Low concentrations and short durations of road runoff are lethal to coho salmon

Urban stormwater runoff contains a complex mixture of toxicants that is acutely lethal to coho salmon (Oncorhynchus kisutch). Although it is unknown what chemicals in the mixture are responsible for the toxicity, the land use most strongly related to rates of mortality in the field is roads -- particularly busy arterials. Juvenile and adult coho salmon experimentally exposed to 100% collected road runoff die within a few hours of exposure. In field studies, dead coho spawners are observed following rain events, but it is unknown how dilute are the chemicals causing the mortality, nor how quickly these mortalities occur after exposure to runoff. To better understand the lethality of road runoff to coho salmon, we experimentally exposed juvenile coho salmon to dilutions of road runoff collected from a busy arterial in Seattle, Washington, USA, varying the concentration and exposure duration. Runoff collected from three storm events showed 24-h median lethal concentrations (LC50) that were remarkably similar (6-10%). Runoff from an additional storm event (24-h LC50 = 4%) was used to determine minimum exposure durations to cause mortality. Factorial exposures of 1, 2, 4, and 8 h were conducted for runoff concentrations of 5%, 11.2%, and 25% and survival monitored up to 24 h. Nearly all mortality occurred after fish were transferred to clean water. Mortality (24-h) occurred for exposures as brief as 1 h at 25%, 2 h at 11.2%, and 8 h at 5%. No mortality occurred following 1-h exposure to 11.2% and following ≦4-h exposures to 5%. All other combinations of exposure time and concentration caused mortality for at least some individuals. By linking these results to water chemistry and observed rates of mortality in urban creeks, we may begin to estimate the amount of treatment required for runoff entering streams where coho salmon spawn.