Sounds like a winter kill.
Steve Quinn's article in the last issue of In-Fisherman explains winter stratification really well. This time of year, plants will start to die and decay because of low sunlight, so they stop producing oxygen. Fish and invertebrates then breath what's left of the dissolved oxygen so the levels get really low.
Low oxygen and temps below 39f on both the bottom and the upper portion push fish out of the shallows and into the midle zone of the lake, which he calls the Big Squeeze. This layer is often somewhere off the bottom. It makes sence. If a bottom dweller like a gobie can't reach the middle zone of a lake during an exceptionaly cold winter, like the one we're having this year, they're toast!!
cheers