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What is killing coho salmon in urban streams?

Scientists at the University of Washington have finally explained why so many coho salmon are dying in urban streams, and the cause of death is not what you would expect. 

Each year, more than half of the coho salmon that return to the streams of the Puget Sound die before they can ever spawn. In some streams, none of the salmon survive. 

According to the researchers, the killer is in the mix of chemicals that leach from tires. When it rains, stormwater flushes bits of aging vehicle tires onto roads and into neighboring streams. 

“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. 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,” said study co-senior author Professor Edward Kolodziej.

“Here we started with a mix of 2,000 chemicals and 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.”

Coho salmon are born in freshwater streams, where they spend the first year of their lives. Next, the salmon make the journey out to sea where they remain for most of their adult lives.

A small portion of coho salmon return to their original streams to lay their eggs before dying. However, experts recently discovered that most of the returning salmon are dying before they can spawn – especially after a heavy rainfall.

“We had determined it couldn’t be explained by high temperatures, low dissolved oxygen or any known contaminant, such as high zinc levels,” said study co-senior author Professor Jenifer McIntyre. “Then we found that urban stormwater runoff could recreate the symptoms and the acute mortality. That’s when Ed’s group reached out to see if they could help us understand what was going on chemically.”

The researchers analyzed water from the creeks where salmon were dying. All of the samples contained a chemical signature associated with tire wear, but these particles contain hundreds of different chemicals. In a separate study, Professor McIntyre had already confirmed that a solution made from tire wear particles was highly toxic to salmon.

Using lab tests, the experts narrowed the culprit down to just a few chemicals, including one that appeared to dominate the mixture but did not match anything known.

“There were periods last year when we thought we might not be able to get this identified. We knew that the chemical that we thought was toxic had 18 carbons, 22 hydrogens, two nitrogens and two oxygens. And we kept trying to figure out what it was,” said study lead author Zhenyu Tian, a research scientist of 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.”

Tian searched a list of chemicals known to be in tire rubber and pinpointed 6PPD, which is used to keep tires from breaking down too quickly. “It’s like a preservative for tires. Similar to how food preservatives keep food from spoiling too quickly, 6PPD helps tires last by protecting them from ground-level ozone.”

According to the researchers, when 6PPD reacts with ozone, it is transformed into multiple chemicals. These include 6PPD-quinone, which is the toxic chemical that is responsible for killing coho salmon.

The researchers tested roadway runoff from Los Angeles and in urban creeks near San Francisco. They found that 6PPD-quinone was present there as well. This is not surprising, said the experts, because 6PPD appears to be used in all tires and tire wear particles are likely present in creeks near busy roads across the world.

“How does this quinone lead to toxicity in coho? Why are other species of salmon, such as chum salmon, so much less sensitive?” said McIntyre. “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.”

“Tires need these preservative chemicals to make them last,” said Professor Kolodziej. “It’s just a question of which chemicals are a good fit for that and then carefully evaluating their safety for humans, aquatic organisms, etc. We’re not sure what alternative chemical we would recommend, but we do know that chemists are really smart and have many tools in their toolboxes to figure out a safer chemical alternative.”

The study is published in the journal Science.

By Chrissy Sexton, Staff Writer


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