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. A valid Colorado fishing license is required for all persons 16 years of age or older. A Colorado State fishing license is required to fish in Rocky Mountain National Park. No other permit is necessary, however, special regulations exist.