In theory, you should fish at or above the thermocline because that is where dissolved oxygen levels are high enough to allow fish to survive. The problem with this theory is it doesnt hold true across lake types; more specifically it depends on the lakes trophic level. In highly fertile, eutrophic lakes (Clear Lake) DO levels typically are very low below the thermocline (but, the thermocline in Clear Lake can dissolve and reform quickly at any time of year, i.e. polymictic). In relatively infertile, oligotrophic lakes (Pardee), DO levels are usually very high anywhere in the water column even though a thermocline forms. So, trying to eliminate fishable water based on the thermocline is not straight forward and can be very complex depending on how far you want to take it. Ive thought about this stuff before, and thought about how I could use it to my advantage, and the only way to do it is to carry a DO meter and take readings each day you fish. My conclusion was this was way too complicated, would take up too much time, and would take away from the fun of fishing. Its easier to just fish at different depths (and at this level taking into account where the thermocline is provides a good starting point) until you hit pay dirt, then concentrate on the depth you were getting bit.