The acutely lethal toxicity of urban stormwater runoff to juvenile coho salmon

Abstract

Stormwater runoff is considered the leading source of non-point source pollution in watersheds under pressure from urban development. In urban creeks around Puget Sound, WA, several lines of evidence indicate that toxic urban runoff is responsible for high rates of premature mortality among adult coho salmon (Oncorhynchus kisutch) that return from the ocean in the rainy fall months to spawn. The current study is a detailed examination of the behavioral and physiological aspects of the urban mortality syndrome in juvenile coho. Juveniles were exposed to either clean (control) hatchery water or stormwater collected from a high traffic volume arterial roadway. Runoff-exposed fish showed the same progression of symptoms (distress) as previously documented for adult spawners in past field surveys. This behavioral sequence was characterized in detail, and discrete stages of distress were then used as phenotypic anchors for physiological analyses of blood and tissue chemistry along a gradient of symptomology, from outwardly normal (presymptomatic) to a several loss of equilibrium (near death). These analyses showed that urban stormwater exposures caused a dysregulation of blood ion and osmoregulatory homeostasis in juvenile coho salmon, as previously observed in adults. The commonalities across life stages suggest that juveniles can provide an experimental platform for identifying the chemical agents and toxicological mechanisms underlying the mortality syndrome in adults. This is an increasingly important conservation challenge for wild coho populations throughout rapidly urbanizing regions of the Pacific Northwest.