A relatively simple, inexpensive method of filtering urban stormwater runoff dramatically boosted survival of newly hatched coho salmon in an experimental study. That's the good news for the threatened species from the Washington State University-led research. The bad news: unfiltered stormwater killed almost all of them. The findings, published in the journal Science of the Total Environment, are consistent with previous research on adult and juvenile coho that found exposure to untreated roadway runoff that typically winds up in waterways during storms resulted in mortality of 60% or more. For the coho hatchlings in this study, mortality from runoff exposure was even higher at 87%. When the stormwater was run through a biofiltration method -- essentially layers of mulch, compost, sand and gravel -- nearly all the coho hatchlings survived, though many of resulting fish had smaller eyes and body sizes than a control group.

 

To understand runoff impacts and possible interventions, researchers set up experiments with about 8,400 fertilized coho eggs, placing a subset under direct exposure to runoff produced by 15 storms in the Seattle area. That water contained a host of contaminants including dissolved metals and polycyclic aromatic hydrocarbons, a class of chemicals associated with fossil fuels. Coho eggs have a thick shell and all the embryos survived under the different treatments, but once they hatched, most of young fish, called alevins, in the set exposed to the runoff died. This high mortality, at various freshwater coho life stages might help explain why coho have disappeared from some streams completely.