Over 800,000 Fish Presumed Dead After 'Large Mortality' Event at California River

Hundreds of thousands who were living in a California river are presumed dead from gas bubble disease.

On Saturday, the California Department of Fish and Wildlife (CDFW) announced that 830,000 fall-run Chinook salmon released from its Fall Creek Fish Hatchery in Siskiyou County just days before are believed to have died in the Klamath River.

The young salmon, released on Feb. 26 into Fall Creek near the California-Oregon border, experienced "a large mortality" event the agency determined "based on monitoring data downstream," according to CDFW's release.

The fish had been hatched at the Fall Creek Fish Hatchery as part of the state's "long-term commitment to supporting and restoring both Chinook and coho salmon runs on an undammed Klamath River."

The young salmon are believed to have died as they migrated through the Iron Gate Dam tunnel, where they may have encountered "severe pressure change" resulting from "environmental or physical trauma," per the department. Gas bubble disease, as defined by the National Institute of Health, "is caused by an increase in the dissolved gas pressure above the ambient air pressure."

The dam and tunnel, considered "old infrastructure," are targeted for removal this year, according to the department.

"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," CDFW noted. "The visual appearance of the dead fry detected by monitoring equipment points to gas bubble disease. Monitoring equipment documented other healthy yearling coho and Chinook salmon that came from downstream of the dam."

The California Department of Fish and Wildlife also points to the dam tunnel as "another sad reminder of how the Klamath River dams have harmed salmon runs for generations." Future fish releases planned for later his month will now occur below the Iron Gate Dam.

Fall Creek Fish Hatchery still holds approximately 3.27 million "healthy," fall-run Chinook salmon, per CDFW. It has an annual production goal of releasing 3.25 million of the species.

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California Gov. Gavin Newsom pledged in January to help restore the state's salmon populations by fast-tracking several projects to remove or bypass dams, CBS News reported. A release from the governor's office at the time noted that the governor's priorities included removing barriers, protecting water flows, and modernizing hatcheries.

"Salmon are an integral part of our shared history in California. Some of my youngest memories were of seeing the iconic fish up close with my father when I was growing up – an experience all Californians deserve," Newsome said. "We're doubling down to make sure this species not only adapts in the face of extreme weather but remains a fixture of California's natural beauty and ecosystems for generations to come."