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. The bad news: unfiltered stormwater killed almost all of them. The findings 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.
