The problem, explained Pat Bigelow, a fisheries biologist at Yellowstone National Park, Wyoming, emerged in 1994 when lake trout were discovered in Yellowstone Lake. Why lake trout were introduced into the lake is a mystery. Perhaps, biologists speculate, some anonymous angler wanted to diversify the fish population in the lake but didn't think through the consequences. "It's an example of bucket biology," said Todd Koel, supervisory fisheries biologist for the park. (Related pictures: Trout vs. Trout in Yellowstone Lake.)

A Yellowstone cutthroat—coppery in color, back peppered with spots, a blush of pink by the gills—is a thing of beauty. But it is outmatched in size and outlived by the lake trout, which weighs in at over 50 pounds