Waist-deep in the fast-running Gunnison River, below the towering rock cliffs of Colorado's Black Canyon, angler John Duncan drifted a tiny brown imitation caddis fly on the filmy surface of the crystal-green water. Suddenly a form rose from below and took the hook. As Duncan played in his leaping, twisting catch, he could tell by its green back, silvery sides and blazing red stripe that he had hooked a rainbow trout. Then Duncan saw something else: a jet-black discoloration on the fish's tail and rear section. The trout was clearly diseased. "I was shocked," says the veteran sport fisherman. "You don't...
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