And the researchers note that while 25 to 65 percent of migrating smolts make it successfully through the Snake-Columbia's entire hydropower system, only a tiny fraction—as little as 0.5 percent some years—return as adult salmon.
The findings are based on technology developed by the Pacific Ocean Shelf Tracking (POST) project, which is headed by the Vancouver Aquarium.
Sound signals from almond-size transmitters surgically implanted in the juvenile salmon were picked up by lines of underwater detectors placed along their migration route.
"For the first time we've tracked a critter the size of a hot dog, 1,553 miles (2,500 kilometers) from the Rocky Mountains up to southeast Alaska," said Jim Bolger, executive director of POST, part of the International Census of Marine Life, a global research network.
"One of the other surprises is how quickly they made it—these are the Michael Phelps of small fishes," Bolger said.
Ray Hilborn, a professor of fishery sciences at the University of Washington in Seattle, said the new findings will be controversial, not least because hundreds of millions of dollars are spent annually on salmon-driven fisheries issues in the Columbia River.
For instance, dam modifications to assist migrating salmon have led to much-improved survival rates since the 1970s, the study team suggests.
But Hilborn, who wasn't involved in the research, said he isn't surprised by the results.
Plummeting Chinook salmon numbers in the Columbia in the 1980s and 1990s were "often blamed exclusively on the completion of the Snake River dams and ongoing losses from hydropower," Hilborn noted.
But, during the same period, "the Fraser River Chinook declined just as rapidly and just as much," he said. "The Fraser had no dams, so what was the cause there?"
"I believe that everyone now accepts that all the Chinook salmon in the Pacific northwest [apart form Alaska] have been very negatively influenced by ocean conditions," Hilborn said.
Kintama Research's Welch agrees: "My personal judgment is that the real survival problems are out in the ocean, after the fish leave the rivers."
Possible negative factors include ocean warming and changes to salmon prey distribution, increased salmon predation by seals and sea lions, and lethal parasite infestations of wild smolts spread by coastal salmon farms.
Future tagging studies aim to pinpoint where Pacific salmon are perishing at sea, and unravel further mysteries of their migration.
"This really neat new technology allows us for the first time look at questions that were never before possible," Welch, the study author, said.
SOURCES AND RELATED WEB SITES