I wouldn't blame painted for attacking fish on a stringer. Sounds like a snapper to me, I had the same thing happen to a walleye in the Bwcaw. When ever we camp we put the stringer on about 10 feet of rope, that gives the fish plenty of roome to avoide the snappers. And remeber just because you don't seem them doesn't mean they aren't there. They breath by stiking just their nostrils out of the water. We watched for them with binaculars less than 50 yards away. Ive never heard of a painted turtle big enough to kill a catfish.