Growth and survival of pacific coho salmon smolts exposed as juveniles to pesticides within urban streams in western Washington, USA

Pesticides are frequently detected in urban streams, with concentrations often exceeding those reported in surface waters within agricultural areas. The authors studied growth, survival, and return rates of coho salmon (Oncorhynchus kisutch) smolts exposed to a pesticide mixture ("cocktail") representative of the pesticides most frequently reported within urban streams in western Washington State, USA, in fall through early spring. Exposure concentrations were selected to represent a reasonable worst-case scenario based on field monitoring data. Smolts were continuously exposed to pulses of the cocktail either from fertilization through swim-up (2007-2008) or from fertilization through smoltification (2007-2008 and 2008-2009), coded wire tagged, and released in 2008 and 2009. Pre-release endpoints (growth, survival, sex ratio, brain acetylcholinesterase activity, and gonado- and hepatosomatic indices) were not affected. However, the number of returning adults exposed to the cocktail to swim-up (0.90%, n = 42) was more than double that of unexposed controls (0.38%, n = 26) in 2008, whereas in 2009, fish exposed through smoltification returned in lower numbers (0.15%, n = 18) than controls (0.37%, n = 30). Variability in return rates among treatments between years was comparable to that observed in previous whole life cycle studies with Pacific salmon and other contaminants. Results suggest that exposure to pesticides in urban streams does not directly impair early life stages of coho salmon, and that additional studies incorporating releases of larger numbers of smolts across several years are necessary to adequately quantify effects on return rates.