The only trout native to the park were the greenback cutthroat and the Colorado River cutthroat. The National Park Service stocked non-native Yellowstone cutthroat trout as late as 1968. These efforts to enhance recreational opportunities in National Park areas have been reconsidered. Since 1975, exotic or non-native fish have been removed and native greenback cutthroat and Colorado River cutthroat trout are being restored to park waters. Populations of at least four species of trout exist in the park - brown, brook, rainbow, and cutthroat.
Some suckers also inhabit the streams and lakes.
Only 48 of the 156 lakes in the park have reproducing populations of fish. Cold water temperatures and lack of spawning habitat prevent reproduction in high altitude lakes. Supplemental stocking is done only to restore native species to altered waters. Fishing success at high altitudes varies, even from waters known to contain fish. Restoration efforts have kept the possession limits to a minimum, and have resulted in increased trout populations in many areas within the park.