Thermocline Fishing Tips and Diagrams
Finding the thermocline and how fish relate to it is very important for summer fishing. This subject is fairly dense, but I’ve included only the necessary details in this article. Make sure you check out the visuals below!
Thermocline Layer Basics
As the summer wares on, three layers of water develop on most lakes and reservoirs. The top layer, called the epilimnion, gets the most sunlight and is the warmest layer of the three. The bottom layer, called the hypolimnion, is the coldest and is least affected by the sunlight. Between these two layers lies the thermocline. All of these water conditions have specific features that attract fish for different reasons.
The top layer holds the highest amount of oxygen and, depending on wind and current, does not disperse much of that oxygen to lower layers. The hot top layer is the least dense and thus lies on top of the other two layers.
The thermocline is defined as a rapid or abrupt change in temperature relative to surrounding water. This sandwiched and relatively small layer can experiences drops of 10° within just a few feet and thus bridges the gap between the fairly consistent epilimnion and hypolimnion layers.
The bottom layer holds the least amount of oxygen and is the densest of the three layers. This layer of water has very little available oxygen because lake organisms die and sink to the bottom, and the process of decaying these dead organisms uses up all the available oxygen in the layer. In addition, the low light penetration means the lower layer does not have very much oxygen-producing aquatic life.
Fish Location Along the Thermocline
For reasons described earlier, fish don’t like it too deep or too shallow in summer. The thermocline is the best location because it offers oxygen, bait, and cool temperatures. In my experience fish generally locate on the upper side of the thermocline. If I am marking fish at 12’ the thermocline can exist anywhere between 12’ and 15’. Other cold water species like trout and pike will relate to the lower end of the thermocline due to the more preferred lower temperatures.
In the heat of summer fish will remain in the top layer too, but it is important to note that fish will rarely go below the thermocline. Basically treat the thermocline like the bottom of the lake. Never fish deeper, it is a barren wasteland void of oxygen—suitably called the ‘Dead Zone.’ Sometimes catfish and other bottom feeders will temporarily dive below the thermocline, but they can only tolerate short trips.
Locating the Thermocline
Thermoclines can be located a few different ways. The most reliable method is to dive off the boat and swim down until you sense a sudden temperature change…just kidding. Most depth finders, if you turn their sensitivity up, will present to you a fine line that represents the thermocline. Below is an example of how the thermocline usually looks on a depth finder. Note how the fish are normally found in bulk slightly above the thermocline. If you do not have a depth finder that works well, investing in a thermometer and recording the temperature at different depths is the next best way. In most lakes expect the thermocline to exist somewhere between 15 and 25 feet.
An excellent technique for fishing is to find places where the thermocline intersects with structure. Trees, humps, dips, or rock piles—if these intersect with the thermocline you’ve found yourself a gem. This takes some practice with the depth finder. Try putting around for a while in the boat to get used to how your depth finder records the thermocline and other structure.
Thermocline fishing is useful and unique because it takes work to get good at it. Most fishermen would rather not leave the shallows and will cast endlessly in the shallows—when most the fish are under the boat or behind them (and a lot less pressured too). In the dog days of summer you provide yourself a big edge to whatever other fishermen are doing by using the thermocline.
Changes in the Thermocline
Factors such as current, weather, wind, and temperature can change the thermocline depth. Some days the thermocline can be shallower than other days. Higher water visibility can make the thermocline exist deeper and vice versa. Keep in mind that thermoclines generally do not exist in lakes shallower than 15 feet and never in creeks or rivers.