Did you keep a hole open in the ice (assuming you had ice)? To degass the water? I run a pump all winter pulling water from the deeper end where the fish sleep to the shallow end, 'lofting' it by about 2' to introduce oxygen. The fish sleep in deep caves (I have no idea how big but 20 fish sleep there, ranging from the big guy who is over a foot long and girthy to the smaller fellows about 3-4" long.) All survived this last winter.
