From what I understand is flatheads go dormant during the winter. I watched an in-fisherman or linder's angling edge segment about how they will literally stack up on top of each other. They put underwater cameras down the hole and the fish didn't move, they put different bait right in front of them and they didn't move at all. On occasion people do find them and snag them through the ice. Flatheads can be very vulnerable if people are able to find them in there wintering hole and snag them. Two winters ago, a bait shop posted a picture on facebook of two guys who "caught" a bunch of large flatheads through the ice. However, the picture was taken down after people started calling them out for snagging. They had like 10 fish that covered the back part of their pickup bed.