Much has been written about the thermocline in lakes and how fish relate to it, but the topic still seems to create some confusion among fishermen. I’d like to take this chance to explain what the thermocline is as clearly as I can, and explain how it affects the creatures that live in lakes.
Let me begin with what the thermocline is and tell you how to determine where it is. The thermocline is the depth in a lake at which the water temperature changes very quickly, with warm water above and much cooler water below. This temperature change at the thermocline will happen very quickly as you descend in the water column. For example if you lowered a thermometer over the side of the boat you might see something like 65 degrees at the surface, 62 degrees from 2’ deep down to 10’, a rapid change from 62 down to 45 degrees in the 10 to 13’ depths, and then finally a fairly uniform 40 degrees from 13’ to the bottom of the lake.
The thermocline in this example would be at the 10-13’ level. With most good sonar units you can see this change if you adjust the sensitivity to the 95-100% level. You can also find it by using an underwater camera which has a temperature sensor, drop the camera until you notice this rapid change and then measure the amount of cable you had let out to find it.
The thermocline begins to develop in the spring when the sun warms the shallow areas of a lake. Warm water is less dense than cold water (in this instance- as I’ll explain later) and the warm water effectively "floats" above the cooler water. If it stays sunny and calm the thermocline can develop quickly, but if it is windy the warm upper layer will be mixed with the colder water below and the separation will not be as pronounced. At some point in the early summer the thermocline will establish itself at a relatively shallow depth. As the summer continues the constant warming and mixing will cause the upper layer of the lake to expand and push the thermocline deeper and deeper.
The thermocline is important because it separates the water in a lake into two layers. Wind can effectively mix the upper layer of the lake and continue to supply the water with oxygen throughout the summer, but the thermocline acts as a barrier to the re-supply of oxygen to the lower layer. The amount of dissolved oxygen in the lower layer at the time the thermocline develops is essentially all that will be available for the rest of the season. Depending on the type of lake you fish, you may or may not have to worry about oxygen depletion in the lower layer. Lakes which have high fertility and a lot of biomass (vegetation) are more susceptible to oxygen depletion. The biomass in a lake typically settles to the bottom of the main basin and the organisms which decompose this material as their food supply consume oxygen in the process, thereby depleting the amount available for fish.
This is important to walleyes since they require a certain level of dissolved oxygen to "breathe". The amount of oxygen in the lower layer of the lake will decrease throughout the course of the summer, and in some lakes it will reach a level that is not comfortable for walleyes to survive. There’s no point fishing below the thermocline in a lake that doesn’t have any oxygen down there.
The thermocline is also important to the lesser organisms in the food chain. Algae, being at the bottom of the food chain, in some species have developed the survival tactic of rising up in the water column to gather more sunlight and at other times of the day dropping back down to the thermocline to avoid predation. The food chain follows the movement of the algae, and it is almost always above the thermocline in the oxygen rich warmer waters. This cycle of walleyes rising up at some point in the day is something that I’ve noticed before, and have always wondered if this was a plausible explanation for this behavior.
At some point in the early fall the thermocline will reach its deepest level and the surface waters will begin to cool. The temperature of the upper layer will begin to drop, and its temperature profile will begin to resemble the lower layer. A strong wind will create turbulence causing the two layers to mix, and the guy at the bait shop will announce that the lake is "turning over".
What’s happening is that the mixing water will become an almost uniform temperature and consequently a uniform density. So you’ll see things like leaves and weeds which were stirred up from the bottom of the lake floating throughout the water column.
As the fall progresses the water will continue to cool and the stratification of the water will flip flop. Water is most dense at 39 degrees, and once the water at the surface drops below that temperature it becomes less dense and will "float" on the warmer 39 degree water. While that doesn’t sound like good warm swimming water to me, it’s true that the warm water will now be on the bottom of the lake allowing the surface to freeze. Then it’s time for tip ups until the ice begins to melt again and the cycle repeats itself.
Understanding how a lake works and how it affects the fish you’re after may help you to catch more fish. Good luck.