Actually, I have a bunch of pic's of trout from the Wood river and Agency lake that look exactly like that! Some of them were 7-8 lbs. and seemed healthy enough other than the deformity. I can't remember what exactly the cause was.....it was the late 80's and early 90's so it has been a while! The native fish in the wood got infected with some sort of bacterial disease from hatchery fish that were stocked in Fort creek on the Take it easy ranch. We were advised to kill all the infected ones we caught!