Water temperature positions fish, and the thermocline is about temperature. Understanding the thermocline can put more fish in your boat.
Any waterbody in the South without flow can, and probably will, stratify in the summer. A stratified lake or pond will have a layer of cool water on the bottom and a layer of warm water on the top. The thickness of each layer will vary depending on several factors including depth, location and water clarity.
The cool, bottom layer will be approximately the same temperature throughout, usually around 65 to 70 degrees Fahrenheit in most Mississippi lakes.
The warm, upper layer, too, will be the same temperature throughout, but the temperature will steadily increase from June through August or September.
Separating these two layers is the thermocline, a zone or layer of relatively rapidly changing water temperature as you move down through the thermocline.
The thickness of the thermocline can vary from as little as a foot to more than 10 feet. The depth of the thermocline might be as shallow as 3 feet in a shallow pond or as deep as 35 or 40 feet in a deep, clear lake.
The oxygen factor
Initially, when a lake stratifies, the upper and lower portions are well oxygenated. Mississippi lakes are productive, and that means moderately dense phytoplankton that limits light penetration into the water. Light penetration may also be limited in muddy water. The phytoplankton are the base of the food web and produce life-supporting oxygen. With only shallow light penetration, all the oxygen is produced in the upper water layer.
When plant and animal life in the upper water dies, it sinks to the bottom. Decomposition of this organic matter consumes oxygen. Because the lower and upper water layers in a stratified lake don't mix, the lower water soon becomes devoid of oxygen, and fish are forced into the warmer water.
Because the thermocline is a gradient of cooling temperatures and lacks the sharp density difference between the upper and lower water layers, some oxygen produced in the upper layer mixes into the thermocline. The thermocline now becomes a habitat where fish can find cool, oxygenated water.
Ideal summer habitat differs among fish species and among lakes. Crappie, for example, prefer vertical cover such as standing timber for shelter. Bass, on the other hand, seek vegetation because it provides a buffet of forage fish and increases the bass' predatory efficiency.
However, water temperature becomes a critical habitat factor because a fish's preferred temperature is where the fish is energetically most efficient - where consumed food is most effectively converted to growth.
Here is where the thermocline becomes important in locating fish.
Preferred water temperature for crappie is in the upper 70s; preferred temperature for bass is the low 80s. By midsummer, the temperature of the upper water in many ponds and lakes exceeds these temperatures, and the fish seek shelter in the cooler, oxygenated water of the thermocline. Lacking the convenience of a thermostat to change temperature to their comfort zone, fish move to find their preferred temperature.
Biologists call this behavioral thermoregulation.
If food doesn't come to bass and crappie occupying their preferred-temperature zone, they may move to find food. But they will return to their comfort zone to conserve the energy they just ingested.
Finding the thermocline
Scientists studying lakes use sophisticated, expensive equipment to measure temperature and dissolved oxygen in the water column. Anglers can locate the thermocline with a modern depth finder using one of two approaches - "seeing" the thermocline or "seeing" the fish in the thermocline.By increasing the sensitivity of your depth finder, you can often see the bottom of the thermocline as a fuzzy line overlying a lot of bottom clutter, as the figure below shows. Note that after you have determined the depth of the thermocline you might want to change the sensitivity of your depth finder to a level more appropriate for detecting other things in the water, like shad schools or your quarry. A second way to locate the thermocline involves interpreting fish signals. In the figure pictured, the bottom of the thermocline is visible at about 35 feet. But note how the band of fish signals ends at 30 feet. The lack of fish below 30 feet probably results from low dissolved oxygen in the lower portion of the thermocline.
The reality is that a lot of fish are concentrated in the thermocline between 20 and 30 feet, and there is no reason to fish deeper.
The thermocline will occur in every lake and pond without flow-through. Do not expect stratification in the flowing portions of hydroelectric and navigation impoundments, like Pickwick or the pools of the Tenn-Tom Waterway. However, even in these "flowing" reservoirs, large coves and embayments lateral to the channel can stratify.
Always consider the thermocline in your summer fishing efforts. It may not always be the solution to finding fish, but in many lakes it is as much a part of good fish habitat as visible cover.