Dams Not Main Cause of Salmon Collapse,
Study Says
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| James Owen for
National
Geographic News
October 27, 2008 |
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The first ever tracking of young Pacific salmon ocean
migrations suggests the main barriers to their survival aren't
river dams—but lethal obstacles at sea.
The controversial finding, which makes use of the latest in
fish-tagging technology, investigated the seaward migration of
Chinook salmon and steelhead—a type of trout—in North
America's two largest West Coast rivers.
Juvenile
fish equipped with tiny sound-emitting tags were released
in 2006 in the headwaters of Columbia River, which has
numerous hydropower dams on its system, and Canada's dam-free
Fraser River.
A team tracked the fish during the "smolt" stage of their life
cycle, when the juvenile fish begin to migrate—as they headed
for the Pacific Ocean and distant feeding waters off Alaska.
Salmon reach maturity in the sea, then later swim back
upstream to their hatching site to spawn.
Dramatic declines in Pacific salmon and steelhead in the
Columbia system have been blamed in part on eight hydropower
dams on the Snake.
Conservation groups are campaigning to have four of these dams
removed.
But David Welch of Kintama Research, Nanaimo,
British Columbia, reports in the journal PLoS Biology
that the Columbia's young salmon and steelhead have as good or
a better chance of survival as those in the Fraser River.
"This doesn't mean that dams are good for salmon, but it's a
very different result than what the science community would
have expected," Welch said.
"We'd have expected to see the survival a lot lower [in the
Columbia], but we're actually seeing it's somewhat higher," he
said.
Navigation Aid
While the new study focused only on the river stage of the
smolt migration, mortality levels at sea are also being
investigated by the team.
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.
Dams Blamed
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. |
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