How deep have you searched for them? If you are getting 50 degree water (I'm assuming at the surface), I'm suspecting that it's a lot cooler down deep. Up in my neck of the woods they have tendency to suspend and/or sit on the bottom (15' - 25') during the cold water periods. But I don't have any experience with the areas you are talking about?
We also find that during the cold water periods sometimes you just don't "see" fish on your sonar very often. Best to look for bait balls relating to, or on structure.