From The North Woods Call
Biologists suspect illegally hatchery-reared brook trout have brought the potential for deadly whirling disease to the Au Sable River. Currently dormant whirling disease spores have been found in brook trout in the North Branch. They have not shown up in brown trout in the North Branch, or browns, brook, and rainbows in other branches of the river.
Conservation officers will investigate the possibility that someone purchased brook trout from a trout farm and released them in the North Branch without a permit. Mio district fish chief, Dave Smith, says the DNR routinely rejects applications for private trout plantings in the Au Sable and other rivers, to avoid introduction of diseases.
Biologist Steve Sendek points out the whirling disease could also have come from other sources. The disease organism, he explains, can survive for months on dry waders. Many Au Sable anglers fish western waters that are infested with this disease. The disease is known to exist in Michigan's Sturgeon and Tobacco river, both from private hatchery operations in the watersheds.
"It is an interesting situation, but we just don't know what it means," Smith says. He, however, is confident it is not the cause of the unexplained decline in the number of larger trout in the Mainstream, and North and South Branches. No fish, he explains, have been found with clinical symptoms of whirling disease.
Smith points out it can remain dormant for a long period, or forever. The greatest potential for an outbreak is if the spores are transferred to the Mainstream's resident rainbow population. Rainbows, Smith says, are far more vulnerable to whirling disease than brooks and browns.
The brook trout found with the spores were healthy, robust fish, according to Smith. If conservation officers trace the disease to introduced fish, the DNR will go to the fish farm where they were purchased to test for the disease. If it is discovered, all fish will be destroyed and the ponds disinfected.
Au Sable River trout will continue to be collected for routine disease testing, Smith says. RWOL
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