I think you will see a drop in activity when the snow cover has been on for a while.Oxygen content in the water drops and the fish naturally shut down for survival reasons.Plus if the ice gets thicker at the same time it adds to the problem.That is why most people that ice fish a lot say first ice and last ice are the best times to get good action.But there is a way to beat that.Fish buy all those spots that a small spring along the edges of the lakes our ponds you fish,most of the fish will move toward this kind of winter relief. Sometimes you can really find them stacked all in a very small spot!