Juvenile salmon transported downstream on barges can lose the ability to migrate back to their breeding grounds, reducing their survivorship and altering adaptations in the wild, according to a study by University of Idaho researchers that appeared in the November issue of Ecological Applications.

Matthew Keefer, a biologist at the University of Idaho and lead author, and his colleagues Christopher Caudill, Christopher Peery and Steven Lee, tracked the movement patterns of adult salmon and steelhead trout along the Columbia and Snake rivers in Washington and Oregon. They found that, when compared to fish that migrated naturally, transported juveniles had lower survivorship as adults and were less likely to find their way home.

"Adult fish usually move steadily upstream toward their spawning grounds, but some will instead move back downstream over dams," says Keefer.

This phenomenon, called fallback by fisheries managers, occurs more often in adults that were barged out as juveniles than in those that migrated naturally, according the study.

"It's not clear if they're just running out of steam swimming up the river or if they get disoriented and move back downstream in search of cues from their home river," Keefer said.

The scientists believe that being carried on a barge prevents young fish from learning about important environmental signals during a formative time of their juvenile lives. A barge can take them the same distance in two to three days that would normally take them several weeks, Keefer said. The study suggests that traveling great distances by boat – in this study, at least 215 miles – appears to garble the natural cues these fish use to find their way home.

Keefer's results also suggest that transported fish are more likely to stray from their home tributary. The study notes that if these lost fish – often from hatchery populations – breed with another wild population, the resulting gene flow can reduce that population's evolutionary fitness.

"Salmon have a life history that represents a long legacy of adaptation to local conditions," Keefer said. "The fish are well-adapted to specific rivers, and when you dilute their unique genetic makeup, it can reduce the productivity of the overall population."

A satisfactory solution is difficult to find, Keefer said. Managers could barge fewer juveniles, but then more fish would die while trying to pass the dams. Another option is to slow down the barges so the trip resembles the time it takes juveniles to swim to the ocean. But boats are a stressful environment for fish, and the close quarters within the ships increase the risk of disease.

"It's tough to find a solution that could handle all the challenges in this system," Keefer says.

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