Salmon deaths and a tyre pollution trial

A US judge will decide if, as research suggests, a chemical tyre additive is harming endangered fish species

A coastal brown bear, also known as a grizzly bear (Ursus Arctos) carries a silver salmon or coho salmon (Oncorhynchus kisutch) that it caught. Pictured at Cook Inlet, South Central Alaska.

At the end of January a district judge in San Francisco, California, presided over a three-day trial brought by west coast fishers and conservationists against US tyre companies. The fishers allege that a chemical additive used in tyres is polluting rivers and waterways, killing coho salmon and other fish. If successful, the case could have implications far beyond the United States.

How did the case come about?

The case was initiated after the apparent solving of a decades-old mystery: what was causing mass deaths of endangered coho salmon in the Pacific north-west as they returned to streams to spawn. The deaths happened after heavy rain.

Before dying, the fish would exhibit unusual behaviour, swimming in circles, their mouths gaping, as if gasping for air. Scientists, suspecting storm runoff, described the phenomenon as “urban runoff mortality syndrome”.

It took years for scientists from Washington State University to pinpoint what they now allege is the chemical culprit.

In 2020, they published a study in Science that claimed to solve the mystery: they found a toxic substance in leachate from car tyres that killed the fish [exa.mn/tyre]. Known as 6PPD-quinone or 6PPD-q, it is an oxidation product from 6PPD, a chemical added to car tyres to prevent them breaking down. This transformed chemical, 6PPD-q, leaches into rivers and creeks with, scientists say, devastating results for the protected and endangered species.

Who brought the case and why?

The case was brought by the Institute for Fisheries Resources (IFR) and the Pacific Coast Federation of Fishermen’s Associations (PCFFA) represented by environmental group, Earthjustice, after a slew of scientific studies linking mass deaths of coho salmon and harms to other salmon with a chemical tyre additive, 6PPD. The fishers say they depend on the health of salmon populations for their livelihood.

Glen Spain, general legal counsel and northwest regional director of the IFR and the PCFFA, said: “The use of 6PPD in tyres has been shown to harm vulnerable salmon populations. Whether or not this should continue will be up to the court.” What is the central question to be answered?

The judge will determine whether the tyre manufacturers are violating the Endangered Species Act by harming fish species, including coho salmon, protected under the legislation. The fisher’s case is that the tyre companies are violating the act by harming 24 populations of fish species protected under it.

Tire manufacturers cast doubt on lab studies as trial over toxic roadway runoff continues @mhattridge_ https://t.co/0Rn280DsdG

— Courthouse News (@CourthouseNews) January 28, 2026

What testimony did the judge hear?

Edward Kolodziej, a co-author of the 2020 Science study and a primary witness for the fishers, told Judge James Donato that tyres are the main source of 6PPD-quinone in waterways and that road runoff is likely to have high 6PPD-q concentrations. Just one car could do tremendous damage, he said.

Four tyres contain enough of the chemical that, after interacting with ozone, could produce enough to 6PPD-q to kill more than 11 million salmon, said Kolodziej.

His testimony, which explained how 6PPD-q was discovered, followed a video of a salmon flailing in the water in distress. His team identified a mortality signature for the chemicals in the water when the fish perished, identifying most of them as derivatives of tyre rubber. “It opened up our eyes that there are a lot of abundant tyre rubber chemicals we knew little about,” he said. “Learning more about tyres could explain this case of mortality.”

Do all scientists agree?

No. Tiffany Thomas, a scientist and defence witness from consultancy firm Exponent, told the court that the findings presented by the plaintiffs’ scientists were based on laboratory conditions rather than real life and were “limited and speculative”.

She said 6PPD-q is quick to degrade and has the opportunity to react with different chemicals and undergo physical reactions before reaching the road and water surface.

“The data available on 6PPD-q is inadequate to reliably predict the presence, phase, and concentration of 6PPD-q in the streams and rivers in the salmonid populations identified in the complaint,” she said.

Interest in tyre composition was sparked when the effects of the 6PPD preservative in tyres was linked to the mass die-off of coho salmon. Come and hear the US perspective on this in Prague, 28-29 February. https://t.co/M1emXkHMDK@tyreconsortium @EPA #tyreemissions pic.twitter.com/79tqh8fYzj

— Emissions Analytics (@E_Analytics) February 1, 2024

What are the tyre companies saying?

In a statement, Sam Singer, a spokesperson for the tyre companies, said that 6PPD is “critical for tyre safety” as it prevents cracking and degradation. There is “no suitable alternative”, he said. He questioned whether the chemical “even reaches any fish habitat” in the “distinctively different locations” listed in the complaint.

He added: “The tyre companies remain committed to working with regulators and are actively participating in regulatory processes to identify a viable alternative to 6PPD. Any alternative must meet established standards for safety.”

A ruling is expected at a later time.

— Guardian