Scientists think they've figured out why so many coho salmon in the Pacific Northwest are dying—it's because of car tires. More precisely, researchers say that a lethal substance moves from tire particles on the road into storm drains and then ends up in streams, reports Popular Science. The issue has been vexing researchers for years, and the problem is alarming. For example, more than half of the salmon that return in the fall to Puget Sound streams die before they can spawn, notes a release from the University of Washington. For their study in Science, researchers exposed salmon to stormwater samples and eventually zeroed in on a tire preservative known as 6PPD. It works by interacting with ozone in the atmosphere, but that interaction also produces a chemical called 6PPD-quinone that is particularly lethal to salmon, explains the Seattle Times.
“We pretty much figured out that anywhere there’s a road and people are driving their car, little bits of tire end up coming off your tire and end up in the stormwater that flows off that road,” Ed Kolodziej of the University of Washington tells the Los Angeles Times. "We were able to get all the way down to this one highly toxic chemical—something that kills large fish quickly and we think is probably found on every single busy road in the world." The finding meshes with the fact that the die-offs tend to occur after it rains, when fish are often seen gasping for air at the surface and swimming erratically. So what now? An official with the National Oceanic and Atmospheric Administration calls the discovery "amazing" and says the tire industry can look for a less-toxic alternative, with pressure from government agencies if necessary. (Read more salmon stories.)