Among the benefits to Klamath Basin fish following the largest dam removal project in the world will be fewer mass die-offs from parasites and bacteria, scientists say.
A team of researchers, led by Oregon State University fish parasitologist Sascha Hallett and Michael Belchik, a fisheries biologist with the Yurok Tribe in California, found that removing dams and restoring natural flows to other Northwest and Eastern U.S. rivers eliminated hot spots of infection for fish, leading to fewer large-scale die-offs.
Hallet, Belchick and their team, including scientists from state and federal wildlife and climate agencies, and from the Hoopa, Klamath and Karuk tribes, published their findings and predictions Oct. 27 in the journal Frontiers in Ecology and Evolution.
For more than 100 years, hydroelectric dams on the Klamath River between Oregon and California blocked native salmon and steelhead from reaching hundreds of miles of habitat. Once the third largest salmon-producing river on the West Coast, it is now home to several endangered salmon and steelhead runs. Four of the dams on the river, built between 1908 and 1962, are being decommissioned following decades of advocacy by tribes and environmentalists in the region. One of the dams was removed earlier this year, and the other three will be decommissioned in the coming year.
Although bacteria, parasites and their hosts in the Klamath Basin are native to the basin, the dams have disrupted their natural dispersion and the natural migration of native fish, causing spots where infection can spread in crowded fish habitats and spawning grounds, Hallet said.
“We don’t expect them to disappear,” she said of the pathogens. “But we anticipate the ecosystem will get back into equilibrium – to a time before the dams were put in place and disease was not such an obvious issue and salmon populations were thriving,” she said.
The Klamath Basin is expected to return to pre-dam conditions following the reconnection of hundreds of miles of river for the first time in a century. That will reduce many of the current parasitic and bacterial threats to the fish as water flows and the distribution of sediments are restored, the scientists found. But timing is key, and climate change could stymie improvements.
“Dam removal is only one component of the river restoration story,” the researchers wrote. “Following dam removals, the basin will continue to evolve in an altered climate realm.”
Fish reconnected to longer migration routes will have more exposure to bacteria and parasites, but they won’t be stuck in infectious hot spots anymore. The scientists said a particular hot spot of bacteria and parasites below the Iron Gate Dam five miles south of the California-Oregon border will likely be flushed out following the removal of that dam in early 2024. A mass die-off of fish in 2002 was caused by bacteria and parasites that proliferated near the dam following low water, high temperatures and crowding near the Iron Gate Dam.
“I think you are going to see fish accessing new habitat right away, and that is going to be a cause for celebration,” said Belchik in a news release.
The researchers will continue to document the health of fish species in the basin and the threat of parasitic and bacterial infections following dam removal, including the impact of rising water temperature and changes in the timing and availability of rain due to climate change.
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