|Document Type:||Journal Article|
|Title:||Coho salmon spawner mortality in western U.S. watersheds: bioinfiltration prevents lethal storm water impacts|
|Author:||J. A. Spromberg, D. H. Baldwin, Steve Damm, J. K. McIntyre, Michael Huff, C. A. Sloan, B. F. Anulacion, J. W. Davis, N. L. Scholz|
|Journal:||Journal of Applied Ecology|
|Keywords:||Pacific salmon,non-point source pollution,Stormwater runoff|
Adult coho salmon (Oncorhynchus kisutch) return each fall to freshwater spawning habitats throughout western North America. The migration coincides with increasing seasonal rainfall, which in turn increases stormwater runoff, particularly in watersheds with urban and suburban land cover characteristics (e.g., impervious surfaces). For more than a decade, field assessments in urban stream networks have shown that adult coho are dying prematurely, often at rates exceeding 50% of the entire fall run. The syndrome is characterized by a loss of orientation and equilibrium, leading to death on a time scale of a few hours. Such high levels of mortality are a significant concern for the long-term conservation and recovery of wild coho, particularly those distinct population segments vulnerable to ongoing and future development pressures in northern California, Oregon, Washington, and southern British Columbia. Although indirect evidence from forensic investigations and geospatial land use analyses has implicated toxic runoff as causing the mortality syndrome, this has not been directly demonstrated. In the present study we exposed otherwise healthy coho spawners to 1) artificial stormwater containing mixtures of metals and petroleum hydrocarbons, at or above concentrations previously measured in urban runoff; 2) undiluted stormwater collected from a high traffic volume urban arterial (i.e., highway runoff); and 3) highway runoff that was first pre-treated via bioinfiltration through experimental soil columns to remove pollutants. We find that mixtures of metals and petroleum hydrocarbons – conventional toxic constituents in urban stormwater – are not sufficient to cause the spawner mortality syndrome. By contrast, untreated highway runoff collected over nine distinct storm events was universally lethal to adult coho relative to unexposed controls. Lastly, the mortality syndrome was prevented when highway runoff was pretreated by soil infiltration. These results are the first direct evidence that toxic runoff is killing adult coho in urban watersheds. However, the causal chemical constituents remain as-yet unidentified.