Whirling disease forces destruction of trout at two DFW hatcheries

By: Jim Matthews

Up to 3 million rainbow and brown trout could be destroyed at two Department of Fish and Wildlife hatcheries in northern California after the discovery of whirling disease at the Darrah Springs and the Mt. Shasta hatcheries in May. This would be a significant blow to the state’s recreational trout rearing program which produces 10 million trout annually.

During routine annual testing, the whirling disease parasite was discovered at Darrah Springs, and fish from the hatchery had been transferred from an infected raceway to a single pond at Mt. Shasta before the testing data showed positive for whirling disease.

“Prior to the May 15 quarantine date [for further testing], we had negative results for the last 25 years at all of our hatcheries statewide. This is a bad deal,” DFW fisheries branch chief Stafford Lehr said.

Lehr said 14,000 pounds of broodstock trout from the single pond with infected fish at Mt. Shasta were destroyed late in the week; 5,500 pounds of trout at Darrah Springs were also destroyed. The DFW is hopeful, especially at the Shasta facility, the parasite was restricted to isolated ponds and raceways at the two hatcheries and that the DFW will not have to destroy more fish.

Since the fish at Shasta had just been moved to the facility from Darrah Springs and were at the extreme downhill side, it would have been impossible for the parasite to spread to upstream raceways and ponds unless it was also at Shasta before the fish transfer. Lehr said Darrah Springs was a serious concern and more than 1 million trout there are in serious jeopardy, pending the results of extensive testing of fish from throughout both facilities.

“In a worst-case scenario, we would have to destroy all of the trout in both facilities,” said Lehr.

While he was gloomy about the prospects for Darrah Springs, he was “keeping my fingers crossed” about the fate of Shasta trout.

The disease is caused by a protozoan parasite that destroys the cartilage in the skull and spinal column on young trout and salmon. While the disease can be fatal in serious infections, most trout and salmon survive as the cartilage ossifies into bone as the fish age. The spoors of the parasite are released when the fish die and decompose. The spoors then enter tubifex worms found in the muddy areas of lakes or river backwaters and grow back into parasites in the gut of the tubifex worm. The parasites are then released and infect the next generation of trout.

Whirling disease is present in the watershed where both of the hatcheries are located. Darrah Springs is in the Battle Creek watershed east of Redding and Mt. Shasta sits in the upper Sacramento River drainage at the base of the mountain with the same name. While both hatcheries are on waters that have been free of the disease, the parasite can easily be moved between nearby waters. The parasite is believed to have entered Darrah Springs through a 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 while moving between waters. Lehr said drought increased the possibility that the parasite was transferred by a bird to the hatchery waters from nearby waters were going dry.

Whirling disease evolved with brown trout in Europe. When first inadvertently introduced into this country, there was concern that it would seriously impact or even wipe out rainbow and cutthroat trout populations. That did not happen. After initial population declines in infected watersheds (like the Madison River in Montana), the rainbow trout populations rebounded as trout developed immunities and/or were selected for fish that inhabited portions of the water as young where they didn’t come into contact with the parasite.

“California has never had a population affected in the wild,” said Lehr.

The parasite is found throughout the state, including throughout the Eastern Sierra Nevada.

Even knowing that the disease has had no impact on wild populations of trout, the DFW is reluctant to continue to raise and plant these trout into waters in the region fearing lawsuits would again cripple the state hatchery system. Whirling disease could be considered a ‘biological pollutant’ under the Federal Clean Water Act, and stocking out these fish into waters where it is not already prevalent would be grounds for a lawsuit. With no recourse, up to a third of the state’s trout production could be in jeopardy of being destroyed.

Because of this incident, Lehr said the DFW would likely be phasing out popular strains of rainbow trout that have shown greater susceptibility to whirling disease. That means Coleman, Shasta, and Eagle Lake strains of fish, all developed by the DFW for specific purposes, will be raised in lesser numbers or dropped entirely in favor of Hofer-strain rainbow, which is highly resistant to whirling disease.

The complex dual-host life cycle of whirling disease it is harmless to humans and all other wildlife; the DFW is looking for ways to utilize the trout that must be destroyed, including donations to food banks and public fish-outs.

Once testing is final and fish from infected raceways and ponds are killed, the DFW will face expensive and time consuming projects to remove the disease from the infected hatcheries. Lehr said such a process at Darrah Spring would cost in the neighborhood of $400,000 and could take three to four years to complete, and no trout would be produced from the facility during that time.

The DFW announced late last year it would have to cut trout production by 50 percent for 2015 because of budget reductions, and the whirling disease outbreak will likely reduce even further the number of trout the agency will be able to stock for anglers in 2015 and beyond.

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