Juvenile coho salmon (Oncorhynchus kitsutch) sensitivity to environmentally realistic concentrations and durations of urban stormwater runoff

Jasmine Prat

Coho salmon (Oncorhynchus kisutch) are a sentinel species for the threat of urban stormwater runoff to aquatic ecosystems. Laboratory studies demonstrate that juvenile coho and adult spawners exposed to stormwater runoff display matching behavioral symptomology, blood dysregulation, and acute mortality. Research to date has focused on the pathophysiology and geographic distribution of the syndrome, as well as clean water soil infiltration strategies to remove pollutants. However, no assessment of coho salmon sensitivity to stormwater chemicals as a function of dilution had been conducted. The current study explores how real-world variations in storm duration and runoff dilution influence the vulnerability of juvenile coho (and, by extension, adult spawners) in freshwater rearing and spawning habitats. Juvenile coho were exposed to static runoff (1-24 hours) from multiple storms, each diluted in clean water to 1-25% stormwater concentrations. Results indicated that stormwater is highly lethal to juvenile coho and variable storms produce similar toxic effects. For runoff collected during three storm events, 24-hour exposures containing 25% stormwater caused (mean) 97.2% (SD = 5.25%) mortality. Mortality was observed at 5% but not 2.2% stormwater. The slopes of the concentration-response curves were not statistically different. Estimates of the median lethal cncentration (LC50) were 6.1 – 9.9% stormwater. Additionally, higher stormwater concentrations resulted in more rapid mortality; estimates of the median lethal time (LT50) ranged from 5.1 – 6.2 hours at 25% stormwater to 22.8 – 39.8 hours at 5% stormwater. Experiments manipulating duration of runoff exposure followed by transfer into clean water revealed that shortest (1-hour) exposure to 25% stormwater resulted in (mean) 3.3% (SD = 5.8%) mortality. The lowest tested concentration of stormwater (5%) caused (mean) 20% (SD = 10%) mortality in the 8-hour exposure period. Most behavioral symptomology and mortality in the duration exposures occurred after transfer into clean water, demonstrating a delay in adverse response particularly at exposure to lower stormwater concentrations. This study indicates that brief exposure to diluted stormwater results in significant mortality to juvenile coho and suggests that coho experiencing lethal exposures of stormwater in the field may develop symptomology and mortality decoupled from active exposure to stormwater toxics.

Juvenile coho salmon (Oncorhynchus kitsutch) sensitivity to environmentally realistic concentrations and durations of urban stormwater runoff

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