(CN) — Scientists revealed Thursday they have solved the mystery of what’s killing more than half of the Coho salmon in Puget Sound’s urban streams before they can spawn.
The toxin decimating the salmon population comes from the mix of chemicals that leach from automobile tires and drain into streams whenever it rains, researchers at the University of Washington Tacoma and Washington State University Puyallup.
Specifically, the culprit is a molecule related to a preservative that keeps tires from breaking down too quickly, according to the study published in the journal Science.
“Most people think that we know what chemicals are toxic and all we have to do is control the amount of those chemicals to make sure water quality is fine,” said co-senior author Edward Kolodziej, an associate professor in both the UW Tacoma Division of Sciences & Mathematics and the UW Department of Civil & Environmental Engineering, in a statement. “But, in fact, animals are exposed to this giant chemical soup and we don't know what many of the chemicals in it even are.”
Coho salmon spend the first year of their lives in freshwater streams before swimming out to sea where they spend most of their lives. Only a tiny fraction of the population returns to their birth streams to spawn before dying.
For years, scientists have known that many returning salmon were dying before they could spawn, especially after a big rain. But no one knew what was killing the Coho. So researchers began studying the water quality of the creeks, a multi-agency effort led by NOAA-Fisheries and including the U.S. Fish and Wildlife Services, King County, Seattle Public Utilities and the Wild Fish Conservancy.
“We had determined it couldn't be explained by high temperatures, low dissolved oxygen or any known contaminant, such as high zinc levels,” said co-senior author Jenifer McIntyre, an assistant professor at WSU's School of the Environment in Puyallup. “Then we found that urban stormwater runoff could recreate the symptoms and the acute mortality.”
Starting with a mix of 2,000 chemicals found in stormwater runoff, researchers compared water from creeks where salmon were dying to identify common trends. By sectioning the tire wear particle solution according to different chemical properties, they began to identify which ones were toxic to salmon in the lab. But key questions remained unanswered.
“We knew that the chemical that we thought was toxic had 18 carbons, 22 hydrogens, two nitrogens and two oxygens,” noted lead author Zhenyu Tian, a research scientist at the Center for Urban Waters at UW Tacoma. “Then one day in December, it was just like ‘Bing!’ in my mind. The killer chemical might not be a chemical directly added to the tire, but something related.”
Refining their process, researchers were able to whittle down the toxin list to one highly toxic chemical called 6PPD that kills large fish quickly and is found on every single busy road in the world.
“It's like a preservative for tires,” Tian said. “Similar to how food preservatives keep food from spoiling too quickly, 6PPD helps tires last by protecting them from ground-level ozone.”
Ozone is a gas created when pollutants emitted by cars react in the sunlight, breaking the bonds that hold tires together. 6PPD helps by reacting with ozone before it reaches the rubber. Unfortunately, when 6PPD reacts with ozone, it’s transformed into multiple chemicals, including 6PPD-quinone, the toxic chemical responsible for killing salmon.
Now that 6PPD-quinone is known to kill coho salmon, researchers intend to understand why this chemical is so toxic.
“We have a lot to learn about which other species are sensitive to stormwater or 6PPD-quinone within, as well as outside, of the Puget Sound region,” McIntyre explained.
One way to protect salmon is to treat stormwater before it hits the creeks. But while there are effective stormwater technologies for removing 6PPD-quinone, it’s practically impossible to build a treatment system for every road.
Another salmon-safe option is to change the composition of the tires.
“Tires need these preservative chemicals to make them last,” Kolodziej said. “It’s just a question of which chemicals are a good fit for that and then carefully evaluating their safety.”
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