Trout resist disease in Missouri River

Montana Fisheries crews found slightly fewer rainbow trout below Holter Dam on the Missouri River this fall, but say the findings are a good sign that whirling disease has not hit the fish population as hard as some feared.

The number of rainbows there remains near the long-term average recorded before whirling disease, drought and a tremendous spike in fish numbers in the late 1990s.

Crews also found that the population of trout 17 inches and longer remains high, while production of younger, 2-year-old fish increased.

Steve Leathe, Fish, Wildlife and Parks Department regional fisheries manager, said surveys documented a decline inrainbow production, but years of drought have affected the river, and the true effect of whirling disease cannot be known until the drought eases.

“It didn’t crash as deeply as we thought. There is resilience in the system,” Leathe said.

Craig Madsen, owner of Montana River Outfitters, said his business of guiding anglers on the Missouri was off a little last summer and he attributes that to changes in the fishing.

“People compare things to their memory of the golden years,” he said, and guides can’t match expectations formed six or eight years ago.

In the Wolf Lodge Creek-to-Craig section this year, electro-shocking crews found 2,500 rainbow trout per mile that were longer than 10 inches, compared with 2,900 in 2005. Leathe considered the decline barely significant.

“In the pre-whirling disease average from 1982 to 1998, there were 2,600 fish 10 inches and longer in that same section of river,” Leathe said.

The number of big fish is down slightly from 2005, but the count of 1,500 lunkers per mile is three times greater than the average during the pre-whirling disease period of 1982-99.

Also in the Craig section, production of yearly rainbow trout 7-10 inches long appears to be up to 1,000 per mile. The last couple of years there were 600- 700 per mile.

In the Pelican Point area several miles upstream from Cascade, crews found more 10-inch and longer fish than last year: 1,800 per mile, compared with a long-term average of 1,500 per mile.

“The yearlings looked better than we thought,” Leathe said.

This year, they found 850 yearling fish per mile compared with 890 per mile in 2005. The pre-whirling disease average was 1,100 per mile.

Researchers discovered whirling disease in North America in 1956. The disease is caused by a parasite that attacks cartilage in fish, resulting in deformities that cause fish to whirl.