Biologists in the Puget Sound region documented urban runoff mortality syndrome from the 1990s: adult coho entering city creeks after rain would surface, lose orientation and die within hours, often before spawning, at rates reaching most of the run in some streams. Coho were affected far more than chum in the same water.
The cause was traced in 2020. Tyres contain 6PPD, an antiozonant that protects rubber from cracking. As tyres wear, particles enter road dust, and 6PPD reacts with ozone to form 6PPD-quinone, which washes into streams during storms. Coho proved acutely sensitive to it at very low concentrations, far below those affecting many other tested species.
The finding reframed a long-standing mystery as a chemical problem with a supply chain attached, and prompted work on tyre reformulation and on roadside soil filtration that removes the compound before it reaches fish. Why coho are so much more sensitive than closely related salmon, and what the mechanism of toxicity is, remain open questions, as does the effect on other freshwater species not yet tested.