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Safe Passage for Salmon?

28 October 2008 (All day)
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Pacific Northwest Laboratory

Saved from damnation? Dams might not be taking a heavy toll on Chinook salmon like these, a new study says.

A surprising new study finds that massive hydroelectric damns in the U.S. Pacific Northwest might not pose as great a risk to migrating young salmon as previously thought. If the results hold up, they could mean that hundreds of millions of dollars spent on providing ways for fish to bypass dams are paying off.

Salmon populations once numbered in the millions in the Columbia River, but now 13 populations of the fish there are listed as threatened or endangered. Hydroelectric dams have long been blamed as a major cause for that decline because they supposedly prevent young salmon from traveling to the sea. In hopes of increasing survival rates, fishery managers have modified spillways on dams, changed the timing of water release, and even gone so far as to barge and truck young salmon around eight major dams on the Columbia.

To pin down the reasons behind the decline, fishery researchers have spent the last decade implanting rice-grain-sized radio frequency transmitters called PIT tags into migrating fish. The transmitters allow scientists to gauge fish survival rates at each stage in the journey. But to pick up the radio signals, PIT tag detectors must be within about a half a meter of the tags. So researchers have placed the readers in fish passageways around dams. That made it impossible to compare the survival rates of juvenile salmon in heavily dammed rivers, such as the Columbia and Snake, to undammed rivers, such as the Fraser and Thompson in British Columbia, Canada.

For their current study, David Welch, a fisheries biologist at Kintama Research in Nanaimo, British Columbia, and colleagues turned to a newer version of tagging technology that is part of the ongoing Pacific Ocean Shelf Tracking (POST) Project, which is tracking the abundance of 10 kinds of fish and invertebrates. The POST effort in the western United States has deployed a network of acoustic sensors from Alaska to California that can detect signals from almond-sized transmitters surgically implanted in fish. The acoustic signals from these tabs can be picked up by sensors as far as 800 meters away, which allowed Welch and his team to monitor fish in dammed and undammed rivers.

From 2004 through 2006, Welch’s team implanted acoustic tags in 1000 juvenile Chinook salmon and followed their journeys in the Columbia and Fraser Rivers. In a paper published today in PloS Biology, the team reports that salmon migrating past the Columbia River dams were as likely to survive their journey to the ocean as were fish in the undammed Fraser River. “It completely surprised all of the co-authors,” Welch says, adding that they expected to see lower survival rates for fish that needed to pass the dams.

“It’s a very, very intriguing paper,” says Peter Moyle, a fisheries biologist at the University of California, Davis. One possible explanation for the counterintuitive result, Moyle says, is that habitat loss or other factors are lowering survival rates in the Fraser River. The study also doesn’t look at survival rates of salmon swimming back up the rivers to their spawning grounds. “This is not a message that dams are okay and have no effect on survival,” Welch agrees. But at least for salmon swimming down stream, those dams may not be as dangerous as previously thought.