An urban stormwater runoff mortality syndrome in juvenile coho salmon

•Freshwater phase juvenile coho salmon are sensitive to urban stormwater runoff.•Symptoms of increased surfacing progressed to loss of equilibrium and mortality within hours of exposure.•Advanced symptomology was associated with a loss of ion homeostasis and increased hematocrit.•Fish transferred to clean water upon appearance of behavioral changes did not recover.•Coho salmon are a vulnerable indicator for urban non-point source pollution. Untreated urban runoff poses significant water quality threats to aquatic organisms. In northwestern North America, ongoing development in coastal watersheds is increasing the transport of toxic chemical contaminants to river and stream networks that provide spawning and rearing habitats for several species of Pacific salmon. Adult coho (Oncorhynchus kisutch) are particularly vulnerable to a stormwater-driven mortality syndrome. The phenomenon may prematurely kill more than half of the coho that return each fall to spawn in catchments with a high degree of imperviousness. Here we evaluate the coho mortality syndrome at the juvenile life stage. Freshwater-stage juveniles were exposed to stormwater collected from a high traffic volume urban arterial roadway. Symptoms characteristic of the mortality syndrome were evaluated using digital image analysis, and discrete stages of abnormal behavior were characterized as the syndrome progressed. At a subset of these stages, blood was analyzed for ion homeostasis, hematocrit, pH, glucose, and lactate. Several of these blood chemistry parameters were significantly dysregulated in symptomatic juvenile coho. Affected fish did not recover when transferred to clean water, suggesting a single runoff event to stream habitats could be lethal if resident coho become overtly symptomatic. Among coho life stages, our findings indicate the urban runoff mortality syndrome is not unique to adult spawners. Therefore, the consequences for wild coho populations in developing watersheds are likely to be greater than previously anticipated.