The issue of prespawn mortality in Coho Salmon was identified as early as the 1990s. Salmon are a valuable resource to Pacific Northwest fisheries and ecosystems. However, adult coho, which spawn in their birth streams, showed a suite of symptoms that rapidly resulted in death following fall storm events. Streams where prespawn mortality occurs are generally receiving waters for urban stormwater, conventional conditions in the streams support salmon (for instance, temperature and dissolved oxygen), and the dead fish showed no sign of disease. The signs pointed to stormwater runoff, and a study that was ongoing in December shows that stormwater is likely to blame. However, the exact compound or mixture of compounds responsible for prespawn mortality has yet to be identified. In the study, the U.S. National Oceanic and Atmospheric Administration and the U.S. Fish and Wildlife Service exposed fish to highway runoff with varying levels of pollutants. Even runoff collected well after the first flush still resulted in prespawn mortality. Read more.
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