I think appleye has it right. It does depend somewhat on the lake, however, all things being equal they like structure. It seems to me the smaller the lake the more they stay put in a small area. There are also times when they go cruising as well, I caught fish this weekend jigging in 40' of water and walter was suspended at 25'. This brings up another point, if you are jigging and want to see if there are fish in the area, rip the jig-stroke hard from rod tip on the ice to as high up as you can reach. Even if they aren't in a biting mood it may bring them in for a look. This last weekend it was the only way I could get a bite.