Coho salmon returning from the ocean are dying before they can spawn because the water they’re swimming in is killing them.

Stormwater from fall rains picks up pollution like copper from brake pads and pesticides before flowing into coho spawning streams.

Almost 200 miles of streams around Seattle have an estimated pre-spawn mortality of one-third or greater, according to the recently released State of Our Watersheds Report. The report was compiled by the the treaty tribes in western Washington.

From the report:

NOAA and USFWS researchers have developed a model to predict areas of (pre-spawn mortality) PSM in Puget Sound using spatial analyses of land-use and coho PSM data. Based on their model, 269 stream miles or 56% of known coho distribution in the Green-Duwamish and Lake Washington basins below the major dams are predicted to have a PSM rate of 5% or more, with 141 miles or 29% predicted to have 35-100% PSM.

Because of poor ocean conditions in recent years, coho runs have dipped. Lack of good spawning and rearing habitat deprive wild coho populations of the flexibility to ride out bad ocean years.

The reduced spawning success that results from PSM has detrimental impacts on the persistence of local salmon runs. As human populations grow and urban centers expand into less developed regions, coho salmon in currently unaffected watersheds may also be affected. Therefore, an understanding of the cause of pre-spawn mortality is essential for the protection of salmon populations today and into the future.