Reaching spawning grounds is hard work for salmon and researchers from the University of British Columbia say fish forced to "sprint" through fast-moving water or other obstacles can suffer heart attacks.
The study found sockeye are more likely to die in the hours after being forced to swim fast and hard to push through turbulent waters below dams.
Researchers say so-called "burst swimming" creates severe stress that could lead to heart failure.
Female sockeye are especially prone, in part because females may be using more energy to produce eggs, hiking their sensitivity to other environmental challenges.
Biologist and lead author Nicholas Burnett says the study demonstrates how vital it is for salmon to have easy access around obstacles in the river.
The study is published in the latest issue of the journal Physiological and Biochemical Zoology.
