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Each fall, more than half of the coho salmon that return to Puget Sound’s urban streams die before they can spawn. In some streams, everyone dies. But the scientists didn’t know why.
Now a team led by researchers from the University of Washington Tacoma, UW and Washington State University Puyallup has uncovered the answer. When it rains, rainwater dumps bits of old vehicle tires onto the roads in nearby streams. The killer is in the mix of chemicals that leach from tire wear particles – a molecule related to a preservative that prevents tires from breaking down too quickly.
This research was published on December 3 in Science.
“Most people think we know which chemicals are toxic and all we have to do is check the amount of those chemicals to make sure the water quality is good. But, in reality, animals are exposed to this. giant chemical soup and we don’t know what many of the chemicals it contains are also, “said senior co-author Edward Kolodziej, associate professor in both the UW Tacoma Science and Mathematics Division and the UW Department of Civil and Environmental Engineering. .
“Here we started with a mix of 2,000 chemicals and were able to work up to this highly toxic chemical, something that quickly kills large fish and we think is probably on every single busy road in the world.”
Coho salmon are born in freshwater streams. After spending the first year of their life there, these fish make an epic journey to the sea where they live most of their adult life. Some – about 0.1% – return to their original streams to spawn, or spawn, before they die. But the researchers began to notice that, especially after a large rain, the returning salmon were dying before they could spawn. The search for the coho-killer began with the study of inlet water quality, a multi-agency effort led by NOAA-Fisheries and which includes the U.S. Fish and Wildlife Services, King County, Seattle Public Utilities and Wild Fish Conservancy.
“We had determined that it couldn’t be explained by high temperatures, dissolved oxygen or any known contaminants, such as high levels of zinc,” said co-senior author Jenifer McIntyre, assistant professor at WSU’s School of the Environment, based in Puyallup. “Then we found that urban stormwater runoff could recreate symptoms and acute mortality. That was when Ed’s group contacted to see if they could help us understand what was happening chemically.”
First, the team narrowed down what might be behind the symptoms in stormwater runoff. The researchers compared the water of inlets where salmon were seen dying to look for common trends. All the creek samples contained a chemical signature associated with tire wear particles. Additionally, a study by McIntyre found that a tire wear particle solution was highly toxic to salmon.
But tire wear particles are a mixture of hundreds of different chemicals, so the team had a challenge ahead: how to find the culprit?
The researchers began by dissecting the tire wear particle solution based on different chemical properties, such as removing all metals from the solution. Then they tested the different solutions to see which ones were still toxic to the salmon in the lab. They repeated this process until there were only a few chemicals left, including one that seemed to dominate the mixture but didn’t match anything known.
“There were times last year when we thought we wouldn’t be able to identify it. We knew the chemical we thought was toxic had 18 carbon atoms, 22 hydrogens, two nitrogens and two oxygens. And we kept trying to find it. understand what it was, “said lead author Zhenyu Tian, a researcher at UW Tacoma’s Center for Urban Waters. “Then one day in December, it was just like bing! In my mind. The killer chemical might not be a chemical added directly to the tire, but something related.”
Tian searched a list of chemicals known to be in tire rubber for anything that might be similar to their unknown – giving or taking some hydrogen, oxygen, or nitrogen – and found something called 6PPD, which is used to prevent that. tires break too quickly.
“It’s like a tire preservative,” Tian said. “Similar to how food preservatives prevent food from deteriorating too quickly, 6PPD helps tires last by protecting them from ground-level ozone.”
Ozone, a gas that is created when pollutants emitted by cars and other chemical sources react to sunlight, breaks the bonds that hold the tire together. 6PPD helps by reacting with the ozone before it can react with the tire rubber, saving the tires.
But when 6PPD reacts with ozone, the researchers found that it has been converted into multiple chemicals, including 6PPD-quinone (pronounced “kwih-known”), the toxic chemical responsible for killing salmon.
This chemical is not limited to the Puget Sound region. The team also tested road runoff from Los Angeles and urban inlets near San Francisco, and 6PPD-quinone was also present there. This finding comes as no surprise, the researchers said, because 6PPD appears to be used in all tires, and tire wear particles are likely to be found in inlets near busy roads around the world.
Now that 6PPD-quinone has been identified as the “smoking gun” behind coho’s death in freshwater streams, the team can begin to understand why this chemical is so toxic.
“How does this quinone lead to toxicity in coho? Why are other salmon species, such as chum salmon, much less sensitive?” McIntyre asked. “We have a lot to learn about which other species are sensitive to rainwater or 6PPD-quinone within and outside the Puget Sound region.”
One way to protect salmon and other creatures living in the creeks is to treat rainwater before it hits the creeks. However, while tests have shown that effective and environmentally friendly rainwater technologies exist for 6PPD-quinone removal, it would be nearly impossible to build a treatment system for every street, the team added.
Another option is to change the composition of the tires themselves to make them “safe for salmon”.
“Tires need these chemical preservatives to make them last,” Kolodziej said. “It’s just a question of which chemicals are suitable for this and then carefully evaluate their safety for humans, aquatic organisms, etc. We are not sure which alternative chemical we would recommend, but we know that chemists are really smart and they have many tools in their toolboxes to find a safer chemical alternative. “
This research was funded by the National Science Foundation, the US Environmental Protection Agency, the Washington State Governors Funds, and the San Francisco Bay Regional Monitoring Program for Water Quality.
Grant numbers: NSF: 1608464 and 1803240, EPA: # 01J18101 and # DW-014-92437301
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