You really CAN catch anything on a mop fly!
I was fishing Dutch Fork lake a couple of weeks ago and with 6 trout in the net was feeling okay about my results but of course had to make that 'one last cast'.
Tossing my rig of a Wooly Bugger and trailing Mop Fly up against the shore I immediately felt a tug on the line. However, when the tug wouldn't move I assumed my fly was hung up on one of the numerous sunken trees in the lake.
Paddling over to my line I noticed it had started to move! Reeling in the slack line I reset the hook and saw my 5wt bend in half due to the weight of whatever was on the other end.
Well as they say 'a picture is worth a thousand words' so check out the video to see what I hooked with a mop fly. Can't say I landed him (didn't really want him in the kayak with me) but he was most definitely on the line.