Pre-spawn mortality is a phenomenon where adult coho salmon, Oncorhynchus kisutch, die before spawning when returning to freshwater streams to spawn.[1][2] It is also known as Urban Runoff Mortality Syndrome in more recent studies.[3][4] This occurrence has been observed in much of the Puget Sound region of the Pacific Northwest.[5] During fall migration, salmonids (trout and salmon) pass through urban watersheds which are contaminated with stormwater runoff.[2] As the coho salmon pass through these waters, many will show symptoms of lethargy, loss of equilibrium and disorientation, and die within a few hours of showing these symptoms.[2][5] These symptoms and behaviors are prevalent after rain events.[5] Mortality often occurs before salmon have the opportunity to spawn, which is determined by cutting open female carcasses and observing for unfertilized eggs.[5] Rates of pre-spawn mortality could impact the local salmon populations.[2] Based on model projections, if rates continue, populations of coho salmon could become extinct within the next few decades.[2]
Weight of evidence suggests coho salmon pre-spawn mortality is due to exposure to stormwater runoff. A ubiquitous tire rubber–derived chemical induces acute mortality in coho salmon.[6] [7] A negative correlation exists between the incidence of pre-spawn mortality and urban land use, specifically with the impervious surfaces, commercial property type, and local roads.[1][5] Exposure to metals and petroleum hydrocarbons displayed some response to pre-spawn mortality, while exposure to pesticides, diseases, and water quality variables did not elicit this effect.[7] This trend has not been observed in any other life history stage of the coho salmon nor in other salmon species in the Puget Sound region.[1]