Arctic grayling and bull trout, native Montana fish species of special concern, appear to be resistant to whirling disease, University of California at Davis researchers have reported.
In tightly controlled laboratory tests designed to rate and monitor specific levels of whirling disease exposures, researchers examined the reaction and vulnerability of Arctic grayling, bull trout, Yellowstone cutthroat trout, westslope cutthroat trout, and Deschutes rainbow trout.
Researchers found that arctic grayling demonstrated a natural resistance to the parasite and bull trout, while continuing to carry whirling disease spores, were able to ward off disease after initial infections.
"Arctic grayling appear to have an innate resistance to whirling disease," said FWP fisheries chief Larry Peterman. "The parasite has virtually no physical affect on grayling and bull trout can apparently defend themselves by somehow containing the parasite before it causes disease. This is very positive news for everyone who cares about these Montana natives."
The tests were conducted under the direction of Ronald P. Hedrick, a professor of Veterinary Medicine and Epidemiology at the University of California at Davis. The research was funded by Montana Fish, Wildlife & Parks and the U.S. Fish and Wildlife Service.
The Deschutes rainbow trout was tested because it is resistant to a parasite similar to the one that causes whirling disease. It was hoped the Deschutes rainbow trout, an Oregon native, would be armored against fatal whirling disease infection. Those hopes were dashed, however, upon Hedrick's discovery that the Oregon rainbow trout is as susceptible to whirling disease as other rainbow trout.
Whirling disease is a potentially fatal ailment of trout and salmon caused by a microscopic parasite that attacks the cartilage of young fish. Of all the fish in the trout family, rainbow trout appear to be least capable of defending themselves against the disease.
Hedrick's research team also established that Yellowstone and westslope cutthroat trout are clearly susceptible to infection, yet appear to be sightly less susceptible than rainbow trout to contracting fatal doses of whirling disease.
"This tells us that two of Montana's native fish--Arctic grayling and bull trout--can clearly go toe to toe with the parasite and win," Peterman said. "It also tells us that we have reason be concerned about our native cutthroat trout."
Peterman said the best hope for cutthroat trout and rainbow trout now appears to be managing around the disease. "The research tends to confirm what we're seeing in Montana," he said. "There is a broad spectrum of susceptibility to whirling disease that includes a number of factors, including how, when, and where the fish and parasite interact. It includes things like water temperature, time of year, and the age of fish. All of these factors relate in some way to that spectrum of resistance to whirling disease."
In recent years, Montana has embarked on extensive native fish recovery plans for bull trout, Arctic grayling, and westslope cutthroat trout. Each are fish species of special concern in Montana that have come under increased scrutiny for federal listing as endangered or threatened species.