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Hundreds of California salmon die from pressure change

March 18, 2024  By Julia Hollister

The California Department of Fish and Wildlife (CDFW) announced that the mass deaths involved an expected 830,000 Chinook salmon.

The juvenile Chinook salmon released on Feb. 26 from its Fall Creek Fish Hatchery in Siskiyou County died in the Klamath River. According to CDFW spokesperson, Jordan Traverso, the cause of the disaster on March 2 is two-fold.

“It is isolated,” said Traverso deputy director of communications. “They suffered gas bubble disease from the diversion tunnel that goes through the Iron Gate dam.

It is believed the salmon died as they migrated through the Iron Gate Dam tunnel, where they may have encountered “severe pressure change” resulting from “environmental or physical trauma.”


Gas bubble disease is caused by an increase in the dissolved gas pressure above the ambient air pressure.

Traverso said gas bubble disease has occurred before, in 2014, but not in this area of California.

This tunnel diversion hasn’t been used for decades. It was built when the dam was constructed. The recent rains were not relevant to the death of the salmon. Traverso explains that the fish were released into Fall Creek alive, then died when going through a tunnel in the Iron Gate dam that is being removed.

“These 830,000 fish were fry, about one to two inches in length. It is unclear how many of them died, but we know it was a large mortality event,” he said.

Their bodies will remain in the river where they will return to the ecosystem either by being eaten (the disease does not pose a risk to other animals that might eat them) or decomposing.

“For all future releases from Fall Creek Hatchery, we will truck the fish around the dam and release them downstream to avoid this,” he said.

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