(CNN)For decades, scientists say something alarming has been happening in the streams and rivers where coho salmon return from the Pacific Ocean to spawn along the West Coast.
After heavy rain events each fall, the fish have been turning up dead in huge numbers before they spawn, a mysterious phenomenon that has been the subject of intense research for years.
Now, scientists think they have found a key piece to this morbid puzzle -- and according to a new study, it's strewn all over North America's roadways.
It starts with a chemical antioxidant known as 6PPD, used in tires around the world to make them last longer.
However, as tire treads break down over time and leave behind bits of microplastics on roads, the 6PPD in them reacts with ozone to become a different chemical -- a previously unreported byproduct called 6PPD-quinone, scientists say.
This chemical is toxic to coho salmon. Researchers found its presence in roadway runoff samples taken from across the West Coast, leading them to conclude it's likely the main cause of the population decline.
The findings were published Thursday in the journal Science, marking a crucial step toward ensuring the survival of these salmon, the scientists said.
"We believe that 6PPD-quinone is the primary causal toxicant for these observations of coho salmon mortality in the field," said Ed Kolodziej, the lead investigator for this study. "It's exciting to start to understand what is happening because that starts to allow us to manage these problems more effectively."
Coho salmon return from the Pacific Ocean each fall to spawn in streams and rivers, and can be found from Alaska all the way down into California.
One of five salmon species f