Based on results from research conducted on the Missouri River and Little Prickly Pear Creek in 1997, state fisheries biologists have discovered that the potential for whirling disease infections appears to be most prevalent when water temperatures range between 50 and 60 degrees Fahrenheit.
"Real cold water appears to be safe," says Dick Vincent, whirling disease coordinator for Fish, Wildlife & Parks. "Infection seems to peak around 55 degrees. It then declines to near zero when water temperature exceeds 60 degrees."
The results could mean strains of wild rainbow trout that hatch and spend the first few months of life in water colder than 50 degrees stand a good chance of survival.
Whirling disease is a potentially fatal ailment that infects members of the trout family. Trout are more susceptible to infection during the first year of life. But there appears to be some indication that a small exposure to the disease when young allows trout to build up an immunity to it.
Discovery of the water temperature-whirling disease connection comes from data collected last year on the Missouri River and a key spawning tributary, Little Prickly Pear Creek. Both are located downstream of Holter Dam between Helena and Great Falls. The critical water temperature appears to be 55 degrees.
"A little above 60 degrees and the infection rate goes to zero," Vincent says. Of course, as water temperatures climb in the 60s, rainbow trout don't do so well, either. Water warmer than 65 degrees is not very trout friendly. That's why the other end of the temperature scale is more important.
"The real key is below 50 degrees," Vincent says. "In the first few months, trout have to stay out of water in the 50 to 60 degree range."
Whirling disease attacks cartilage in young fish before it can turn to bone. It's a disease of the young.
From April to October last year, FWP placed young trout in cages, one cage in the Missouri, another in Little Prickly Pear. The fish were changed every 10 days. Fish in Little Prickly Pear were found to have the highest infection rates in late May to early June, when the water temperature is about 55 degrees. Another small increase in whirling disease infections occurs in late September to early October.
"From mid-June to late July," Vincent says, "infection rates declined to zero. It seemed to be directly related to increasing water temperatures."
Vincent has developed a whirling disease severity scale of 0 to 4, with 0 representing no disease and 4 the most severe cases. Fish in Little Prickly Pear infected last spring had an average level of 3. By mid-summer the infection rate had dropped to 0. The peak of infection in the Missouri registered about 1, or very low.
Fish put in cages in the Missouri also became infected, although not as severely as those in Little Prickly Pear, which plays host to an estimated 15,000 spawning rainbows each spring.
"The rate of infection peaked in caged fish on the Missouri at 23 percent," says Steve Leathe, FWP regional fisheries manager in Great Falls, "compared with 87 percent in Little Prickly Pear."
That's good news, Leathe says. "It indicates that young fish migrating from whirling disease free tributaries last summer probably suffered minimal impacts from whirling disease when they reached the river."
In addition, Lyons and Wolf creeks, tributaries of Little Prickly Pear, showed no signs of harboring the disease. That could mean a ray of hope for rainbows that hatch in the early summer in those small creeks.
"Fish that hatch in the mainstem of Little Prickly Pear probably got high doses of infection," Leathe says. "But fish coming out of Lyons and Wolf creeks later in the summer encountered much lower doses when they passed through Little Prickly Pear on their way to the river."
Biologists have found no evidence of the disease in other important Missouri spawning tributaries. The Dearborn River and Sheep Creek are on the clean list.
Nonetheless, the disease is slowly moving down the Missouri, though at a low infection rate. The disease has turned up in both brown and rainbow trout.
In 1996 biologists found the disease in the river near the town of Craig, but not farther downstream at Pelican Point near the town of Cascade, which is considered the end of the blue-ribbon trout section, about 35 miles below Holter Dam. One year later, however, the disease appeared at Pelican Point.
Rainbow trout numbers in the Missouri, so far, show no sign of declining from whirling disease. Population estimates of rainbow trout in the Missouri downstream of Holter Dam have remained at or above the river's long-term average since 1994.
Last year's data showed 4,361 rainbow trout per mile in the river near Craig. That's for fish larger than 9 inches long. The long-term average is 3,689 per mile.
Downstream near Cascade the numbers are 2,222 rainbows (9 inches and larger) per mile, compared with a long-term average of 1,817.
"This fall's yearling trout sample in the Craig area is critical," Leathe says. Yearling trout measure about 10 inches long. "Most Craig-area rainbows spawn in Little Prickly Pear Creek. We know the fish that will show up in the Craig area on the river as yearlings this fall were exposed to high whirling disease levels in Little Prickly Pear last summer, when they were young of the year. If the numbers of yearlings this fall are way below average, we'll start to get nervous."