PORTLAND, Ore. -- Pollution from our cars may be killing Coho.

A new study links car pollution and runoff from high traffic roads and highways to a rapid die-off of the threatened fish.

Scientists studying the threatened fish in the Puget Sound found large numbers of the fish dying.

They took video that showed Coho struggling in the water, gasping for air. They ultimately traced the die-off to contaminants from roads and cars running off during the rains and winding up in the watersheds.

The more the traffic, the higher the Coho mortality rate.

"In the most densely urbanized areas, when the runoff reached the streams upwards of 40 percent of the adult Coho were dying before they could spawn," explained Michael Milstein with NOAA Fisheries Northwest Fisheries Science Center.

The research showed the contaminants impacted only Coho. Other species of salmon were not affected.

And while scientists don't know why that is, they did discover a way to protect the fish.

"If you take this runoff and filter if through a column of soil and gravel, it completely eliminates the toxic effects on the fish," said Milstein.

It's something the city of Portland and state of Oregon are already adding to new and improved roads. They call them bioswales or "green streets."

"We have Coho in every stream in Portland, for us the effect of storm-water on those fish and our ability to actually protect and recover them is a really big deal," said Kaitlin Lovell, with the Bureau of Environmental Services.

The city of Portland has been closely following the study up in Washington. The researchers who did the study plan to come to Portland in a couple weeks to meet with the Bureau of Environmental Services.

They will discuss potentially expanding the research study into Portland.