I have a 1-1/2 to 2 acre pond in central michigan. As far as I can tell it's about 12 to 15' deep. Every spring I see fish all around the banks big and small and have no problem catching slab bluegills and crappie. This spring after the hard winter I'm not seeing any fish at all and only catching bullheads wich I didn't even know was in there up intil now. What happened to all my bluegills and bass? Did they kill off this winter? How do I know if they killed off I'm not seeing any dead fish?