Technically a thermocline refers to the thin layer of water where the temperature changes more rapidly with depth than it does in the layers above or below.
As far as oxygen goes, a nearby creek flowing into the lake could cause better mixing and result in fish deeper than the levels of oxygen reported from the dam.
A very likely reason could be a nearby weed bed, they produce oxygen during daylight just like the plants that grow on land, but after dark they have the opposite effect resulting oxygen depletion at much shallower than reported. Either way if your catching fish, your in the right spot.
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