My first Catfish, a 10 lb. Channel Cat, hit a white Marabou Streamer fished on a Floating line.. I was working some Impoundments near my Hometown in Alabama with My cousin Ricky for Bass.. and suddenly this Channel Cat ate the Marabou streamer.. what a Good fight on the 8wt...
Since then I've caught a number of Catfish on flies... about a 50/50 on using sinking lines verses Floating lines (thought all the Floating line catches where in water not over 5 feet deep)
Flies of Choice are BH Wool Buggers, Woolly Worms, Crawdad imitations & Minnow patterns in sizes #2 thru #6. I'm sure my #1 Carp fly, the Rusty Brown Clouser swimming Nymph, would produce Catfish as well.....
For patterns like Woolly worms, Crawdads and BH Woolly Buggers keeping the fly on the Bottom provides the most effective.. But remember Channel Cats will actively pursue Baitfish and have been caught by Bass Anglers on crank baits... so if there's baitfish nearby a faster retrieve oughta get their attention....
I'd go minimum a 7wt. on Catfish...because some of the local lakes can produce Big ones (besides if a Larger Bass eats your Catfish offering, you want to rip them out of cover). Fluorocarbon leaders and Tippets are best....
The Only time I've ever caught Catfish on Dryflies was years back, I took a Student of mine to Malibu Creek State park.. figured if they'd hook into some Bluegill they'd get more into the Flyfishing... Well we found the Bluegill where I expected they'd be.. But what came to the surface and grabbed Our Dryflies were not Bluegill but Bullhead Catfish?!?!?!?!???? It was soo weird to see them swim up off the bottom (About 2-3 feet deep in this area) and gently suck down the Adams or Black Gnat dries we were fishing.... They fought nicely on the 5wts. we were using..... This is the One & Only time I have ever caught Catfish on the Surface.... ( I talked with Ken Hanley about this episode recently and he said he's only had it happened a few times for him)...
Catfish for me have been predominately an incidental catch.. but often I think about focusing on trying nothing but catching them one day...