called 6PPD, a common rubber additive aimed at making car tires last longer, that transforms into deadly 6PPD-quinone when unleashed in nature, reports Lynda V. Maps of the Seattle Times.

Sick coho salmon
A sick coho salmon that was exposed to roadway runoff. (K. King / USFWS)

Painstaking analysis allowed the researchers to pluck the poisonous chemical from a lineup of pesticides, pathogens, heavy metals and other potential explanations like warming waters or low oxygen levels.

“We pretty much figured out that anywhere there’s a road and people are driving their car, little bits of tire end up coming off your tire and end up in the stormwater that flows off that road,” Ed Kolodziej, a chemist at the University of Washington whose lab led the study, tells Rosanna Xia of the Los Angeles Times. “We were able to get all the way down to this one highly toxic chemical—something that kills large fish quickly and we think is probably found on every single busy road in the world.”

Per the New York Times, the team’s early attempts to find the coho’s killer came up empty, but McIntyre, one of the paper’s lead authors, decided to take a closer look at the components of the slurry washing off of roadways.

In 2012, heavy rains provided an opportunity for the researchers to collect buckets of storm runoff, fresh from the pavement of the parking lot of the Northwest Fisheries Science Center. When they poured that water on baby salmon, the results were much like what had been observed in streams impacted by the die-offs: the salmon started circling, gasping at the surface of the water and then dying.

“What is it in that mixture?” McIntyre tells the New York Times she wondered at the time. “This is just water that’s on the road, it’s what we tramp through in our rain boots.” She supposed the deadly ingredient must be something they weren’t looking for in their initial analyses.

Per the Los Angeles Times, once the researchers pegged stormwater as containing the killer chemical, they were able to discern that the creeks with the worst die-offs were near busy roadways. Using a scientific instrument called a high-resolution mass spectrometer, the researchers conducted a chemical comparison of stormwater runoff from highways and the water from streams with dying coho. Both the waters from the road and the stream featured chemicals related to car tires.

Researchers wading through Longfellow Creek
Researchers Jenifer McIntyre (left), of Washington State University; Edward Kolodziej (center), of the University of Washington; and Zhenyu Tian (right), of the Unversity of Washington wading through Longfellow Creek near Seattle. (Mark Stone / University of Washington)

To hone in on which compound in car tires might be poisoning the fish, the researchers essentially made a car tire smoothie. A few more poor salmon sacrificed their lives to confirm that this mixture was indeed deadly for cohos, and then the team set about narrowing the field of roughly 2,000 chemicals present in the mix, according to the New York Times.

By testing batch after batch of the various chemicals in the tire smoothie, the researchers narrowed it down from 2,000 to 200 compounds. But even then, the scientists’ quarry eluded them.

Finally, Zhenyu Tian, a research scientist at the University of Washington and the paper’s first author, had the idea that the killer might be a transformed version of one of the chemicals in the shredded tire solution. That approach finally cracked the case and delivered 6PPD, which, when it reacts with ozone gas, becomes the toxic 6PPD-quinone, per the Los Angeles Times.

“It's a brilliant piece of work,” Miriam Diamond, an environmental chemist at the University of Toronto, tells Erik Stokstad of Science. “They've done a tremendous job at sleuthing out a very challenging problem.”

More studies are needed to determine how sensitive other salmon species are to the chemical, not to mention whether it’s harmful to humans, McIntyre tells the Los Angeles Times.

The Seattle Times reports that there are roughly 3.1 billion tires globally manufactured each year, painting a picture of a dauntingly pervasive pollutant. But Dan Kent, the executive director of an environmental non-profit called Salmon Safe, tells the Seattle Times he sees potential for salmon-safe tires in the not so distant future.

Alternately, cities may decide to improve their stormwater’s filtration by funneling it through basins of plants and soil, according to Science. The problem is that installing enough of these so-called bioswales to make a dent in the problem would be hugely expensive—a cruel irony for an intervention that mimics the passive filtration once provided for free by the many wetlands humans have destroyed in pursuit of dry land for buildings and agriculture.

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