For 25 years I have fished the same lake in Ontario. At the beginning there were plenty of crayfish in the rocks, now not so much. In the beginning (when I was beginning too) the successful flies were buggers in black (leeches) or olive (crayfish). For the last 3 or 4 years a SMALL minnow - mostly white - has been the answer, because the new perch minnows are more abundant than crayfish. I agree with the above, bass will key on the dominant forage, but mostly you can entice them to eat when they see something Twitch, Tremble and Twinkle.