Written by: Stephanie Blair, PhD student
Pacific salmon are an enduring icon of ecological resilience and adaptation, though population numbers are plummeting in the Puget Sound region. Pollution from urban stormwater runoff is a major threat to salmon recovery. Coho salmon are dying at high rates in urban streams before having the chance to spawn- a phenomenon known as coho “urban runoff mortality syndrome” (URMS). In 2011, research lead by NOAA’s Northwest Fisheries Science Center concluded that an unknown contaminant in stormwater was linked to the coho die-offs (Scholz et al. 2011). After nearly a decade, the mystery contaminant has finally been found: a common tire chemical called 6PPD was recently linked to coho deaths (Tian et al. 2021). 6PPD is a type of rubber preservative that prevents weathering and cracking along the surface of the tire. When 6PPD reacts with ozone, it forms a product toxic to coho called 6PPD-quinone.
Coho salmon are highly sensitive to 6PPD-quinone. A lethal dose is less than 1 part per billion, which is like adding ½ teaspoon to an Olympic-size swimming pool. Roadway runoff frequently contains 6PPD-quinone well above the lethal dose, leading to coho deaths within just a few of hours after exposure (Tian et al. 2021). As a newly discovered chemical, toxicology studies have not yet been performed to explain what makes 6PPD-quinone so deadly to coho salmon. However, a recent study published in the Canadian Journal of Fisheries and Aquatic Sciences points to damaged blood vessels in the brain as the primary mechanism of toxicity in URMS (Blair et al. 2021).
Prior to this discovery, behavioral and physiological clues pointed to cardio-respiratory impairment as a likely cause of death. Coho exposed to roadway runoff displayed a consistent suite of symptoms, such as surface swimming and loss-of-equilibrium, as well as a severe rise in hematocrit that dramatically thickens the blood (McIntyre et al. 2018). Hematocrit is the percent volume of whole blood taken up by red blood cells (the remainder of blood is composed of plasma). Normal hematocrit values in healthy fish vary from 20-40%. However, hematocrit levels observed in coho exposed to roadway runoff were well above normal levels- as high as 70-80%. This observation lead researchers at WSU Puyallup Research and Extension Center and Evergreen State College to examine how rises in hematocrit levels may be related to cause of death.
Fluorescent-labeled plasma proteins were introduced into the coho’s circulatory systems to test the “leakiness” of blood vessels in their brains following exposure to roadway runoff. The passage of plasma proteins into brain tissues is prevented by the blood-brain barrier, a highly selective border of endothelial cells lining cerebral blood vessels that protect the central nervous system from exposure to toxicants and pathogens. When the blood-brain barrier is compromised, plasma constituents can leak into brain tissues and destroy the delicate environmental conditions necessary for neurons to function. Depending on the size and location of blood-brain barrier disruption, the effects can range from slight to severe. For coho salmon exposed to roadway runoff, the entire brain demonstrated widespread accumulation of the fluorescent-labeled proteins, indicating that severe blood-brain barrier disruptions occurred shortly before death (Blair et al. 2021). The labeled proteins could also be seen leaking from the gills, contributing to a loss of blood plasma and rise in hematocrit.
It is likely that coho salmon are not the only species that are susceptible to urban stormwater runoff and 6PPD-quinone, but they appear to be the most sensitive species. Chum salmon do not develop URMS symptoms that lead to rapid death, as seen in coho, but may be impacted sublethally (McIntyre et al. 2018). For example, both chum and coho populations are rapidly declining in representative urban creeks, such as Miller Creek in West Seattle (King County, 2021). Sublethal effects in fish exposed to urban runoff include impaired growth and cardiac function, which decreases success in capturing prey and avoiding predators and increases mortality risk. Ongoing research will test whether threatened Puget Sound Chinook salmon may be affected sublethally by 6PPD-quinone. Additional studies will explore how environmental variables, such as temperature and pH, may influence the toxicity of 6PPD-quinone and the triggering events that may lead to blood-brain barrier disruption in coho. Unlocking the mechanism of toxicity in coho urban runoff syndrome is a key step in the search for answers on how environmental contaminants may be contributing to salmon declines in the urban Pacific Northwest. Visit the Washington Stormwater Center Research Page (WSC, 2021) for more information on stormwater science, salmon toxicology and low impact development.
References:
Blair, S. I., Barlow, C. H., & McIntyre, J. K. (2021). Acute cerebrovascular effects in juvenile coho salmon exposed to roadway runoff. Canadian Journal of Fisheries and Aquatic Sciences, 78(2), 103-109. https://cdnsciencepub.com/doi/full/10.1139/cjfas-2020-0240
King County (2021). Salmon Monitoring Program- Community Salmon Investigation (CSI) for Highline. Accessed online, https://kingcounty.gov/services/environment/watersheds/central-puget-sound/miller-walker-creeks/salmon-monitoring.aspx
McIntyre, J. K., Lundin, J. I., Cameron, J. R., Chow, M. I., Davis, J. W., Incardona, J. P., & Scholz, N. L. (2018). Interspecies variation in the susceptibility of adult Pacific salmon to toxic urban stormwater runoff. Environmental Pollution, 238, 196-203. https://www.sciencedaily.com/releases/2018/04/180418100511.htm
Scholz, N. L., Myers, M. S., McCarthy, S. G., Labenia, J. S., McIntyre, J. K., Ylitalo, G. M., Rhodes, L., Laetz, C., Stehr, C., French, B., McMillan, B., Wilson, D., Reed, L., Lynch, K., Damm, S., Davis, J., & Collier, T. K. (2011). Recurrent die-offs of adult coho salmon returning to spawn in Puget Sound lowland urban streams. PloS one, 6(12), e28013. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0028013
Tian, Z., Zhao, H., Peter, K. T., Gonzalez, M., Wetzel, J., Wu, C., Hu, X., Prat, J., Murdock, E., Hettinger, R., Cortina, A., Biswas, R., Kock, F., Soong, R., Jenne, A., Du, B., Hou, F., He, H., Lundeen, R., Gilbreath, A., Sutton, R., Scholz, N., Davis, J., Dodd, M., Simpson, A., McIntyre, J., & Kolodziej, E. P. (2021). A ubiquitous tire rubber–derived chemical induces acute mortality in coho salmon. Science, 371(6525), 185-189. https://science.sciencemag.org/content/371/6525/185.abstract
Washington Stormwater Center. 2021. https://www.wastormwatercenter.org/research/research-projects/
This article appeared in the Spring 2021 issue of Water Currents News. Subscribe today!