Two decades ago, rainbow trout in the upper Madison River were struggling. Whirling disease had been found in the stream, caused by a microorganism that latches onto fish. Brown trout aren’t affected by it, but rainbow trout are, and numbers of adult rainbows dwindled. In one section of the river, estimates of adult rainbows — those larger than 14 inches — fell to fewer than 150 a mile. That was about the time Tim Weiss, a fisheries technician with Montana Fish, Wildlife and Parks, started working on the Madison. A little more than two decades later, the density of adult rainbow trout in that river has changed dramatically.
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