You can catch fish on flies if you can find open water. Last winter/spring I got out when the ponds were still 1/2 covered with ice. Caught crappies. Bigger bass hit earlier than the smaller bass. Or maybe its once the smaller bass get active, they just beat the bigger bass to the fly? Bluegills will hit anytime as well. Fish slow for panfish when the water is cold...Using a microjig under an indicator works great. I've actually done well on bass in cold water by using a much more aggressive retrieve....lots of aggressive jerks and twitches, but try to move the fly forward as slowly as possible.