Surprising study shows more salmon survive West's dammed rivers
Researchers are calling them the Michael Phelps of fish — spirited little salmon that are turning scientific theories upside down by swimming great distances at rapid speeds, while somehow surviving a series of dams once thought to be their greatest threat.
The scientists studying juvenile chinook and steelhead salmon in Canadian and U.S. waterways say they've discovered that fish that go through dammed rivers actually have a higher survival rate than those that travel in rivers without dams.
The finding, to be published Tuesday in the online journal PLoS Biology, challenges long-held beliefs that west coast salmon stocks are being harmed largely by a series of dams that line the Columbia River.
"This really stood conventional wisdom on its head," lead author David Welch said from Nanaimo, B.C. "Where we assumed the problems were isn't necessarily where the major problems are, so that may mean we need a re-think on what is the best approach to conserving these salmon stocks."
Using sound-emitting tags the size of an almond implanted in the fish, scientists tracked about 1,000 salmon as they moved over sensors in the Columbia, Fraser and Thompson rivers and eventually north along the continental shelf to Alaska.
They found some of the salmon - most just the length of a hot dog - could swim distances up to 2,500 kilometres in only a matter months, putting their pace at about a 'Phelpsian' two body lengths a second - a reference the researchers made to the record-setting Olympic swimmer Michael Phelps.
The researchers compared the survival rates of salmon navigating the Columbia's eight dams versus those that travel the dam-free Fraser River, finding that more survived the Columbia.
"The dams are universally thought to be the cause of the decline of salmon in the Columbia River," said Welch. "We can now show that most of the mortality happens after the fish leave the river for the ocean."
Dams have been blamed for destroying certain salmon stocks by cutting them off from their spawning grounds and chopping them up as they pass through turbines. In one area, an estimated 40,000 to 60,000 sockeye returning to the Coquitlam River in British Columbia were blocked as they made their way to a lake to spawn and virtually vanished overnight.
Welch said over the years, billions of dollars have been spent to retrofit dams to reduce the impact on salmon survival.
The finding raises questions about what is causing the steady decline of the stocks, suggesting there are broader environmental factors at play.
Jim Bolger of the Pacific Ocean Shelf Tracking project said scientists knew in the 1990s that something was happening to the fish in the ocean, but this research shows that the mortality is occurring in the near shore soon after they leave the river system.
Grant Snell of the B.C. Salmon Marketing Council salmon stocks rise and fall naturally year to year and that hanging too much importance on one study is risky. And he's concerned that this might send the message that dams aren't harming the species.
"It sets off a couple of alarm bells if they're trying to say dams aren't a problem, I'm concerned," he said from Vancouver."The numbers of salmon always fluctuate within nature, so it's difficult to find the smoking gun."
Both Welch and Bolger say they're not suggesting dams are harmless to salmon, but that they must look at the whole picture.
Previously, scientists had only tagged larger marine species, like sharks, sturgeon and tuna to follow them in the ocean.
For the first time, this study has allowed them to tag such small animals and is giving greater insight into what happens to species that travel vast distances through many different waterways.
"It's really kind of turning the overhead lights on in the ocean," Bolger said from the Vancouver Aquarium. "We're able to understand where animals are moving and we will be able to understand better how they interact with each other."
Scientists expect to expand the tagging system to even smaller species as the technology gains a foothold around the world and global waterways become wired for study.
There are 300 sensors that run from San Francisco Bay up to Alaska, at an initial cost of about $3 million. Bolger said Canada is leading the movement by developing the technologies and installing curtains of sensors on both coasts, in rivers and bays.
It's not clear what the research might mean for future conservation efforts, but the researchers say fisheries managers should be looking more closely at what happens to the fish when they reach the ocean, an area Snell said is sorely lacking.
"This is going to start shifting the debate to saying what's happening in the ocean," said Welch. "The next stage is to focus on what is actually causing the mortality."