Be they orange, pink, or red, salmon share one color in common: green. According to a study published earlier this year, salmon were the third most-valuable type of seafood, worth $598 billion in 2018.
Salmon doesn't just taste good — it is good for us, too. These fish are nutritional powerhouses, laden with omega-3 fatty acids and essential minerals. Eating a diet rich in salmon could help stave off cognitive decline, improve men's reproductive health, and perhaps even treat depression.
The demand is so high some types of salmon struggle to keep up. But overfishing and human appetites are not the sole danger to these fish — another human desire may be driving their numbers down dramatically, a new study suggests.
The danger comes when salmon return from the ocean to rivers to spawn. Trapped in rivers, as many as 40-90 percent of salmon may be killed not through fishing, but by driving.
The culprit, it seems, is not the exhaust fumes coming from cars, the new research suggests. Rather, it is the car itself — its tires.
Tires are universally made out of a combination of natural rubber, synthetic rubber, steel, and a variety of other materials. Within the textiles and fillers lies a little-known chemical compound called 6PPD, the primary antioxidant chemical used in tires. When combined with ozone, it forms what’s known as 6PPD-quinone.
The study zoomed in on one species of salmon, coho salmon — these salmon commonly end up on American plates, but certain populations of coho salmon are considered endangered under the Endangered Species Act. The 6PPD-quinone chemical is deadly to coho salmon, according to the new study.
Smoking gun — Edward Kolodziej is an associate professor at the University of Washington and an author on this study. He describes the study as “a classic detective story,” in which the researchers turned to forensic techniques to “find the smoking gun" behind the salmon deaths.
Kolodziej and his team study what is known as urban runoff mortality syndrome, which occurs when salmon return from the ocean and meet untreated stormwater from urban areas in rivers. The symptoms aren't pretty: The syndrome starts with salmon acting lethargic, swimming on the surface of the water, seeming to gasp for breath and losing their motor abilities. Eventually, the salmon die. No fish has ever been found to recover from the syndrome.
Urban runoff mortality happens after rain or snowfall moves water through storm drains to larger bodies of water, like lakes and rivers. When rain and snow lands on city streets the water picks up a wide variety of chemicals, and these are what end up in rivers. Most frequently along for the ride are those associated with cars. The chemicals range from motor oil to antifreeze, leaving scientists with a poisonous stew to sift through to find the exact killing mechanism.
“We've known for 30 or 40 years now that something was in stormwater killing these fish, but we didn't know what that was.” Kolodziej himself has worked on the problem for the better half of a decade.
Identifying the exact chemical killer was “definitely an interesting moment.”
In the new study, Kolodziej and his colleagues used an imaging technique called high-resolution mass spectrometry to search for molecular clues to what was killing the salmon.
“We've known for three or four years now that the only thing that was killing coho salmon was little bits of tire rubber and water. That was the only chemical source we looked at, from vehicle fluids and things like that, that could actually kill the fish.” Tiny pieces of rubber and plastic have become inescapable on the planet.
There was only one way to determine if the 6PPD-quinone was actually the culprit: look inside fish. The team obtained salmon which died of urban runoff mortality syndrome in the wild, closely examining them for signs of the chemical. The team also took coho salmon from two local creeks and reared them at the Puyallup Research and Extension Center of Washington State University, where they could control the water supply with exact precision. Then, they were able to add in tiny amounts of 6PPD-quinone and observe reactions in a controlled environment. The results were the same as the wild.
Tire change — Kolodziej says the study presents “a tough question. Because we can’t control what’s in our tires.”
Changing tire composition would require large-scale shifts in manufacturing. Ultimately, Kolodziej hopes society can find “a way to come up with safer ingredients for tires.”
Meanwhile, Kolodziej says there are “obvious things” people can do now to improve salmons' prospects when they swim upstream.
“Don’t park your car in a river. Don’t throw your used tire in a river," he says.
It's unlikely that coho salmon are uniquely affected by the 6PPD-quinone. Further study is needed to fully understand how much damage this chemical does across the ecosystem. It is also unclear if the salmon we eat also has 6PPD-quinone in it.
The history of coho salmon intertwines with the history of America before the country was even born. The fish, which lives primarily in the Pacific Ocean, has been part of indigenous mythologies and livelihoods among the Coast Salish, Squamish, and Spokane for centuries. Beyond the money that salmon bring in, or the bellies they fill around the world, salmon are a crucial part of American culture. Their survival might depend on finding that safer ingredient.
Abstract: In the U.S. Pacific Northwest coho salmon (Oncorhynchus kisutch), stormwater exposure annually causes unexplained acute mortality when adult salmon migrate to urban creeks to reproduce. By investigating this phenomenon, we identified a highly toxic quinone transformation product of N-(1,3-dimethybutyl)-N-phenyl-p-phenylenediamine) (6PPD), a globally ubiquitous tire rubber antioxidant. Retrospective analysis of representative roadway runoff and stormwater-impacted creek of the U.S West Coast indicated widespread occurrence of 6PPD-quinone (<0.3-.1.9 µg/L) at toxic concentrations. These results reveal unanticipated risks of 6PPD antioxidants to an aquatic species and imply toxicological relevance for dissipated tire rubber residues.