In a new study that’s already generating controversy, researchers tracked more than 1,000 young Pacific salmon on their first journey to the sea, and found that those battling dams on the Columbia River fared no worse than the young fish with an easier path to the sea on Canada’s free-flowing Fraser River. The findings seem to contradict many previous studies about dams: Conservationists have blamed these obstacles for a large share of the shrinking salmon populations in the Pacific Northwest, and engineers have spent billions trying to make the dams less damaging to salmon [Science News].
The study used implanted transmitters to follow the juvenile salmon, called smolts, on their trips downriver, and found that only about 25 percent of smolts in both the Columbia and the Fraser survived the voyage and made it to the ocean. But environmentalists and several salmon biologists pounced on the study, suggesting that industry funding might have biased the results. These critics question the value of comparing the two rivers and say that the study doesn’t even address what many think is the dams’ biggest effect: stressed smolts dying after they reach the ocean [Nature News].
The lead author of the study, which was published in the journal PLoS Biology, stands by his results, but says people have to be careful about the conclusions they draw. “We’re not saying that the dams have never had an effect,” said David Welch, the lead author…. “What we all have to ask ourselves is, if survival is up to the level of a river that doesn’t have dams, then what’s causing survival problems?” [The Seattle Times]. Welch suggests that the dam bypass systems built for salmon may be proving effective, although other experts note that the Columbia’s dam operators have been under a court order since 2005 to release extra water in the spring and summer to help migrating fish.
Welch partially agrees with critics who say that his study didn’t account for stressed out salmon that died once they reached the ocean, saying: “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 [National Geographic]. Welch, who founded the company that makes the fish tracking devices, says he hopes future studies will follow the salmon into their feeding grounds in the Pacific to discover their ultimate fates.
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Image: PLoS Biology