Bullhead are very active under the ice, at night I will often have large schools of them swim through and readily hit a jig... I don't think a little cold water will slow them down much, maybe they feed somewhere else when the temp drops?
Have caught a ton of them under the ice in late March. Water temps 37-40 about a foot under the ice.
Water has the unusual property that it is most dense at a temperature of 39 degrees F. (I know, it seems counter intuitive that water at 39 degrees is denser than colder water at 33 degrees.)