I would think they would retreat to the middle, deeper parts of the lake because it's presumably warmer there. But hopefully someone who understands fishing better will answer.
shareThe bottom part of a lake that doesn't freeze remains a solid 4 degrees Celcius no matter how cold the air of the territory anyways is.
During such conditions the fish almost hibernate; moves slowly and as little as possible. Even eats little...as feeding/ chasing a bait also requires energy that is better perserved as body heat.
You may still catch fish ice fishing by bait or nets
But don't expect as plentiful cathes and results as during summer period