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Whirling Disease Update


Fri Jan 21 00:00:00 MST 2000

FWP fisheries biologists learned recently that five of 86 young rainbow trout placed in test cages in the Smith River for a brief period last spring developed low level whirling disease infections. While unwelcome, the discovery was not unexpected.

"We're not surprised to find it in the Smith," said Steve Leathe, regional fisheries manager for FWP in Great Falls. "We expected it would be just a matter of time before it showed up there, since it's already in one of its major tributaries. The irony for us is that the Smith River wild trout populations appear to be on the increase." The disease has not been verified yet in the river's wild fish.

Whirling disease affects the cartilage of young trout and can kill them. It was first discovered in Montana in 1994.

Whirling disease was previously found in Hound Creek, a tributary of the Smith, and in the Missouri River, which the Smith joins near the town of Ulm, south of Great Falls. The Hound Creek finding came in 1996 in wild brown trout and was confirmed last year in caged rainbow trout. So far, about 350 Montana waters have been tested and infected fish have been found in 83. Whirling disease can occur in any water where the parasite that causes the disease completes its life history.

For 10 days last spring, young of the year rainbow trout, a couple of inches long, were set in cages in the Smith River at Camp Baker and 59 miles downstream at Eden Bridge. Then the fish were taken to a control-test location where they lived in tanks and are supplied with parasite free waters for three months to allow whirling disease -- if present -- a chance to develop. Finally, the fish were sent for analysis to a laboratory in Washington.

At Camp Baker, the popular launch site for the river's 60-mile float, whirling disease was discovered in three of 36 fish. At Eden Bridge, the take out spot, the disease developed in two of 50 fish.

It may or may not hurt populations in the long run. Only time and fisheries surveys will tell. In the short term, the disease should have no affect on the river's increasing trout populations.

"Provisional estimates from fall 1999 surveys suggest brown and rainbow trout population levels are at some of the highest levels ever seen," said George Liknes, FWP fisheries biologist. "Anglers should have great fishing over the next couple of years if the winter is mild and good water flows return to the Smith next spring and summer."

Meanwhile, Pat Byorth, FWP's Madison River biologist based in Bozeman, has documented a significant jump in yearling rainbow trout numbers in the Pine Butte section of the Upper Madison River. These 6- to 8-inch wild trout were hatched in the spring of 1998. "Our preliminary 1999 surveys documented the most yearling rainbow trout in the Pine Butte section since 1988."

Byorth cautioned, however, that the yearling spike could be short-lived. "While this could be the best yearling crop in a decade, we're pretty sure the numbers relate to 1998's high spring water flows. We think the high-runoff flows helped minimize exposure to the whirling disease parasite at a critical time during their early development." Byorth noted that yearling rainbow trout numbers farther downstream at Snoball and Varney remained within the 200-500 per mile range. "Most yearling rainbows at Pine Butte probably have low-grade whirling disease infections, we'll be very interested in watching how many of these trout survive this winter."

Dick Vincent, FWP's Whirling Disease Coordinator, said the Pine Butte rebound doesn't mean the Madison is staging a post-whirling disease comeback, but does offer a new clue that could lead to new management solutions. "It's all about timing," Vincent said. "These yearling numbers give us a lot of hope. If we can find a way to get trout to hatch at the right temperature, when high water levels can actually dilute the parasites' effectiveness, we may soon be able to fit a management scheme together with habitat enhancements to benefit young trout and get closer to a long-term solution."