As many as hundreds of thousands of newly hatched Chinook salmon released into the Klamath River have died due to “gas bubble disease” caused by extreme changes in water pressure.
The young salmon fry were released amid the largest dam removal project in US history along the 257-mile-long river, which flows across Oregon and California. Four hydropower dams are being removed, reconnecting the lower and upper portions of the Klamath River for the first time in a century and allowing fish free passage along the river.
The removal is the result of a decades-long campaign by tribes, environmentalists and fishers to restore the river and its ecosystem to their natural state. One dam has been fully demolished and three others are scheduled to come down this year.
Last week, the California department of fish and wildlife (CDFW) released 830,000 hatchery-raised fall-run Chinook, only to discover days later that they were dying downstream of the 173ft Iron Gate dam, which is scheduled to be demolished soon. A tunnel at the dam’s base had been opened to allow the river to pass freely across it for the first time in a century, a step before the structure could be fully removed.
Many of the young fish, which were only 1-2in long, appear to have died passing through that tunnel, because the water pressure inside was too great for them. Officials do not know exactly how many of the fry have died, but expect a “very high mortality rate”, according to Jordan Traverso, deputy director of communications for the CDFW.
Dead fish caught downstream of the tunnel, in a trap that officials use to monitor their health and numbers, showed classic signs of gas bubble disease, including popped eyes, according to the CDFW. The deadly condition is caused when high pressure aerates water, saturating it with natural gases that form microbubbles inside the bodies of the fish.
Officials have verified that Chinook and coho salmon residing downriver from the dams were doing well, further affirming that the tunnel was the issue.
“The problems associated with the Iron Gate dam tunnel are temporary, and yet another sad reminder of how the Klamath River dams have harmed salmon runs for generations,” the CDFW said in a news release on Saturday.
The agency has more than 3 million fish, raised at its Fall Creek fish hatchery, that it plans to release downstream of the tunnel. Next year, when the dams and related infrastructure have gone, so too will the threat of gas bubble disease, Traverso said.
The agency has countered concerns that the deaths were caused by sediment stirred up by the emptying of reservoirs behind the dams, which some critics feared could be toxic to fish, noting that the water turbidity and dissolved oxygen levels were suitable before releasing the salmon.
Officials had planned for sediment to be disturbed in the process of demolishing the dams, said Mike Orcutt, the fisheries department director at Hoopa Valley Tribe. And scientists have modelled how long it could take for the sediment to flow and disperse through the river and its tributaries, planning for contingencies.
Orcutt said it was important that federal and state agencies continue to keep tribal governments informed, and to consult with them as the river’s restoration unfolds. The Fall Creek hatchery, which was built as part of a settlement agreement with PacifiCorp, the former owner of the dams, is expected to remain operational for about eight years – but that might not be long enough to fully restore salmon populations throughout the river and its tributaries, Orcutt said.
Moreover, the two most northerly dams on the Klamath, the Link River dam and the Keno dam in Oregon, will not be removed. “Those are still major barriers for passage of Chinook salmon,” Orcutt said. The dams have fish ladders to allow the salmon to pass, but scientists are still studying the effectiveness of such infrastructure.
“There are a lot of challenges still before us,” Orcutt said.
As global heating brings more intense droughts, heatwaves, wildfires and other extreme weather, tribal, state and federal agencies will have to continue adapting new strategies to restore the ecosystem.
Before the damming of the river during the first half of the 20th century, the river’s fall and spring salmon runs were among the largest in the country, sustaining Indigenous tribes across its watershed. The dams blocked salmon-spawning habitats and disrupted the natural flow of the river. This, along with water diversions to agriculture upstream, decimated salmon populations.
In 2002, more than 70,000 salmon died in one of the largest fish kills in the US west, after the federal government allowed water to be diverted to farmers and ranchers, despite concerns that cutting the rivers’ flow would harm endangered fish. The event galvanised tribes, including the Klamath, Karuk, Hoopa Valley and Yurok, and environmentalists, who pushed for dam removals and other efforts to restore the river.
Removing the dams and restoring the river’s natural flow will also eliminate hotspots of parasitic and bacterial infections, leading to fewer large-scale fish die-offs, according to a study published in October.
