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Whirling Disease

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Trout populations rebound after two decades of battle
By Taylor Anderson
Published: Aug. 15, 2011

It starts as a tiny parasite in water,
infects worms, and eventually makes
its way onto the bodies of fish before
infecting their nervous systems and
killing the fish. It’s called Whirling
disease, and for the past 17 years it’s
infected Montana’s waters and fish.
Whirling disease is found in both the
eastern and western U.S., and was
particularly devastating to anglers in
Southwest Montana because of its affects
on the trophy rainbow trout fishing
on the Madison and East Gallatin
rivers. Montana biologists discovered it
in the early 1990s on those rivers after
deformed fish, some with black tails,
were swimming in circles, being eaten
and dying.
Fish populations dropped by up
to 90 percent in some stretches of
river (though numbers are disputed).
Conservation biologists rushed to find
solutions, but stopping it proved difficult
because the dying fish released
the parasite into the river, restarting the
parasitic cycle.
The only thing to do was wait for the
natural path of the disease to run its
course, said Mike Vaughn, a fisheries
biologist with Fish, Wildlife and Parks.
“It was so widespread, and it was out
there in the wild, it’s not like we could
deliver some kind of knockout drug to
get the bad guys,” Vaughn said.
A 2009 report by several fisheries biologists
found mixed results and a complex
disease. Some trout, for instance,
died from the effects of the parasite,
while others weren’t affected.
After two decades, the disease seems to
have ebbed drastically, and populations
during the late 2000s have been strong,
according to FWP fish counts.
Pre-summer estimates pegged the
Upper Madison at 2,500 rainbow trout
per mile, and around 2,000 browns; the
Lower had an estimated 1,500 rainbows
and 1,000 brown trout per mile. East
Gallatin numbers near Bozeman have
been a bit lower, which Vaughn said
had been the case before the disease.
“We may not have the numbers we
had before Whirling disease,” Vaughn
said, “but the numbers are now fairly
strong…your average angler wouldn’t
notice a difference.”

Taylor Anderson is a staff writer at the Big Sky Weekly

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