The Chemicals in Your Car Tires are Killing Coho Salmon

(Creasman., 2022)

The West Coast of North America is known for our flourishing populations of Pacific salmon including Pink, Sockeye, Coho, Chum, Chinook, Steelhead, and Cutthroat salmon. Just think about how many people look forward to sitting down at a nice restaurant right along the water in the Downtown of Vancouver to eat a grilled piece of salmon with a maple glaze laying over a bed of an organic green salad. Yum. Sounds delicious to me. However, this luxury may become limited due to the declining populations of Pacific salmon. About half of the populations of Pacific salmon are in a state of decline with some populations being classified as endangered and others being classified as threatened. For example, as of 2016, Interior Fraser Coho have been termed as a threatened species. 

Pacific salmon are anadromous, which means that they begin their life in freshwater habitat, they migrate to the ocean, and then they return back to freshwater streams to spawn and die. This means that these populations are threatened by both the stressors affecting marine and freshwater ecosystems. In this article, I am only going to focus on the effects of tire-wear compounds on freshwater ecosystems but keep in mind that there are many other factors which affect the health of Pacific salmon. 

The life cycle of Pacific Salmon. (Washington Forest Protection Association., 2018)

Following precipitation and storm events (aka the typical rainy day in Vancouver) rainwater will run off of impermeable surfaces such as concrete roads and metal roofs to get directed into urbanized streams and creeks. This stormwater collects many of the pollutants sitting on these surfaces and introduces them directly into freshwater habitats as there is no current treatment plan for the filtration of the water. This introduced pollution has shown to be problematic to Coho salmon when considering a specific class of chemicals. Tire-wear compounds are used in the manufacturing process of the tires which go on our cars. As these tires age, they leak chemicals onto roads and eventually into freshwater ecosystems where salmon return to spawn. See this diagram I sketched out to visualize this process:

There is one specific tire-wear compound which has shown to cause the mortality of Coho salmon. This compound is called: N-(1,3-Dimethylbutyl)-N′-phenyl-p-phenylenediamine-quinone. Try saying that 10 times. For short, we just refer to this chemical as 6PPD-quinone. 6PPD-quinone is formed from the oxidation (just a chemical process) of 6PPD which is used as an antioxidant in tire rubber. Researchers have found that at toxic concentrations, 6PPD-quinone can cause the lethality of Coho salmon in only a matter of 2-6 hours. This is concerning as on average, 6PPD-quinone is at toxic concentrations in freshwater urban streams. 

The oxidation process of 6PPD in the presence of air. (Tian, et al., 2021)

6PPD-quinone is thought to be a major contributor to the mortality syndrome in Coho salmon. It has been observed that 40 to 90% of Coho salmon returning to freshwater streams die before they are able to spawn. Researchers have concluded that this result is due to the toxicity of stormwater that ends up in urban aquatic environments; however, it is still unknown if the lethality is due to one chemical alone or a combination of chemicals.

Other tire-wear compounds which require further research include 1,3-diphenylguanidine (which we can just call DPG) and hexa(methoxymethyl)melamine (in short, HMMM). Both DPG and HMMM are chemicals involved in the crosslinking process of tire rubber. Both of these chemicals are found in significant concentrations in urban stormwater where the mortality syndrome of Coho salmon is high. While an association does not allow us to conclude a causation, it should bring about speculation. 

I know that it is not possible to get rid of all the cars on the face of the planet. However, there are other mitigation strategies which we should consider. Roadside filtration systems such as Bioretention basins filter stormwater through gravel and soil before it enters urban streams. This system was shown to be effective in preventing the lethal effects of stormwater on Coho salmon and therefore, deserves to be further looked into. 

Bioretention basins. (Jusić, et al., 2020)

Ultimately, this issue should lay within the hands of chemists. It is them who have the capabilities to develop and research alternative chemicals which serve the same purpose as tire-wear compounds such as 6PPD-quinone, DPG, and HMMM but, are not toxic to Coho salmon. With this problem solved or even improved we can achieve healthier freshwater habitats and therefore, healthier Pacific salmon populations!

References:

Bendriem, N., Roman, R., Gibson, D., & Sumaila, U. R. (2019). A review of the fate of southern British Columbia coho salmon over time. Fisheries Research, 218, 10–21. https://doi.org/10.1016/j.fishres.2019.04.00

Creasman, J. (2022). Coho salmon. Wild Salmon Center. https://wildsalmoncenter.org/salmon-species/coho-salmon/ 

Johannessen, C., Helm, P., & Metcalfe, C. D. (2021). Detection of selected tire wear compounds in urban receiving waters. Environmental Pollution, 287, 117659. https://doi.org/10.1016/j.envpol.2021.117659

Jusić, S., Hadžić, E., & Milišić, H. (2020). Urban stormwater management – new technologies. In I. Karabegović (Ed.), New Technologies, Development and Application II (Vol. 76, pp. 790–797). Springer International Publishing. https://doi.org/10.1007/978-3-030-18072-0_90 

Peter, K. T., Tian, Z., Wu, C., Lin, P., White, S., Du, B., McIntyre, J. K., Scholz, N. L., & Kolodziej, E. P. (2018). Using high-resolution mass spectrometry to identify organic contaminants linked to urban stormwater mortality syndrome in coho salmon. Environmental Science & Technology, 52(18), 10317–10327. https://doi.org/10.1021/acs.est.8b03287

Species & lifecycle « pacific salmon foundation. (n.d.). Pacific Salmon Foundation. Retrieved May 16, 2022, from https://psf.ca/learn/species-lifecycle/ 

Spromberg, J. A., Baldwin, D. H., Damm, S. E., McIntyre, J. K., Huff, M., Sloan, C. A., Anulacion, B. F., Davis, J. W., & Scholz, N. L. (2016). Coho salmon spawner mortality in western US urban watersheds: Bioinfiltration prevents lethal storm water impacts. Journal of Applied Ecology, 53(2), 398–407. https://doi.org/10.1111/1365-2664.12534

State of salmon « pacific salmon foundation. (n.d.). Pacific Salmon Foundation. Retrieved May 16, 2022, from https://psf.ca/salmon/ 

Tian, Z., Zhao, H., Peter, K. T., Gonzalez, M., Wetzel, J., Wu, C., Hu, X., Prat, J., Mudrock, E., Hettinger, R., Cortina, A. E., Biswas, R. G., Kock, F. V. C., Soong, R., Jenne, A., Du, B., Hou, F., He, H., Lundeen, R., … Kolodziej, E. P. (2021). A ubiquitous tire rubber–derived chemical induces acute mortality in coho salmon. Science, 371(6525), 185–189. https://doi.org/10.1126/science.abd6951

Washington Forest Protection Association. (2018). Pacific salmon depend on a healthy, connected stream system. The Seattle Times. https://www.seattletimes.com/sponsored/pacific-salmon-depend-on-a-healthy-connected-stream-system/