Controversial Study Says Dams Aren't Killing Off the Pacific Salmon

By Eliza Strickland | October 28, 2008 5:53 pm

Pacific salmonIn 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.

Related Content:
80beats: California’s Water Management Threatens Salmon With Extinction

Image: PLoS Biology

CATEGORIZED UNDER: Living World
  • peter

    I understood the primary issue was not so much with reaching the ocean as returning to the river.

    In my mind I just see little fishies swimming through the dams with the water flow while they are running, but swimming back up PAST the dams is much more difficult. Even if fish can get through the dams while they are generating, it would create so much of a bottle neck as to effectively destroy the population. That’s what fish ladders are for!

    Next study: implant transmitters in mature fish caught near the mouth of the river as they return for smolting, and track how far each fish gets. THAT, I suspect, will be the much more telling number.

  • Brian

    Peter, several groups have been doing similar studies with adult fish as they come back to the rivers for the last 10 years. Survival of adult fish is relatively high. The vast majority of mortality occurs when fish are in the river or soon after they enter the ocean.

  • Dan

    Science–even environmental science–should be unbiased. Those who just assume that the dams are causing all (or most) of the problems could wind up hurting rather than helping the salmon. It’s vital that we perform studies like this one, to find out what is really going on.

    With salmon populations as dangerously low as they are now, it is imperative we take meaningful steps, not emotional steps, to fix the problem. It would be pretty stupid to tear down all the dams, only to find (as this study suggests) that we haven’t thereby helped the salmon runs.

    What if, instead, banning coastal-water salmon farming is the magic bullet?

  • Tuco

    The scope of the problem with most of the cold water species (esp. anadromous) is larger than one study can provide meaningful data. Its the overfishing, dams, siltation, overgrazing in there headwaters, disruption of riparian buffers that ultimately put them at a position where they cannot recover on there own. We are the only hope for them. It will take a large cultural shift to put our natural resources at a high priority and lots of time. Humans living high on the razorback can be reluctant to change.

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