Benjo:
The Montana regulations generally allow cutthroat to be kept. For example, the standard regulation for the western district allows anglers to keep 3 cutthroat of any size from lakes, and 3 under 10 inches from streams/rivers. Those general rules are subject to exceptions for certain water bodies (most of the main rivers) where no cutthroat are allowed to be kept. I see people keeping lake-caught cutthroat all the time, and less often in the rivers. (I don't keep any fish, but would if I caught a nice halibut someday). Idaho and Utah also allow anglers to keep cutthroat caught in certain places, according to the regulations.