The largest dam removal project in U.S. history has a problem: Dead salmon. As dam destruction continues on the Klamath River on the Oregon-California border, Oregon Public Broadcasting reports mass die off of salmon downstream. 

According to the California Department of Fish and Wildlife (CDFW), the fish kill happened last week after workers released a load of young fall-run Chinook salmon from the newly established Fall Creek Fish hatchery. All 830,000 individual fish died after passing through a tunnel inside the Iron Gate Dam, according CDFW.

“Gas bubble disease results from environmental or physical trauma often associated with severe pressure change,” the agency states in a recent press release. “There is no indication the mortality is associated with other Klamath River water quality conditions such as turbidity and dissolved oxygen, which were reading at suitable levels on Feb. 26 and the days prior to release.”

Gas bubble disease can be fatal, especially in small, developing fish called fry. In a sprawling watershed—the Klamath flows from southern Oregon into northern California—the diseased fish become disoriented, lingering on the the surface. They also develop abnormally dark skin that has a tendency to hemorrhage. Younger fish usually die before exhibiting extreme symptoms. 

The Klamath flows through southern Oregon and northern California before emptying into the Pacific Ocean. Adobe Photostock.

The recently built $35 million hatchery in Siskiyou County, California is part of the region’s salmon recovery efforts. In a bold effort to clear passage for migrating fish, the state is removing four dams on the Klamath River—including the Iron Gate Dam, where the hundreds of thousands of diseased hatchery salmon recently died. In its press release, CDFW called the incident “a sad reminder of how the Klamath River dams have harmed salmon runs for generations.”

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Future fish releases will happen below Iron Gate Dam, until the dam is fully removed in late 2024. According reporting from KRCR, the hatchery has an additional 3 million salmon set for release in late March.