Why does this fin look like this?

Looks to me like this is most likely a Largescale Sucker – one of Idaho’s native species of Catostomidae (sucker family!). The small white bumps on this fish are breeding tubercules, which are common on suckers (and many minnow species) during the spawning season. According to Dr. Peter Moyle’s excellent book, Fish: An Enthusiasts Guide, breeding tubercules “function much like antlers on a deer – for defense, real and ritualized, and for attracting females.” -Martin Koenig