Simply put, the thermocline is a thin layer of water in a lake, which is sandwiched between the upper layer of water and the deeper, colder water. During the spring and summer months, the sun heats surface water which floats over a layer of colder water called the hypolimnion. The hypolimnion is far denser than the warm water layer, so it tends to remain fairly deep especially as the water temps rise. Now, between these two layers you'll find what I like to call "the living room," otherwise known as the thermocline. The thermocline is important because the temperature variations don't swing too wildly, the light penetration is just right, and there's plenty of oxygen. So if you're a bass, it's the perfect comfort zone. Kind of like you feel when you are relaxing in your... you guessed it... living room.
So why is the thermocline so important? Well, without trying to get too scientific because I'm definitely no scientist, there are two primary reasons why you'll find bass in greater concentrations within the thermocline, especially if the lake is stratified. The first, and possibly most important reason, is sunlight penetration. As you probably have noticed, bass have no eyelids... poor guys, and they can actually see ultraviolet light waves, which have to be annoying. So, they seek a little shade under the water. Obviously, there are several factors that will affect sunlight penetration like the wind, water clarity, and even the time of day since morning and evening sunlight is much more indirect. So, if the lake you're fishing is gin clear, better look a little deeper for the thermocline.
Stratification and oxygen levels are a little trickier. Stratification is the process where the lake literally divides itself into sub layers. The upper level retains oxygen and the lower level loses it for several different reasons. First, the upper level replenishes oxygen levels through turbidity, or the simply action of waves washing around and stirring up the water. Secondly, feeder streams feed oxygen into the upper layer continually throughout the year. And finally, vegetation rarely grows much deeper than say 25 feet, so there's no plant life to oxygenate deep water. However, prior to stratification, you will find greater levels of oxygen in the deeper end of a lake because the water is colder, and oxygen can't disperse as easily in cold water as it can in warm water. But after the stratification... or as many of us know it, the turnover; it's a whole different story. The oxygen will get out of the deep water faster than I'll try to get out of a day of shopping with my wife. Now during this process, the thermocline will change depths as the stratification process progresses, so it may not be at the same water level two days in a row.
So how do you find the thermocline? Well, you can use a temperature probe and watch closely for subtle temperature changes until you come to a depth where the temp seems to be fairly consistent, or you can do what I do, which is simply pay attention to your fish finder. If you're marking fish at a particular depth, and there seem to be a lot of them, then guess what, you've found the thermocline!
So if you are lucky enough to be able to fish throughout the year, without having to drill a hole in the ice, then look for the thermocline and that's where you'll find the fish. Good luck!
