Results from comprehensive testing at Mt. Shasta Hatchery show the whirling disease parasite is confined to a smaller group of fish than originally assumed. As a result, the California Department of Fish and Wildlife (CDFW) will lift quarantine and resume operations at the Mt. Shasta Hatchery.
Initial fears were that all of the approximately 1.1 million fish being raised at the Mt. Shasta Hatchery would have to be destroyed. The recent test results indicate that only approximately 2,500 fish need to be destroyed to curb spread of the disease.
“This is wonderful news,” said Stafford Lehr, CDFW Fisheries Branch chief. “We have raised and nurtured these trout to provide fishing opportunity, and now we can resume planting fish for anglers from Mt. Shasta Hatchery.”
During routine hatchery and fish health checks in May 2015, CDFW fisheries pathologists found that captive-raised fish at the Darrah Springs Trout Hatchery (located in the Battle Creek watershed east of Redding) tested positive for whirling disease. Prior to the discovery, trout were transferred from an infected raceway at Darrah Springs to Mt. Shasta Hatchery. CDFW placed 3 million rainbow and brown trout under quarantine out of concern that trout transferred from Darrah Springs Hatchery to Mt. Shasta Hatchery may have encountered environmental conditions that would allow the whirling disease parasite to complete its lifecycle at that location.
Whirling disease is caused by Myxobolus cerebralis, a protozoan parasite that destroys cartilage in the vertebral column of trout and salmon. It can be fatal to infected trout and salmon but does not affect humans or other wildlife or fish. Trout and salmon that test positive for the disease are still safe for human consumption, but fish that tested positive for the disease in the Mt. Shasta and Darrah Springs hatcheries will not be stocked for recreational fishing.
The whirling disease parasite is naturally present in some streams and rivers in California. Hatchery outbreaks are unusual (there have been none in northern California for two decades). The disposal of infected hatchery-raised trout is a necessary precaution to protect wild trout and salmon and to prevent the spread of disease to non-infected state waters where the fish typically would be planted.
The whirling disease parasite is believed to have entered Darrah Springs Hatchery via the water source supplying a portion of that hatchery. Some species of fish-eating birds can transmit the parasite and river otters can carry it on their fur from one water body to another. The parasite may have been transferred to the hatchery from nearby waters known to have whirling disease, as drought conditions cause wildlife to move to available water sources.
Testing at Darrah Springs Hatchery is still in progress, and the quarantine is still in effect. CDFW pathologists routinely inspect each of the 13 state-run trout hatcheries which raise approximately 10 million trout for California anglers statewide, as well as the nine hatcheries that raise over 31 million young salmon and steelhead.
For more information on whirling disease, please visit http://whirlingdisease.montana.edu.