The demand is so high some types of salmon struggle to keep up. But overfishing and human appetites are not the sole danger to these fish — another human desire may be driving their numbers down dramatically, a new study suggests.

The danger comes when salmon return from the ocean to rivers to spawn. Trapped in rivers, as many as 40-90 percent of salmon may be killed not through fishing, but by driving.

The culprit, it seems, is not the exhaust fumes coming from cars, the new research suggests. Rather, it is the car itself — its tires.