Gas Bubble Disease Claims Fall-Run Chinook Salmon Fry in Klamath River

The California Department of Fish and Wildlife (CDFW) disclosed in early March that fall-run Chinook salmon fry, newly released from its Fall Creek Fish Hatchery in Siskiyou County, are presumed to have succumbed to gas bubble disease in the Klamath River.

 

The newly hatched fish, called alevins, live in the gravel for several weeks until they gradually absorb the food in the attached yolk sac. These juveniles, called fry, wiggle up through the gravel by early spring.

 

On Feb. 26, the CDFW released around 830,000 fall-run Chinook salmon fry into Fall Creek, a tributary of the Klamath River located above Iron Gate Dam. These fish were hatched at the recently constructed $35 million Fall Creek Fish Hatchery, reflecting California’s commitment to bolstering both Chinook and coho salmon runs on an unobstructed Klamath River.

 

Subsequent monitoring revealed a significant mortality rate among the salmon fry downstream. It is suspected that gas bubble disease was the cause, likely occurring during the fry’s passage through the Iron Gate Dam tunnel, aging infrastructure slated for removal alongside the dam later this year.

 

Gas bubble disease, also known as gas bubble trauma or gas embolism, is a condition that affects fish when they are exposed to sudden changes in water pressure. This can occur when fish pass through dams or other structures with rapidly changing water conditions, such as spillways or turbines.

 

When fish experience a rapid decrease in pressure, gases, which are usually dissolved in the fish’s blood and tissues, can come out of the solution and form bubbles. These bubbles can block blood flow, causing tissue damage and potentially leading to death.

 

Gas bubble disease can affect various organs in fish, including the eyes, gills and swim bladder. Symptoms may include abnormal behavior, such as swimming erratically or floating near the surface and physical abnormalities, like bulging eyes or distended swim bladders.

 

In severe cases, gas bubble disease can result in mass fish die-offs, particularly in areas where fish are concentrated, such as fish hatcheries or downstream of dams. Efforts to mitigate gas bubble disease typically involve managing water flows and reducing the impact of structures that cause rapid pressure changes in waterways.

 

Despite suitable water quality conditions, including acceptable turbidity and dissolved oxygen levels, as observed on Feb. 26 and the preceding days, the visual appearance of deceased fry detected by monitoring equipment suggests gas bubble disease. Conversely, the same equipment recorded healthy yearling coho and Chinook salmon originating from downstream of the dam.

 

The issues linked to the Iron Gate Dam tunnel serve as a temporary setback and underscore the long-standing impact of the Klamath River dams on salmon populations. The CDFW announced its intentions to conduct all future salmon releases below Iron Gate Dam until the infrastructure is removed. Such challenges, alongside habitat degradation caused by dams, necessitate CDFW’s staged releases of hatchery fish.

 

The Fall Creek Fish Hatchery currently houses approximately 3.27 million healthy fall-run Chinook salmon, with additional releases slated for later in the month. The hatchery’s annual production goal for fall-run Chinook salmon is 3.25 million fish distributed across fry, smolt and yearling stages. The surplus stock in the hatchery will help mitigate losses incurred during the initial fry release.

 

Gas bubble disease in fish does not pose a direct health risk to humans who consume them nor does it pose risk to other wildlife that consume them. Gas bubble disease affects the internal organs and tissues of fish but does not typically impact the meat or flesh of the fish itself. Therefore, consuming fish affected by gas bubble disease is generally safe for humans.

 

For more information, please visit https://wildlife.ca.gov.