By Kurtis Alexander, Staff Writer
Oct 26, 2026
Six years ago, scientists in the Pacific Northwest untangled a mystery about why salmon were dying in the region, often in astonishing numbers — with as many as 90% of the migrating adult coho salmon perishing in parts of Puget Sound.
The culprit turned out to be a toxic chemical from tires washing into waterways.
Now, some of the same researchers that made the 2020 discovery have confirmed that this tire-derived compound is present, at worrisome levels, in salmon-bearing watersheds in California.
A study published last month in the journal Environmental Science and Technology Letters finds that at least 10 rivers and creeks in the Bay Area and Santa Cruz region have had potentially lethal concentrations of the chemical 6PPD-quinone after stormwater flowed into them.
While salmon are much less plentiful in these waterways than they are in Oregon and Washington, five of the California rivers and creeks that logged harmful levels of the toxic substance are federally designated as “critical habitat” for endangered coho. These include Marin County’s Corte Madera Creek, San Mateo County’s San Pedro Creek and Santa Cruz County’s San Lorenzo River.
The study didn’t track fish fatalities, but coho salmon are particularly sensitive to 6PPD-quinone, and local fish experts say the chemical is probably exerting a toll on California’s small coho population.
“It’s highly likely that fish have died as a result of this,” said Chris Berry, watershed compliance manager for the city of Santa Cruz, which is conducting a $50 million program to help restore salmon in Santa Cruz County. “We’re putting a big effort into recovering coho and to see something like 6PPD (quinone) pose another challenge for coho is really unsettling.”
Coho, like other types of salmon, have struggled with drought, dams, pollution and over-development of their watersheds, among other things. California’s coho population today is less than 5% of its historical count, experts say.
The compound 6PPD-quinone was discovered to be a menace to coho only after the scientists in 2020 linked it to another chemical, 6PPD. This industrial material is commonly added to plastics and rubbers, including vehicle tires, to increase durability. As tires wear and shed particles, the 6PPD can react with ozone, the scientists noted, to create the more toxic 6PPD-quinone.
The newly created 6PPD-quinone builds up on roads and spills into rivers and creeks when it rains. While coho salmon spend much of their lives at sea, they swim inland, up freshwater channels to spawn, inevitably encountering runoff from roads that can contain 6PPD-quinone.
“There’s always been a question of whether 6PPD-quinone would be an issue for California,” said Ed Kolodziej, one of the authors of the new study and a professor of sciences and mathematics at the University of Washington campuses in Seattle and Tacoma. Kolodziej helped author the 2020 paper that originally identified 6PPD-quinone as a problem. “The coho are certainly off the coast there (in Central California). I know people off Santa Cruz in the ocean catching coho salmon.”
Kolodziej’s research team came to California last November, as a storm approached, and took water samples before and after the rain. These samples were sent to a lab for testing, and the results came back showing that 6PPD-quinone levels generally rose with the stormwater runoff.
Forty-six out of 59 storm samples, representing at least 10 waterways, had concentrations of 6PPD-quinone that exceeded 11 nanograms per liter, which the U.S. Environmental Protection Agency considers harmful to aquatic life.
Branciforte Creek, a tributary of the San Lorenzo River in Santa Cruz County, recorded 230 nanograms per liter, the highest concentration of any of the waterways designated as coho habitat. Carbonera Creek, another tributary of the San Lorenzo River and designated coho habitat, also had extremely high levels of the compound.
“This shows that the situation is quite urgent,” said Rebecca Sutton, senior scientist at the San Francisco Estuary Institute, who works to keep California watersheds healthy for fish and other wildlife. Sutton helped with the 2020 study that brought 6PPD-quinone to light.
Steelhead trout, which also swim in some of the California watersheds that tested at high levels for 6PPD-quinone, can be harmed by the compound as well. The Central Coast runs of steelhead are classified as threatened under the federal Endangered Species Act.
Studies are underway to determine how much, if any, of a health risk the chemical is to humans, whether from direct exposure or by eating fish.
The state of California began taking steps to address 6PPD-quinone after the incriminating findings were published six years ago. While there are no restrictions on the parent material, 6PPD, the California Department of Toxic Substances Control has listed the substance as a “priority product” and is requiring tire manufacturers to begin considering the use of alternatives. Washington state is also taking steps to try to phase out the compound.
“Getting this chemical out of tires is definitely the solution that fixes this problem everywhere at once,” Kolodziej said. “But that takes time.”
In the meantime, he and others recommend measures to prevent and treat stormwater runoff along roads, such as installing catch basins and rain gardens, particularly in areas where salmon are vulnerable.