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> MSU News
Whirling Disease experts note importance of Missouri tributary
March 25, 2003 -- By Evelyn Boswell, MSU Research Office
A thriving tributary between Helena and Great Falls seems to be the key as to why whirling disease hasn't decimated the rainbow trout population in the Missouri River, according to Montana researchers who've been predicting a collapse for years.
The Dearborn River is a tributary of the Missouri River. It has whirling disease, but so far it's at such a low level and its young trout are so abundant that it appears to be making up for a crisis in another Missouri tributary, Tom McMahon said after completing a five-year study in collaboration with Steve Leathe of the Montana Department of Fish, Wildlife and Parks (FWP).
McMahon is an associate professor in fish and wildlife management at Montana State University-Bozeman. Leathe is the FWP's Region 4 Fisheries Program Manager.
The study looked at the Missouri River and three of its main spawning tributaries, the Dearborn River, Little Prickly Pear Creek and Sheep Creek. Together known as one of the top three trout fisheries in Montana, the tributaries sent some 75,000 fry and 150,000 yearlings into the Missouri in 1998-99.
"The low infection thus far in the Dearborn River is particularly significant given that an estimated 8,000 rainbow spawn in this tributary," the researchers wrote in a summary of the project.
Whirling disease attacks the cartilage of young fish and causes them to swirl in a circular pattern. Research shows that it primarily strikes rainbow trout during their first two months of life. Where infections are severe, young fish suffer high mortality and adult populations may plummet.
Whirling disease was first detected in Montana in 1994 and the Missouri River in 1996. It is now found in more than 100 Montana streams and rivers, with the Madison River and Blackfoot River among the hardest hit, said Richard Vincent, FWP whirling disease research coordinator. However, the disease hasn't hurt rainbow trout fishing in the Missouri to this point. Anglers there have been grinning about the 17-inch fish they've caught and the record and near-record numbers of adult rainbow trout they've seen.
"Since the Dearborn is so large and productive for juveniles, it's probably helping sustain the adult rainbow population," McMahon theorized.
But what happens when the Missouri River adults die off naturally? Can the Dearborn produce enough new rainbow trout that, when combined with the increased growth and survival of adults, they compensate for Little Prickly Pear Creek?
"That's the big question we don't really know the answer to," McMahon said.
Little Prickly Pear has lost about 80 percent of its young rainbow trout, one of the most severe infections recorded in Montana. Sheep Creek, like the Dearborn River, has shown little or no infection.
Surprised by the lack of infection in the Dearborn, FWP officials have tried to protect rainbow trout in the Missouri River from future devastation. The daily bag limit for rainbow trout in the upper river near Craig was reduced to only one fish in 2001, for example.
"The Department of FWP and other organizations like Trout Unlimited have done a lot of outreach to stress to anglers that they can do their part to reduce the spread of whirling disease by washing mud off their gear and drying it," Leathe added. "We have done some on-site signing on the Missouri and tributaries to remind folks of this, but the reality is that rainbows are being infected in the Missouri, and some of them will swim up the Dearborn to spawn, bringing the parasite with them. This cannot be controlled."
Evelyn Boswell, (406) 994-5135 or evelynb@montana.edu
Hi-Resolution Images or PDFs Available:
| [View or Download] | 1. | Sentinel fish cages were used to monitor the timing and rate that whirling disease spread through the Missouri River trout population. Photo courtesy of the Montana Department of Fish, Wildlife and Parks.
| | [View or Download] | 2. | Researchers used this and other sentinel fish cages in a five-year study of whirling disease in the Missouri River system. Photo courtesy of the Montana Department of Fish, Wildlife and Parks.
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