The Twenty-Year Mystery of Puget Sound's Dying Salmon

Joseph Shan

In 1999, the Puget Sound community had a problem: their Chinook salmon were recently reclassified as endangered, and their other salmonids were facing similar risks. This decline most directly impacted the Puget Sound tribes, since 80-90% of their diet consisted of fish. Treaties_{1, 2} signed in the mid-1800s gave these tribes the right to fish in Washington waters; treaties that would be null if there were no fish to catch in the first place.

What followed was a massive financial and infrastructural restoration project to reintroduce salmon back into their streams, redirecting them as they traveled to rivers for their spawning season. From 1999-2017, $527 million was spent, in part to restore urban creeks to be hospitable to these salmon. Public green space was increased, culverts and other barriers to fish migration were removed, and native vegetation was reintroduced. 

The results, however, were mixed. While salmon did return in droves, they would swim upstream and die before they could lay their eggs. For certain rivers, coho salmon saw a mortality rate of up to 100%. Tragically, the restoration efforts had unintentionally created an ecological trap for coho salmon. 

Investigating the cause of these deaths, rainwater became a clear culprit. Hours after rain, the salmon would display symptoms of erratic surfacing, gaping, and loss of orientation, before dying within a day. But what specific chemical in the rainwater was causing these deaths? This was a much harder question, and would take over 18 years before the answer was found.

Looking at salmon deaths broadly, it was clear that these deaths were concentrated in urban roads that experienced high traffic — researchers knew, then, to look not just at stormwater runoff, but stormwater road runoff. But these roadwater samples contained thousands of unique chemicals, many of which were unknown, and their toxicological effects on aquatic life were unstudied. As for the known toxic chemicals, like copper or zinc, they were nowhere near concentrated enough to be lethal.

Since the runoff was most deadly in heavily used roads, cars became the main culprit. To isolate the “primary causal toxicant,” Dr. Jen McIntyre and her research team at Washington State University began running experiments in 2002. They fractured runoff samples into smaller and smaller groups, tested their toxicology, and ran them through high-resolution mass spectrometry. Groups, at first, were split into categories such as car motor oil, wiper fluid, and tire wear particles. Tire wear particles soon became the main suspect, with its batch of 2216 unique chemicals. 

Eighteen years later, in 2020, Dr. McIntyre and her team discovered the molecular formula for the toxicant: C₁₈H₂₂N₂O₂. Yet, when comparing the compound to ones found in tires, there was no direct match — this compound wasn’t used in tire manufacturing. It was similar, however, to a known tire particle: C₁₈H₂₄N₂  (also known as 6PPD). 

Eventually, they discovered that the toxicant was formed when 6PPD reacted with O₃ in the air. This new chemical is now known as 6PPD-Quinone. 

6PPD is used in tire manufacturing to improve tire integrity. It protects tires from cracking, but kills salmon. Studies in 2022 have found 6PPD-Quinone to be the second most toxic chemical to aquatic life. The first is parathion, an insecticide banned in most countries due to its harmful effects on humans and animals. Further studies have revealed that 6PPD-Quinone is also found in the urban creeks of Los Angeles and San Francisco.

Since then, several solutions have been proposed, especially in the area of green stormwater infrastructure. Runoff rates are typically high in urban areas because the roads are impervious to water, leading runoff to slide right into rivers. Installing permeable pavements, gardens, or other bioretention systems reduces overall runoff, consequently reducing the chemicals that leach into rivers. King County, Washington, has currently field-tested an effective soil mix to filter out toxic chemicals, with their next steps to field-test in Whatcom Creek.

The ultimate goal is to replace 6PPD with an eco-friendly alternative. Washington took a first step in March 2024, when it became the first state to pass legislation to phase out 6PPD. But the unfortunate truth is that, even if 6PPD were phased out right now in tire manufacturing, it would still be around with us for 10 years, with sheddings from tires already on the road. And tire manufacturers are fighting against such replacement, asserting that 6PPD is currently an “essential ingredient” in tires. A viable alternative must be found first before the problem can be removed.

 In the meantime, coho are still dying in the same urban streams they migrated to two decades ago. It took 18 years to identify what was killing them. Saving them may take even longer. This is what makes King County’s soil-filtration so important, as it’s possibly the only defense salmon have against 6PPD for the foreseeable future.

Citations

(2023). Car Tires Pollute by Releasing 6PPD—And Regulators Like the EPA Are Finally Doing Something About It. Slate. https://slate.com/technology/2023/11/car-tires-6ppd-pollution-epa.html

Dunagan, C. (2024). Scientists Worldwide Are Immersed in Studies of a Deadly Tire Chemical. Encyclopedia of Puget Sound. https://www.eopugetsound.org/magazine/scientists-worldwide-are-immersed-studies-deadly-tire-chemical

(n.d.). Factors Limiting Progress in Salmon Recovery. Snohomish County. https://snohomishcountywa.gov/DocumentCenter/View/81540/08a_SSAGWhitePaper_Attachment-1

King County. (2025). King County Scientists Identify a Potential Breakthrough for Treating Salmon-Killing Tire Chemical. King County Newsroom. https://kingcounty.gov/en/dept/dnrp/about-king-county/about-dnrp/newsroom/2025-news-releases/03-27-treating-tire-chemicals

Nairn, C. (2026). Tire Manufacturers Fight Accusations That Toxic Runoff Harms Fish. Courthouse News Service. https://www.courthousenews.com/tire-manufacturers-fight-accusations-that-toxic-runoff-harms-fish/

National Caucus of Environmental Legislators. (2025). Washington Passes Legislation to Phase Out 6PPD. National Caucus of Environmental Legislators. https://www.ncel.net/articles/washington-passes-legislation-to-phase-out-6ppd/

NOAA Fisheries. (2025). Puget Sound Chinook Salmon. NOAA Fisheries. https://www.fisheries.noaa.gov/west-coast/endangered-species-conservation/puget-sound-chinook-salmon

Puget Sound Partnership. (2025). Vital Signs: Salmon. Puget Sound Partnership. https://vitalsigns.pugetsoundinfo.wa.gov/VitalSign/Detail/32

Scholz, N. (2011). Recurrent Die-Offs of Adult Coho Salmon Returning to Spawn in Puget Sound Lowland Urban Streams. PLOS ONE. https://pmc.ncbi.nlm.nih.gov/articles/PMC3237429/

Scholz, N. (2026). The Origin Story for 6PPD-quinone in Ecotoxicology. Environmental Science & Technology Letters. https://pubs.acs.org/estlcu/article/13/5/644/5084004/The-Origin-Story-for-6PPD-quinone-in-Ecotoxicology

Tian, Z. (2022). 6PPD-Quinone: Revised Toxicity Assessment and Quantification with a Commercial Standard. Environmental Science & Technology Letters. https://pubs.acs.org/doi/abs/10.1021/acs.estlett.1c00910

(n.d.). Treaty of Medicine Creek, 1854. Washington State Governor's Office of Indian Affairs. https://goia.wa.gov/resources/treaties/treaty-medicine-creek-1854

(n.d.). Treaty of Point Elliott, 1855. Washington State Governor's Office of Indian Affairs. https://goia.wa.gov/resources/treaties/treaty-point-elliott-1855

Washington Stormwater Center. (n.d.). Tire Chemicals and Coho Salmon. Washington Stormwater Center. https://www.wastormwatercenter.org/research/tiresandsalmon/