For Pacific Northwest coho salmon, returning to spawn in the streams and creeks near urban areas can be a death sentence, thanks to a ubiquitous additive in vehicle tires, a new study reveals. The findings show that 6PPD-quinone - a highly toxic oxidation product of tire rubber particles - turns streams toxic and may be responsible for the annual die-offs observed amongst migrating adult salmon across the U.S. Pacific Northwest. For decades, researchers have observed regular acute mortality events affecting adult coho salmon (Oncorhycchus kisutch) that migrate into urban waterways contaminated with stormwater runoff. In the most urbanized watersheds, urban runoff mortality syndrome (URMS), is estimated to kill 40-90% of returning salmon before they have a chance to spawn. While URMS has been tied to stormwater runoff and potentially linked to tire tread wear particles (TWPs) - one of the most significant sources of microplastics in freshwater - the one or more toxicants responsible for killing salmon have remained elusive. Using liquid chromatography mass spectrometry and nuclear magnetic resonance, Zhenyu Tian and colleagues explored the compounds associated with roadway runoff and TWP to search for toxic compounds. Tian et al. found that 6PPD, the primary antioxidant chemical used in tire rubber, reacts with ozone to form the previously unidentified compound 6PPD-quinone. According to the authors, 6PPD-quinone is highly toxic and is deadly to juvenile salmon at concentrations of roughly 1 microgram per liter. What's more, retrospective analysis suggests that this deadly compound is widespread in stormwater-impacted waterways across the U.S. West Coast. "It is unlikely that coho salmon are uniquely sensitive, and the toxicology of 6PPD transformation products in other aquatic species should be assessed," write the authors.
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Journal
Science