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Rio Grande Cutthroats Tested for Whirling Disease

LAS CRUCES - Researchers from New Mexico State University are trying to protect the state's Rio Grande cutthroat trout from deadly whirling disease.

Bob DuBey, right, a New Mexico State University fisheries specialist, and Colleen Caldwell, a NMSU-based fisheries biologist with the U.S. Geological Survey, examine a 4-month-old Rio Grande cutthroat trout under a microscope. The researchers are searching for a parasite that causes whirling disease. (NMSU Agricultural Communications photo by J. Victor Espinoza)

So far, whirling disease has yet to find its way into New Mexico's cutthroat trout populations. And, fisheries experts want to keep the state from losing this native fish species.

"We don't want cutthroats to go the way of rainbow trout, which have seen a complete collapse of some populations in the Midwest because of this disease," said Colleen Caldwell, an NMSU-based fisheries biologist with the U.S. Geological Survey's New Mexico Cooperative Fish and Wildlife Research Unit. "One thing we do know: Whirling disease is working its way south. That's why it's very important that we identify just how susceptible this native fish is."

The disease is caused by a tiny parasite, which passes from an aquatic worm to the young fish. The parasite attacks cartilage in the head and spine of young fish, causing nerve damage that kills the fish directly or causes them to spin, making them vulnerable to predators.

To find out how susceptible trout are to whirling disease, researchers exposed randomly selected 4-month-old Rio Grande cutthroat trout and rainbow trout to a range of disease concentrations. Scientists check exposed fish for visual and genetic symptoms of the disease.

Classic symptoms include uncontrollable whirling from the tail-chasing motion the disease causes and an abrupt darkening of the trout's tail region to almost black. In the lab, researchers also study tissue samples to confirm the disease is present.

"There's no question that Rio Grande cutthroat can get the disease," said Caldwell, who also serves as a faculty member with NMSU's fishery and wildlife sciences department. "We just need to determine at what exposure level." A final report, partially funded by the Whirling Disease Foundation and the U.S. Forest Service's Rocky Mountain Research Station in Albuquerque, is due next summer.

Whirling disease was first observed in the United States around 1958. It's now found in at least 20 states, including West Virginia, Pennsylvania, New York, Ohio, Michigan and most western states. The disease, first confirmed in New Mexico in 1999, has been moving southward through the introduction of infected fish, and by clinging to birds and fishing equipment.

Caldwell's results could help reintroduction efforts. Rio Grande cutthroat, the state fish, used to thrive in nearly every clear mountain stream and lake across the Rio Grande basin in New Mexico and Colorado. Now, it is limited to less than 7 percent of its former habitat. In a move to rebuild cutthroat trout populations, the New Mexico Department of Game and Fish is reintroducing hatchery-raised cutthroats into their traditional stream beds.

Caldwell and her students are evaluating stream conditions in the Carson and Santa Fe national forests, targeting disease-free streams for reintroduction. The analysis is part of Caldwell's long-term study on effects of whirling disease on native fish.

After three years of collection, the biologists have found several good sites for restoring Rio Grande cutthroat trout, as well as a number of places where there might be a high risk of whirling disease.

Not all streams are connected, which is a saving grace, Caldwell said. This isolation is a way to prevent introduction of the disease. Another precaution calls for anglers and boaters to make sure their equipment is properly disinfected before they get in the water. Boats, fishing rods and waders can help spread the parasite.

"Once whirling disease is introduced into the waters where Rio Grande cutthroat trout live, we won't ever be able to get rid of it," she said. "We have to be proactive."