January 18, 2021


A highly toxic chemical used in the production of millions of tires every year is killing salmon in the Pacific Northwest, and it is being detected in streams across Northern California, a new study finds.

Scientists have known for decades that stormwater runoff from roads, highways and other urban areas has been linked to high rates of coho salmon deaths in Washington state, where as many as 90% of salmon in the Puget Sound area have died before they could spawn.

The new study published in the research journal Science on Thursday has identified a culprit chemical for the first time — a commonly used preservative called 6PPD used to give tires longer life.

“I think the broader impact is as we have already found this in San Francisco (Bay) creek water as well as the road runoff,” said the study’s lead author, Zhenyu Tian, a researcher at the University of Washington Tacoma. “We believe this thing is a prevalent contaminant. Wherever you have a busy highway, you have runoff from there and you probably will detect it. Our detection rate for this chemical in runoff is almost always 100%. For coho salmon, it’s definitely a threat.”

As part of the study, stormwater running into four Bay Area waterways and creeks tested positive for deadly concentrations of the toxin known as 6PPD-quinone. The creeks that tested for high toxicity included Rodeo Creek in Contra Costa County, Elmhurst Creek near Oakland and two locations near Coyote Creek near San Jose.

As tiny pieces of tire break off on roads and highways, the preservative 6PPD interacts with ground-level ozone to create 6PPD-quinone, which in turn washes into creeks and rivers when it rains, according to the study. The study led by the University of Washington and Washington State University found the toxin was highly deadly, killing some young coho salmon in just four hours when exposed to it.

Estimated to have once numbered in the hundreds of thousands, coho salmon numbers in California have been devastated since the mid-20th century by human development, damming, habitat loss and changing climate and ocean conditions. Coho have been federally listed as an endangered species and are at risk of extinction on the Central California coast, where only 1% of the historic population remains.

State agencies and environmental groups have been working for decades to restore coho salmon populations between Santa Cruz County and the California-Oregon border, including remaining coho strongholds on Lagunitas Creek, in San Mateo and further north in the Klamath and Eel rivers.

The discovery of the toxin could help to inform further recovery efforts throughout the state, researchers said.

“Here in the tributaries that discharge into San Francisco Bay, we no longer have coho,” said Rebecca Sutton, a study co-author and senior scientist with the San Francisco Bay Estuary Institute, which tested Bay Area stormwater as part of the research. “But up north we still have coho and we want those populations to thrive. So removing stressors like this particular chemical could


When officials in Seattle spent millions of dollars restoring the creeks along Puget Sound — tending to the vegetation, making the stream beds less muddy, building better homes for fish — they were thrilled to see coho salmon reappear.

a man standing next to a river: From left, researchers Jen McIntyre, Edward Kolodziej and Zhenyu Tian study the stormwater impacts on coho salmon in Longfellow Creek in the Seattle area. (Mark Stone / University of Washington)

© Provided by The LA Times
From left, researchers Jen McIntyre, Edward Kolodziej and Zhenyu Tian study the stormwater impacts on coho salmon in Longfellow Creek in the Seattle area. (Mark Stone / University of Washington)

But when it rained, more than half, sometimes all, of the coho in a creek would suffer a sudden death.

These mysterious die-offs — an alarming phenomenon that has been reported from Northern California to British Columbia — have stumped biologists and toxicologists for decades. Numerous tests ruled out pesticides, disease and other possible causes, such as hot temperatures and low dissolved oxygen.

Now, after 20 years of investigation, researchers in Washington state, San Francisco and Los Angeles say they have found the culprit: a very poisonous yet little-known chemical related to a preservative used in car tires.

This chemical is just one of a vast number of contaminants that washes off roads whenever it rains. This giant soup of pollutants, which includes trillions of microplastics, rushes down drains and into creeks and ultimately into the sea.

“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,” said Ed Kolodziej, an environmental engineer and chemist at the University of Washington (Tacoma/Seattle), whose lab led a study that was published Thursday in the journal Science. “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.”

Coho salmon, also known as silver salmon, are prized among fishermen and an important indicator species — the canary in the coal mine for coastal watersheds along the northern Pacific Ocean. Their range has historically stretched from the creeks nestled in the redwood forests near Santa Cruz all the way north to the waters of Alaska. The few coho populations that still exist in California are either endangered or threatened.

The fish are born in freshwater streams, where they stay for about a year before making the long journey through rivers and estuaries and into the ocean. They return a year and a half later to lay and fertilize eggs before dying. Many obstacles have made this journey across different environments more difficult: Shrinking estuaries, blocked passages from dams and culverts, as well as drought and a changing climate.

“While we often monitor temperature and dissolved oxygen levels, much more could be done to test for toxicity,” said Mariska Obedzinski, a California Sea Grant fisheries biologist who leads monitoring and salmon recovery research on the Russian River and was shocked to see the findings out of Puget


Pollution from car tires that washes into waterways is helping cause a mass die-off of salmon on the US west coast, researchers have found.

a fish swimming under water: Photograph: NOAA/Alamy

© Provided by The Guardian
Photograph: NOAA/Alamy

In recent years, scientists have realized half or more of the coho salmon, also known as silver salmon, returning to streams in Washington state were dying before spawning. The salmon, which reach 2ft in length, are born in freshwater streams before making an epic journey out to sea where they live most of their adult lives. A small number then return to their original streams to lay eggs before dying.

a fish swimming under water: Coho salmon, which can grow to 2ft in length, spend their lives in the ocean but return to the US Pacific coast to spawn.

© Photograph: NOAA/Alamy
Coho salmon, which can grow to 2ft in length, spend their lives in the ocean but return to the US Pacific coast to spawn.

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The cause of the die-off has remained a mystery but a new study, published in Science, has seemingly found a culprit. When it rains, stormwater carries fragments of old car tires into nearby creeks and streams. The tires contain certain chemicals that prevent them breaking down but also prove deadly to the coho salmon.

“The salmon would be inexplicably dead, which is tragic because this beautiful wild animal should be culminating its life and then it’s suddenly dead,” said Jenifer McIntyre, an assistant professor of aquatic toxicology at Washington State University. “The more we look, the more we find it. In some years all of the fish we find dead did not spawn.”

Samples taken from urban streams around Puget Sound, near Seattle, and subsequent laboratory work identified a substance called 6PPD, which is used as a preservative for car tires, as the toxic chemical responsible for killing the salmon. It’s currently unclear how it kills the fish but McIntyre said it was likely to be an “acute cardio-respiratory problem”.

The finding suggests that fish and other creatures elsewhere in the US and around the world are also at risk from the car tire chemical. Animals are being “exposed to this giant chemical soup and we don’t know what many of the chemicals in it even are”, said co-author Edward Kolodziej, an associate professor at the University of Washington.

“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,” Kolodziej added.

The nature of the threat facing coho salmon has been unclear since the fish were first seen “rolling” down streams, unable to swim upright, in the 1990s, McIntyre said. In an undisturbed riparian area it would be extremely rare for a coho salmon to die before laying its eggs but a growing sprawl of roads, cars and buildings near waterways has coincided with a surge in pre-spawning deaths. A reduction in 6PPD use or buffers to prevent the flow of