I saw a couple of catfish while I was carp fishing one day and I took a few casts at them without success. Never really targeted cats, but I'll bet they would eat a properly presented fly. I know they aren't just scavengers -- they will even chase and eat baitfish. I'm sure they also eat crayfish and nymphs and anything else they can get their mouths on! If you can sight-fish for them it would be fun to try even if they don't hit. It seems like blind-casting for them would be much more difficult because I think you'd need to get the fly right in front of them. Good luck!