This is the first study of its kind and is carried out by an international team of scientists that includes researchers from the University of British Columbia. Their findings will appear in the October 28 issue of the open access journal PLoS Biology, published by the Public Library of Science.
The team compared the survival rates of out-migrating, juvenile spring Chinook and steelhead salmon from two river basins: the heavily dammed Snake and Columbia Rivers and the free-flowing Thompson and Fraser Rivers – both critical spawning grounds for numerous salmon species.
"It came as quite a surprise to us that the Fraser River salmon populations studied have lower survival than the Columbia River study populations"
Erin Rechisky, one of the study authors and a PhD Candidate in the UBC Dept. of Zoology.
Using technology that has only recently been available, the team electronically tagged juvenile salmon (smolts) and monitored their journey from freshwater into the ocean via a large-scale acoustic telemetry system called the Pacific Ocean Shelf Tracking (POST) array.
"It came as quite a surprise to us that the Fraser River salmon populations studied have lower survival than the Columbia River study populations," says Erin Rechisky, one of the study authors and a PhD Candidate in the UBC Dept. of Zoology.
Rechisky stresses that there is not yet sufficient evidence to reach any conclusions. "Clearly dams are not good for salmon. What is unclear is whether the Fraser River has a problem that cuts salmon survival to that of a heavily dammed river, or whether factors other than dams play a larger, unsuspected role in salmon survival."
Only in recent years have acoustic tags become small enough for scientists to implant them into juvenile salmon and track them as they migrate downstream. These innovations enabled the team to gather data on salmon smolts in the Fraser River. Before that, it was only possible to measure juvenile survival where salmon gathered in dam bypasses like those in the Columbia and Snake Rivers.
The lower Columbia and lower Snake Rivers currently have eight major hydroelectric dams combined. During the late 1930s when the first dams began to go up, salmon survival rates began to plummet, hitting mortality highs during the 1960s and 1970s due to a combination of warmer waters, fish-grinding turbines and new predators. Since then, the U.S. government has invested in major restoration measures to improve salmon survival rates.
Current conservation efforts have focused on helping smolts pass through the hydropower system. However, the scientific team aims to clarify with future studies using POST technology whether dam passage in itself has long-term detrimental effects that impact salmon's ocean survival.
The researchers note that threats beyond the rivers are taking a heavy toll on salmon. These include habitat destruction, competition with hatchery fish, harvesting and large-scale changes in ocean climate.