actualy the thermocline has little to do with O2 levels but temps. it just so happens that water disolves O2 best at certain temps.

the thermcline is an area of dence water that hovers at a depth where the heat transfer from the top of the water colum stops penetrating to the lower levels. this can range from three to four feet in winter to several feet in summer. the most destintive thermoclines are in early spring and late fall. this is when the water is colder than the air temps in spring and the heat will only go so deep into the cold water. in fall the water is still hot from the long summer days with intence sun light heating it up and then the cool nights and cooler days will only go a few feet down. some times in the fall the fish will hold just under this line and be close to the top in spring the fish will hold on top of this line and be close to the top, BUT and there is always a BUT....... as the water equilizes to the air temps this line can move deeper and even go away. what you need to know is what O2 level is at this line as it may not always have the proper saturation for the fishes liking. In the past the O2 dispersal rate for catfish has thought to have been 6 parts in 1 million but lately the studies have shown it really depends on the habbitat of the fish and what they have acculamated to. just as we as people find it hard to breath in the mountains if we live in the flat lands we can acculamate to the thinner air with time, fish do the same but they try to find the best quality of O2 they can. this is why some areas hold fish at depths and others don't if you see a spring creek in winter that dumps clear water into a river or lake chances are the fish will be close by since that water will have more O2 per part than the bigger body of water. also water temps and O2 saturation do not always go hand in hand as once thought. the way water disperces O2 is directly related to flow and bottom make up. rock and sand in hot water will have a higher O2 level tham a mud or muck bottom, the key to O2 saturation is flow and current. slow hot water has little O2 saturation so the fish has to move deeper to find the sub-currents created by creeks and other influances to get the O2 there fore you find them deeper. a body of water that is deep and little flow will hold fish at brakes in the bottom because structure will cause the water to "churn" around this and therefore stiring up the O2. the same way a fan in a room with out windows and the door closed will move the air around and make it feel cooler. The other thing to take into count is that shallow fast water will move the O2 too fast for some fish with slower motabolism, and that would be like you or I tring to breath in a wind tunel. so all in all think of it as you breath, on a hot day you want a fan blowing the air on you but not too fast where you have to gulp it to breath. on a cool day you want the air to move slow so it is not as cold on you and you have to gulp it to breath. so knowing what depth the O2 is best for fish is the depth to fish and the thermocline really has little to do with it however in atleast two times a year the temp at which O2 saturatates and the temp the fish prefer will be really close to each other.

hope this helps and does not confuse you but it's the best answer I can give with out pulling out the data books from past years and telling you the water temp at the depths and the O2 level at the same depth where fish were cought. the heart of summer to mid fall and the heart of winter to the mid spring is the times when O2 saturation and water temps really play havock on fishermen as the fish are always moving looking for food and good air...