Bronsswyk is right on the mark when he said big flies. You will most likely have to tie them yourself or get someone to custom tie them for you because the guys in most tackle shops don't stock them big enough. You want minimum size 4, but I have found 1's & 2's to be more productive. Sounds out there I know; I didn't believe it myself until the guy who use to run the hatchery on the upper Pitt showed me how to catch those big bull's. I remember him asking my wife and how we were doing (referring to the fishing) and I told him it was slow. He told me that it shouldn't be and asked what I was using. "Big flies," I told him. "How Big," he asked? "Size #6," I answered. "Not big enough," he replied as he showed me his bull flies which went up to size 2/0. Then I challenged us to fish over a run which he claimed was holding three bulls from 15 to 20 lb. We fished and fished without a touch and then he fished the run, with the same pattern in size 1/0 hitting one of the bulls on his second drift. After that he explained that bulls are all out predators and wait in ambush for their prey. He further explained how they find spots where they blend into the substrate making themselves almost invisible. Once finding their position they do not move (give away their position) until a meal worth the effort is within their strike zone. As they say the proof is in the pudding, seeing is believing. You will like bulls they are fascinating predators.