In the 1980s, there were anecdotal reports of something fishy afoot: coho salmon near cities in the Pacific northwest were dying off at higher rates than salmon farther from the city. Since then, scientists have suspected that urban runoff was to blame, and a study published in Science last week (December 3) supports their hypothesis. Urban watersheds contain a chemical called 6PPD-quinone, which is a derivative of tire rubber and toxic to coho salmon. 

The Scientist spoke with University of Washington chemical engineer Edward Kolodziej, a coauthor of the study, about the findings.

The Scientist: Can you tell me a bit about what you were noticing with salmon populations before starting the study?

Edward Kolodziej

Edward Kolodziej: Anecdotal reports that urban stormwater killed coho salmon have been around since the 1980s. Beginning in the 1990s in the Seattle area, much...

TS: Tell me more about these die-off events.

EK: Salmon are born in fresh water and then coho salmon live in fresh water for maybe a year or so and then they go out to the ocean and come back two or three years later. They only come back in a relatively short period of time in the fall—the adults are returning to reproduce. After they reproduce, they die, so people are used to seeing dead salmon in the stream because the assumption was, ‘they’ve laid their eggs, they’ve died, that’s totally natural.’ What people started to figure out in the 1990s was actually they came back to reproduce and then when it rained, they died. They could cut them open they were full of eggs, so they hadn’t reproduced, and so that’s really where these observations began. A lot of work went on to understand where it was happening, how much, and if water was related. 

TS: Can you describe your process of narrowing down what could be in the water and hurting the salmon?

EK: We were pairing water quality analyses and toxicology observations together. And by doing that, we found roadway-derived chemicals in the water, which was very consistent with some of the landscape modeling and the GIS-based statistical analyses. Those are actually present during time periods when the salmon were dying. 

Toxicology observations, meanwhile, were showing that among all these different possible sources of chemicals, the only one that killed coho salmon was tire rubber. If we just soaked tire rubber bits in water, that was very lethal to the coho salmon. That’s a very complex mixture—our instrument sees more than 2,000 chemicals there. We had to just start separating it apart and isolating it into chemically simpler fractions. We just started to follow the toxicity as we kept separating and separating and separating. Eventually, we identified a chemical present in the tire leachate that was highly toxic to the coho salmon. We could then replicate the phenomena with a pure standard exposure. 

TS: I’ve never heard of this chemical. What do we know about it?

EK: All tires need a number of preservative chemicals in them to make them durable, last longer, and be stronger. At least since the 1930s, it’s been known that ground level ozone—this little concentration of ozone in the air—really degrades tire rubbers. There’s always been compounds in tires that are designed to soak up the ozone before it can react with the tire rubber. This antioxidant chemical, 6PPD, was identified in the early-to-mid 1950s. I’m not exactly sure when it became widespread in tire manufacturing, but it’s one of the most abundant, if not the most abundant, antioxidant compound added to tires to soak up ozone. Essentially, what happens is the reaction product—what’s present after the ozone reaction—is what we were seeing in the tire leachate and what proved to be very toxic to the coho salmon. 

Toxicology observations, meanwhile, were showing that among all these different possible sources of chemicals, the only one that killed coho salmon was tire rubber.

TS: In the grand scheme of factors that could be affecting fish populations—climate change or other known pollutants, for example—how big a threat does this novel chemical pose?

EK: I can’t answer that cleanly. It’s a big giant question that people devote their entire lives to. All struggling aquatic organisms often face multiple stressors, multiple threats, multiple difficulties. For systems where there’s roadway runoff contacting habitats that are important for coho salmon, this seems to be pretty important near Seattle. Some of the population modeling predicts localized extinction of coho from these exact creeks within 20 or 30 years because of this constant level of mortality in the returning fish. They just can’t sustain their population, and every year it gets smaller and smaller. 

TS: In this study, you were focusing on the coho salmon. Is there indication that other species that interact with the runoff may also die from the tire runoff?

EK: We do know that in these same watersheds where coho salmon die, chum salmon and cutthroat trout co-occur, and they don’t seem to die from the same storms that kill the coho. That field data tells us that we can anticipate differences in species sensitivity. Beyond coho salmon, we just don’t know how sensitive other species are, so that’s work that requires follow-up study. 

TS: What do you hope to do with these data, and do you hope that your study will inspire change in the way tires are produced in the future?

EK: Pretty immediately, what we’ll be doing is looking at these receiving waters, understanding the dynamics of this compound—how does it persist, what breaks it down, how much of it [there is], what its fate is out in the environment—so we have a lot of work to do just to understand the basic water quality elements of this compound now that we know about it. 

Concurrently, I’m sure there’ll be a bunch of related toxicology studies understanding the biology of these exposures. We work to generate data hoping that we can improve problems and solve problems, and I certainly hope that we can have a conversation about chemical ingredients used in consumer products like tires that is linked to this type of work. I would anticipate people will think about whether or not this chemical is the best choice for the desired attributes in consumer products like tires. 

Disposal of used tires is a giant materials-handling issue. Sustainable products are ones that we can reuse and recycle. For tires, they’re reused in playing fields, they’re mixed into asphalt, they’re used in building materials. To better enable sustainable products, we need to consider their end uses and their reuses as well. Chemical safety is very relevant for the decisions we make with respect to reuse. Enabling product sustainability comes from making sure everything in those products is environmentally benign and benign for people. That is an aspect of this finding that I think is important to think about.

TS: What personal significance does this work have for you?

EK: It’s a very motivating problem to work on. It’s devastating to me, all of my group, and all the people working on this to every single year see adult salmon dying when they return to spawn. I mean, these are really beautiful fish. If you went to the store, you might buy these fish. It’s very motivating for all of us to try and understand what it is that’s harming these fish because they really are that one percent of salmon which actually make it back to survive, and then they just can’t survive our water. All of my group is very motivated and excited by the prospect that we can improve our management of these important species. 

E.P. Kolodziej et al., “A ubiquitous tire rubber–derived chemical induces acute mortality in coho salmon,” Science, doi:10.1126/science.abd6951, 2020.

Editor’s note: The interview was edited for brevity.

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