A recent study has shown that stormwater runoff from urban roadways is very poisonous for coho salmon, and can kill adult fish in just two and a half hours. However, Seattle scientists have also pointed out to a relatively easy fix in the study. They said that filtration through a simple, soil-based system can be helpful.

Julann Spromberg, a toxicologist for the National Oceanic and Atmospheric Administration and co-author of the report said, “It’s basically … letting the Earth do what it does so well, what it has done for eons: cleaning things up”. The report was carried by Journal of Applied Ecology on Thursday.

Since long, scientists have been suspecting that the mixture of oil, heavy metals and grime used in washing off highways and roads can be toxic to coho, but the study is the first ever to prove it.

Scientists began the research more than 10 years ago, when habitat-restoration projects started coaxing a trickle of coho back to a number of urban streams in the Puget Sound area. But several fish lost their lives before they could spawn. And the deaths apparently coincided with rainstorms that sent runoff surging via drainage pipes and into the waterways.

In places like Longfellow Creek in West Seattle’s Delridge area, around 90% of females died. Spromberg said that it seemed like something draining out of those pipes led to the deaths.

She along with her colleagues made an attempt to reproduce similar effect in the lab, but all in vain as the artificial mixture of oil and other chemicals had no effect on the fish. So, next they planned to try the real thing that is an actual runoff, collected at NOAA’s Northwest Fisheries Science Center from a downspout, draining a Highway 520 onramp near Montlake.

“When we brought out the real urban runoff: Bang! They were down, they were sick, they were dead,” said co-author Jenifer McIntyre, a researcher at Washington State University’s Puyallup Research and Extension Center.

“We should be seeing more and more of these systems in the future,” McIntyre said.