Many lakes experience what is called a thermocline. You’ll also hear anglers say that a lake stratifies. On some lakes, the warmer, oxygenated water rises during the summer and leaves the cold, less-oxygenated water in the depths. Fish need oxygen to survive and will seldom venture below the thermocline. Without expensive equipment it is impossible to determine if a lake has a thermocline and, if it does, how deep the dissolved oxygen can be found. It’s been said that divers can feel a drop of several degrees in water temperature when they move below the thermocline but that doesn’t help the rest of us who are still up there in the boat. Locating fish who are holding just about the thermocline is all trial and error. Try fishing shallow and work your way down. When you start connecting with fish on a hot afternoon, they’re probably holding at just above the thermocline. Most of the fish will be right at this depth.