Coho salmon are dying from tire runoff in spring, not just in fall
Adult coho have been dying in urban creeks after autumn rains for decades. A 2026 field study shows juvenile coho die at the same rate during ordinary spring storms, which means the hazard is year-round.
By Timothy Li
The clearest causal story in tire pollution is the coho salmon. Adults returning to urban streams in the Pacific Northwest die within hours of a rainstorm, before they can spawn, and the cause was traced in 2021 to 6PPD-quinone washing off roads. Almost all of that work looked at adults in autumn. A study published in Environmental Science & Technology in January 2026 asks about the fish that live in those creeks all year.
What they did
Researchers from Washington State University, the University of Washington, and the U.S. Geological Survey set up a small field facility at Miller Creek, a runoff-impacted stream south of Seattle. Across three spring storms in 2024 they exposed 720 hatchery-raised juvenile coho to either creek water or clean groundwater, 120 fish per treatment per storm, and measured water chemistry alongside mortality.
What they found
- Peak 6PPD-quinone in the creek reached 96, 110, and 73 nanograms per litre during the three storms, at or above published lethal concentrations for juvenile coho. Between storms it sat near 1 to 2 nanograms per litre.
- About 80 percent of the creek-water fish died in each storm: 79, 80, and 81 percent. Most deaths came within 24 hours, and signs of distress appeared 13 to 15 hours after rain began.
- No control fish in groundwater died.
What it means
Juvenile coho spend more than a year in fresh water before heading to sea, so they will meet many storms. The authors conclude that runoff mortality is a year-round hazard for every freshwater life stage, not a first-flush event in autumn. They are also blunt about the fix: treating enough stormwater to protect every sensitive creek is not practical as the only approach, and reformulating tire rubber is likely to be cheaper and more effective.
What it cannot show
One creek, one spring, one species. Coho are unusually sensitive to 6PPD-quinone, and other salmon and trout species respond differently. Hatchery fish in aquaria may not behave exactly like wild fish that can move within a stream. The study measures death, not the sub-lethal effects that may matter for survivors.
For California, where coho are already protected and the state has listed tires containing 6PPD as a priority product, the study strengthens the case that timing-based fixes, such as diverting the first autumn flush, are not enough.
Read the study (open access) · Brown et al., Environmental Science & Technology, 2026