What is the thermocline?

I keep hearing the term “thermocline”. What is it and how does it influence my attempts to locate catfish in the water?

A body of water actually has several different temperature layers. A thermocline is the layer of the water in which the temperature will change quickly with the depth. The surface layer, for example. will be the warmest layer as it absorbs the sunlight and heats up quickly. Because this heat is circulated by wind and waves, the heat will often raise the water temperature for a couple of hundred feet. That is called the second or mixed layer. Directly below this area, however, the temperature can drop as much as 20 degrees Celsius in as little as 500 feet. It is this pocket of rapidly descending temperature that is known as the thermocline. Below the thermocline, the temperature will continue to decline but much more slowly. When fishing for Catfish, locating the particular thermo cline in the area in which you’re fishing will more than likely help you locate the fish themselves. For example, depending of where you live, the thermo cline in May could be at a depth of 20-30 feet. In late summer such as July and August, it may only be 12 or 15 feet down. Catfish will usually suspend about 1 or 2 feet above or below the thermocline. They won’t stay in water that is either too hot or two cold which makes the thermocline ideal for them. To find the thermo cline, you can use a depth finder- when you start to see the most fish, mark the depth. That is going to be the thermocline and that will be where you’ll find the catfish. Thermoclines are most defined in lakes that are man-made and there is virtually no running water. The best way to catch the catfish in a thermo cline is to drop a lure at various depths, for example if the thermo cline is 20 feet deep, drop a line at 20, 18, and 22 feet. That way you’ll catch the fish above, below and in the thermocline.


Dan Eggertsen is a fellow catfish fishing enthusiast to the point of obsession. :) He's been providing solid advice on catfish fishing since 2004.

One Response to “What is the thermocline?”

  1. dustin hepp says:

    is it to early to catch cats in late march,in rivers, in iowa

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