For years, the mysterious and alarming deaths of coho salmon in Puget Sound creeks after rainstorms have perplexed scientists and conservationists alike. These iconic silver salmon, native to the Pacific Northwest, exhibited troubling behaviors as they suffocated, gaping and swimming erratically at the water’s surface before dying. The phenomenon, often referred to as “coho urban runoff mortality syndrome,” had long resisted clear scientific explanation. However, recent groundbreaking research led by Washington State University (WSU) and the University of Washington (UW) has illuminated the lethal culprit behind these die-offs: a toxic transformation product of a common tire additive known as 6PPD.
The enigma deepened in 2018 when researchers first confirmed a strong link between the unexpected salmon deaths and roadway runoff carrying tiny particles of tire wear. These microscopic fragments, generated by tire friction, wash off roads during rainfall and flow into local waterways where coho salmon reside. The identification of tire particles as a key vector was a breakthrough, yet the precise chemical agent responsible within those particles remained elusive until more recent studies pinpointed 6PPD, a widely used tire preservative. When exposed to atmospheric ozone, 6PPD undergoes chemical conversion to form 6PPD-quinone, a potent and hitherto underrecognized toxin.
Under the leadership of Stephanie I. Blair, a WSU PhD candidate, researchers accomplished a vital step in the investigation by elucidating exactly how 6PPD-quinone impacts coho physiology at the cellular level. Blair’s investigative team demonstrated that this compound breaches critical protective barriers within fish, specifically disrupting the blood-brain and blood-gill barriers. These barriers are essential biological defenses that regulate what substances enter delicate tissues of the brain and vascular system from the bloodstream. Their breakdown caused by 6PPD-quinone leads to catastrophic oxygen deprivation, which manifests as the suffocation symptoms observed in affected salmon.
The research methodology involved exposing coho salmon to stormwater runoff collected from state highways near Tacoma, as well as to laboratory concentrations of 6PPD-quinone representative of real-world exposure scenarios. Behavioral observations confirmed that exposed fish displayed the same surfacing, gaping, and disorientation behaviors characteristic of urban runoff mortality events. Extensive fluorescent marker analysis revealed focal points of increased permeability at the blood-brain and blood-gill barriers, indicating biochemical “leakiness” where 6PPD-quinone crosses into sensitive brain tissues and compromises respiratory function through the gills.
This precise characterization of the mode of toxicity marks a significant advancement. Prior to these findings, the connection between environmental tire chemicals and salmon die-offs was suspected but lacked mechanistic clarity. Now, the cause-and-effect relationship is firmly established: 6PPD-quinone causes targeted disruption of vital physiological barriers, leading directly to oxygen starvation and death. Blair herself emphasized the importance of this discovery, stating that it clears up the fundamental confusion surrounding coho urban runoff mortality syndrome and provides a foundation for the development of safer alternatives to 6PPD in tire manufacturing.
The ecological stakes of this research could not be higher. Coho salmon are not only emblematic of the Northwest’s natural heritage but also integral to its environmental health, economic livelihood, and cultural fabric. Many coho populations are currently threatened or endangered, with federal and tribal protections emphasizing the need to understand and mitigate mortality drivers. The newly elucidated biochemical vulnerabilities raise concerns beyond salmon, as emerging studies suggest that 6PPD-quinone toxicity extends to other fish species, aquatic mammals, and potentially humans through exposure to contaminated waterways.
As research progresses, the focus shifts towards screening and identifying alternative chemical preservatives that can replace 6PPD in tires without the devastating ecological consequences. Blair’s work equips environmental scientists, toxicologists, and regulatory agencies with targeted assays and biomarkers to evaluate candidate compounds accurately. By understanding the precise cellular sites and mechanisms of damage, future testing can be both more efficient and relevant, accelerating the search for sustainable tire chemistries that eliminate harmful runoff.
The collaboration between WSU, UW, and the U.S. Geological Survey Western Fisheries Research Center reflects a growing interagency and interdisciplinary effort to address this urgent environmental challenge. Jenifer McIntyre, associate professor of aquatic toxicology at WSU Puyallup Research & Extension Center and co-author on the study, has long spearheaded investigations into urban runoff toxicity. Their combined expertise in aquatic toxicology, fish physiology, and environmental chemistry ensures that findings translate directly into actionable conservation and regulatory strategies.
From a biological standpoint, the blood-brain and blood-gill barriers in fish serve as critical checkpoints that maintain homeostasis by regulating toxin entry. The newfound evidence that 6PPD-quinone induces disruption at these barriers elucidates why the exposure results in neurological and respiratory failure so rapidly. This insight enhances the broader toxicological understanding of how anthropogenic pollutants interfere with aquatic species’ physiological defenses and survival.
Stephanie Blair’s dedication also encompasses her personal connection to Indigenous environmental stewardship. As an enrolled member of the Sault Ste. Marie Tribe of Chippewa Indians and working with the Confederated Tribes of the Umatilla Reservation, she brings invaluable cultural perspective to the research. This intersection of traditional ecological knowledge and cutting-edge science exemplifies the holistic approaches needed to safeguard threatened ecosystems and species thoughtfully.
With the article published in the highly respected journal Environmental Science & Technology, set for June 17, 2025, Blair’s team has laid the cornerstone for extensive future research and policy reform aimed at mitigating the harmful impacts of roadway runoff on aquatic life. Their revelations about 6PPD-quinone’s role in disrupting protective physiological barriers will undoubtedly inform regulatory decisions and drive innovation in tire manufacturing.
Ultimately, this research is a compelling reminder of how modern industrial chemicals, even those embedded in everyday objects like tires, can have unforeseen ripple effects on natural environments. It underscores the critical importance of interdisciplinary, mechanistic toxicology studies in protecting vulnerable species and ecosystems from emerging pollutants hidden in urban landscapes. The escalating environmental urgency demands swift action, and with these new insights, science has opened a promising path forward to healthier rivers, resilient salmon populations, and smarter chemical use.
Subject of Research: Animals
Article Title: Blood–Brain and Blood–Gill Barrier Disruption in Coho Salmon Exposed to Roadway Runoff and 6PPD-Quinone
News Publication Date: 17-Jun-2025
Web References: https://pubs.acs.org/doi/10.1021/acs.est.5c01559
References: The paper published in Environmental Science & Technology
Image Credits: Not specified
Keywords
Coho salmon, 6PPD-quinone, roadway runoff, blood-brain barrier, blood-gill barrier, urban runoff mortality syndrome, aquatic toxicology, environmental pollution, tire additives, Puget Sound, fish physiology, environmental chemistry
