Harmful Habitats: The Premature Death of Coho Salmon Spawners in Pacific Northwest Urban Watersheds

Adult coho salmon (Oncorhynchus kisutch) return each fall to freshwater spawning habitats throughout western North America. 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. 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. Although toxic runoff is suspected to cause the mortality syndrome, this has not been directly demonstrated.  Here we exposed otherwise healthy coho spawners to 1) artificial stormwater containing mixtures of metals and petroleum hydrocarbons; 2) undiluted stormwater collected from a high traffic volume urban arterial; and 3) highway runoff that was first pre-treated via bioinfiltration.  We find that mixtures of metals and petroleum hydrocarbons are not causal.  By contrast, untreated highway runoff collected over nine distinct storm events was universally lethal to adult coho.  Lastly, the mortality syndrome was prevented when highway runoff was pretreated by soil infiltration to remove pollutants.  These results are the first direct evidence that toxic stormwater is killing adult coho in urban watersheds.