I may be mistaken, but according to my Oregon fishing library, my books do not indicate any cats in the lake. I have fished there for trout. And there are bullheads, crappie and bass (and according to another OFF'er some kokes).
Have you searched the Hagg Lake archives on here? If not, you may find the info you want there.
I dont think there are any channels in there but there are brown bullheads. State record was caught there I think it was about 4 pounds which is huge for a bullhead since they are ussually 10 inches. I have never fished for them there before but I remember reading something that said focus around the mouths of creeks where the bottom was muddy. heard there are crappy in there too but have only caught bass trout and perch.
Yes, I saw a stringer of catfish coming up the bank a few years ago. They appeared to be in the 15-20 inch range- definitely not channels. They must have been fishing from the bank somewhere west of Ramp A.
I have caught both channels and bullheads from Hagg - channels are kinda rare compared to bullhead. Most of the bullhead I've caught over the years have come from the coves around the lake - caught in probably 20' or less of water. We have caught them bottom fishing in deeper parts of the lake toward the middle - I am not terribly surprised though to hear of them being caught by trollers -- catfish aren't just bottom feeders, they feed any part of the water column that their food source is located. If they're eating bait fish, and the bait fish are on the surface - you'll find surface feeding cats. They do tend to gravitate toward the bottom though, like most fish.
The only places on Hagg I've ever caught channels were near the mouth of the Sain Creek cove (just south of ramp C) and up in the Tanner Creek arm of the lake. They were most definitely channels - light colored sides with a few spots, long body with forked tail. Just like the channels we used to catch in Kansas
Worms fished on bottom using a sliding weight setup and short leader always seem to be the best for catfishing out there - then again you can probably get more if you go with stinkier bait like chicken liver, or pickled fish parts...