Twenty years ago, the rainbow trout of the Gunnison River were in serious trouble.

After whirling disease — a parasitic infection that can cause skeletal deformities and whirling swimming behavior that makes trout susceptible to predation — was accidentally introduced, the population of the fish plummeted from nearly 10,000 in a two-mile stretch of the river in the Gunnison Gorge National Conservation Area in 1993 to just 86 a decade later.

But the rainbows have since made a strong comeback, t...