Habitat and Life Stage Affect the Toxicity of 6PPD-Quinone to Coho Salmon (Oncorhynchus kisutch)

Garrett Michael Foster

doi:10.7273/000007180

6PPD-quinone, a chemical byproduct derived from a widely used antiozonant additive in tire rubber, is leached from tire wear particles into surface water through roadway runoff. In Pacific Northwest freshwater systems, during upstream migration to spawning grounds as adults and before outmigration as juveniles, salmon are exposed to runoff containing 6PPD-quinone. This exposure has been correlated with high rates of pre-spawn mortality in coho salmon. In laboratory experiments, juvenile coho salmon (Oncorhynchus kisutch) exhibit acute mortality when exposed to 6PPD-quinone, with a median lethal concentration (LC50) approximating 95 ng/L over a 24-hour exposure period. However, coho salmon encounter 6PPD-quinone in stream habitats that often differ from the standardized laboratory conditions used to derive the LC50. These habitat conditions include temperatures ranging from near freezing to over 20°C, stream pH values that are generally slightly alkaline but can be acidified by peat mosses, and variable ionic strength and concentrations of dissolved organic matter. Furthermore, coho salmon exhibit a range of activity levels, from idling in refugia to navigating rapids and waterfalls. All these ranges of habitat conditions are known to alter the toxicity of xenobiotics and therefore have the potential to improve or worsen the toxicity of 6PPD-quinone to coho salmon. Coho salmon also encounter 6PPD-quinone at all freshwater life stages — alevin, fry, yearling parr, and adults — each with distinct physiological and behavioral characteristics that may alter toxicokinetics relative to the coho parr used in previous studies. Additionally, genetic or population differences among coho from various streams may impact toxicological responses. To elucidate the effects of 6PPD-quinone on coho salmon, juveniles were exposed to the chemical across ranges of six environmentally relevant conditions: loading rate, temperature, pH, ionic strength, dissolved organic matter, and flow velocity. Coho salmon at five different life stages and from two creek populations were exposed to 6PPD-quinone. Temperature significantly altered the toxicity of 6PPD-quinone (likelihood ratio test (LRT); p < 0.01), whereas conductivity, pH, natural organic matter concentration, and flow velocity did not (LRT; p > 0.05). Episodic 6PPD-quinone exposure during embryonic development significantly altered embryonic morphology (t test; p < 0.001) and caused post-hatch mortality (t test; p < 0.0001) and differences in swimming behavior (t test; p < 0.05). Alevin were less acutely sensitive to 6PPD-quinone than swim-up fry (LRT; p < 0.01), and fry were less sensitive than yearling parr (LRT; p < 0.01), while yearling parr and returning adults had similar sensitivity (LRT; p > 0.05). Coho alevin from two different creek populations had significantly different sensitivity to 6PPD-quinone (LRT; p < 0.05). The results will help confirm whether proposed regulations are sufficiently protective for coho across life stages and the habitats they encounter throughout their range.

Habitat and Life Stage Affect the Toxicity of 6PPD-Quinone to Coho Salmon (Oncorhynchus kisutch)

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