The California Department of Fish and Wildlife (CDFW) said that 830,000 young Chinook salmon, released from its Fall Creek Fish Hatchery in Siskiyou County, are presumed to have died due to gas bubble disease in the Klamath River.

On Feb. 26, CDFW released the fish into Fall Creek, a tributary of the Klamath River above Iron Gate Dam.

The fish were hatched at CDFW’s new, $35 million, state-of-the-art hatchery, which CDFW said represents California’s long-term commitment to supporting and restoring both Chinook and coho salmon runs on an undammed Klamath River.

The salmons experienced a large mortality based on monitoring data downstream. CDFW said in a statement Saturday there are indications the fish were killed by gas bubble disease that likely occurred as they migrated though the Iron Gate Dam tunnel, old infrastructure that is targeted for removal along with the Iron Gate Dam itself later this year.

An aerial view of the Iron Gate Dam and California Department of Fish and Wildlife hatchery facility along the Klamath River, south of the Oregon border. CDFW said it plans future salmon releases below the dam after 830,000 newly released Chinook salmon fry perished last week. (USGS)

Gas bubble disease results from environmental or physical trauma often associated with severe pressure change.

The CDFW said there is no indication the deaths are 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.

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

CDFW said 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.”

CDFW said it will plan all future salmon releases below Iron Gate Dam until it is removed.

“Poor habitat conditions caused by the dams and other circumstances such as this are reasons why CDFW conducts releases of hatchery fish at various life stages,” CDFW said.

CDFW’s Fall Creek Fish Hatchery continues to hold approximately 3.27 million healthy, fall-run Chinook salmon. Additional releases are planned later in the month.

The annual fall-run Chinook salmon production goal for the hatchery is to raise and release 3.25 million fish. The additional Chinook salmon remaining in the hatchery exceeds the annual production goal and will help offset losses experienced with the initial release of fry.