Coho Salmon Are Choking to Death on Urban Pollution—but There’s a Fix

A new study finds that filtering storm water runoff through gravel, sand, and compost can save the endangered species.

A coho salmon carcass in a small coastal Oregon river. (Photo: Justin Bailie/Getty Images)

Oct 10, 2015· 1 MIN READ
Taylor Hill is an associate editor at TakePart covering environment and wildlife.

A toxic cocktail of oil, gas, brake dust, and urban sludge that contaminates the Pacific Northwest’s river system during rainstorms can kill the region’s endangered coho salmon in a few hours, a new study has found.

But wait. In the same study, conducted by the National Oceanic and Atmospheric Administration Northwest Fisheries Science Center in Seattle, researchers found that if the same pollution was run through a simple filtration system of sand and soil, the toxic effect on fish was practically eliminated.

“Untreated urban runoff is very bad for salmon health,” Julann Spromberg, a research scientist at NOAA, said in a statement. “Our goal with this research is to find practical and inexpensive ways to improve water quality. The salmon are telling us if they work.”

For more than a decade, scientists have known that urban runoff was most likely damaging fish species such as the coho salmon, which were dying off in high numbers before they could spawn. One report focused on West Seattle’s Longfellow Creek, where each year, up to 90 percent of the female salmon were dying before spawning.

In their initial experiments, the scientists mixed crude oil, metal, and other substances to make a synthetic “urban runoff” cocktail. But when they ran it through the test tanks, the fish did fine—no mortalities.

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So the scientists decided to go to the source, capturing actual runoff from a downspout along highway 520 near Montlake, Washington. When the team ran that through the tanks, the fish became weak and sick—some died within three hours, and most were dead by the following day.

It was an unexpected result, and the scientists theorize it could show that yet-unknown toxins from automobile exhaust, metals, and other substances coating roads could be the real culprits behind the fish declines.

“The recurring coho spawner deaths have been a high-profile mystery for many years, and we’re now much closer to the cause,” said Nat Scholz, manager of the NOAA ecotoxicology program and a coauthor of the study. “Although we haven’t identified a smoking gun, our study shows that toxic storm water is killing coho.”

But whatever the culprit is, the researchers already have a solution—filter runoff through a 55-gallon drum barrel filled with gravel, soil, and compost, and the fish survive. The mixture is basically what filtered rainwater was before the advent of paved, impermeable surfaces that push water swiftly into drains and culverts eventually to rivers and oceans.

The study found that if these “filtration columns” are integrated into urban development, coho salmon could thrive.

“If we can incorporate clean water design strategies into future growth, as some transportation projects are already doing, wild salmon might have a chance,” Scholz said.