Ok, I reread the o.p. Esshup says that the fish were in the bottom. Hey Esshup, at what depth were the fish when seen laying in the bottom. If they were in deeper water, this is my take. Ice cover, combined with reasonably clear water. I.e. small amount of phytoplankton to produce oxygen. Ice cover limiting oxygen absorption at the surface.Calm and overcast days. No sun reduced any photosynthesis that may have normally occurred. Calm = no aeration from the windmill, or oxygen absorption and mixing through wave action.
