Oxygen is also a big factor in finding fish deep. As,these lakes and reservoirs were built and flooded, they covered springs up that flow today along creek channels and sometimes at the bottom of rock outcropping was well as the very bottom. I have scuba dived under 12” of ice before in a rock quarry and strip pits at depth of 70’-80’ ft. And on bottom would be a spring just full of every imaginable kind of fish in that body of water. Same in the hottest time of summer in several strip pits, springs on the sides or bottom at deepest depths full of fish of all kinds together, Water temp and oxygen bubbling up with the current of the spring. Even below the thermocline if the spring is found. Use your electronics to find schools of tightly schooled fish if you can. Some of these Live View videos I have seen show blubbles coming up with the fish in them as the guys are running their lures sometimes catching fish. Could be a spring below causing the bubbles of oxygen. Most of the time, if your electronics are showing a thermocline, in the summer, you will find no fish due to water temp and lack of oxygen.