July 26th, 2009  |   ( 3 comments )

Find the Thermocline using your Sonar Fish Finder

The dog days of summer are here and one of the most important things to know about your lake or reservoir is the depth of the THERMOCLINE. Find the thermocline and it will be much easier to find the fish.

What is a thermocline?

On calm lakes and reservoirs with little if any water movement, the water often stratifies and settles into two distinct layers. The upper layer is warmer, brighter (light) and less dense than the cooler, darker and denser water below. Once the water becomes stratified, there is very little movement of water between the two layers. The dense cool water near the bottom of the lake does not get any oxygen and becomes more and more oxygen depleted as the summer wears on.

Between the warm upper layer and the cold lower layer is a thin layer called the thermocline. The water in the thermocline is cooler than the upper layer and warmer than the lower layer. The light penetration and oxygen level in the thermocline are also improved.

In most lakes the thermocline develops somewhere between 15-25 feet deep. The depth and thickness of the thermocline can vary from place to place on the same body of water, and some areas of a lake or reservoir may not show a distinct thermocline at all.

Why do I care about the thermocline?

The thermocline is important when trying to locate fish because it is the depth that is most comfortable for the fish. The thermocline is the comfort zone for the fish because it contains the coolest water that has abundant oxygen and prey. The light penetration at the thermocline level is also improved. Most game fish (bass, crappie, catfish, walleye, trout, salmon, etc.) will be found near the thermocline, especially during hot summer days. Fish will rise into the upper layer at times, but most fish will not spend much time below the thermocline due to the lack of available oxygen.

When you find the thermocline, you should start your search for fish near that depth. Look for brushpiles, timber, humps, channels, points and other cover at or near the thermocline. If you sink your own cover (brush piles, condos, etc.), you should consider the normal thermocline depth for that lake and sink the structure near that depth (keeping in mind the normal summer depth of the lake or reservoir).

Pay close attention to where you catch fish in relation to the thermocline. Some species may prefer to be slightly above the thermocline while others prefer to be just below it. The time of day, brightness and other conditions also have an effect, so keep these things in mind when you are trying to pattern the fish.

Even catfish, known for living near the bottom of the lake, are affected by the thermocline. They will stay near the thermocline and will not spend much time below it. You might find huge concentrations of catfish in the middle of the lake suspended at the depth of the thermocline. They might be suspended at 20 feet in 40 feet of water.

How do I find the thermocline on my fish finder?

There are two ways to find the thermocline with your fish finder. The first way is to pay attention to the depth where you see the majority of the fish. That depth is probably the thermocline. The second way is to adjust your fish finder to show the thermocline depth on your graph. A quality fish finder will be able to detect the change in water density at the thermocline and will show you exactly where it is. Get into open water that is at least 30 feet deep or so and turn up the sensitivity (gain) of your fish finder sonar unit (above 80%). You should be able to see a fairly distinct line at the depth of the thermocline. If you are running your fish finder with fish symbols turned on, you will probably see a clean line of fish symbols at this depth.

If you can’t find the thermocline with your fish finder sonar, you can slowly lower a temperature probe and look for a fairly dramatic drop in temperature (3 or 4 degrees). That should be the thermocline.

What happens when the lake “turns over” in the fall?

In the fall, the upper level of water begins to cool off when the overnight temperatures drop. When the warm water in the upper level cools enough to become denser than the water below, the lake “turns over” and the layers mix together. This is a dramatic and traumatic event for the fish and the entire lake. Fishing is usually poor for a week or two as the fish adjust to the rapidly changing water conditions.

Pictures of the thermocline

I’ll get some thermocline pictures and post them soon.

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Please leave a comment!

Do you have information or advice on finding or fishing the thermocline? Or pictures of the thermocline on your equipment? If so, please add a comment below.

3 comments to Find the Thermocline using your Sonar Fish Finder

  • netmanOK

    Thanks for this information. I found the thermo-cline on my Eagle 480 yesterday for the first time. Maybe its a conincidense but I also found a lot of fish too. The crappie were in the trees just above the thermo-cline. I also had to deal with a couple nasty bass fish.

    I just wanted to thank you for thie good tips.


  • gcd

    Excellent advise!!!

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