What Does A Thermocline Look Like on a Fish Finder?

During the summer months, many people hit the warm water of the nearest lake trying to land their next trophy catch. Whether you are a summer-only fisherman, or simply love …

During the summer months, many people hit the warm water of the nearest lake trying to land their next trophy catch. Whether you are a summer-only fisherman, or simply love getting out on the water in the heat of the day, knowing where the thermocline is at your favorite fishing spot can help with catching bass, catfish, walleye or other species throughout the late spring and summer.

What is a Thermocline?

As the water temperature rises under the brutal summer sun, various bodies of still water will change in a few different ways. These changes are most commonly seen in reservoirs, lakes, and ponds that have no moving water from spring feeds or connecting rivers or creeks.

The thermocline is a layer of water towards the bottom of the lake where warm water and cold water combine and it’s good for anglers to know where it lies in their favorite fishing spots.

Dissolved oxygen levels are extremely low or even non-existent in this area and while fish can enter the thermocline level, they cannot stay there long due to the lack of oxygen. As summer wears on and temperatures continue to rise, the denser water of the thermocline will be easier to find on imaging sonar. 

Since the thermocline is towards the bottom of the lake, the water temperatures are lower here and most fish will stay in this colder layer to remain comfortable throughout the hottest part of the day.

As an angler, you can determine the location of the thermocline using a sonar fish finder or other fish-finding electronics and have a good idea of the location and depth to cast a lure.

Why is it Important in Fishing?

Also known as the dead zone, the thermocline is an area where fish can make short trips to feed but cannot remain in for extended periods of time due to the lack of dissolved oxygen. The water in the thermocline is often much cooler than the surface water temperatures, so more fish will spend much of their time just outside of the dead zone than they will in surface water.

If you are an angler that loves bottom fishing in the summer heat for catfish, bass, trout, or other fish that stay low in the water column, knowing where the thermocline is located can be important. If you don’t know the depth of the thin layer of the thermocline line, you may end up casting your lure into the dead zone and not getting any bites.

On the other hand, if you know where the thermocline starts, you can let your lure fall just above the line where the water is still cooler than the surface temperatures, but there is plenty of dissolved oxygen for bass and other species to thrive.

In this deep area of the water column, small prey fish and other bait fish will stay suspended. And where there is bait fish, there are also larger predatory fish such as bass and walleye for you to catch throughout the summer months in your favorite lakes.

How to Find the Thermocline on Your Fish Finder

Finding the thermocline in most lakes is not difficult when you are using sonar-based fish-finding electronics, but it’s not uncommon for summer anglers to wonder what does thermocline look like on fish finder. In fact, it can be extremely easy to notice on any type of sonar signal including 2D, side imaging, and down imaging formats. 

Before you can get an accurate reading on the thermocline depths, make sure your fish-finding electronics are properly installed and set up. If you’ve been having issues with your fish-finding electronics due to obstructions in the lower layer of your favorite lakes or reservoirs, trolling motor interference, or a layer of marine life growing on the external transmitter, you may end up with a weaker sonar reflection and not get a clear view of the water density changes in the thermocline.

Additionally, if you are in water that is too shallow, you may not be able to find the thermocline at all. You want to look for water that is 20 to 25 feet deep or deeper that also has the preferred temperature layer regardless of the summer heat at the surface of the water body. Once you find the right depth, switch your sonar on and start following a set course or contour on the lake bottom.

Over time, you will start to notice a line appearing on the lower part of your display. If you are using a color display, the band may be easier to see than if you were using a monotone display. You’ll potentially see several bass suspended above and below the band, but not directly within the line. This line is the upper layer of the deep water thermocline.

If you are unable to see the thermocline band, you can adjust the sensitivity setting on your sonar mode until it starts to appear. The thermocline will be visible as long as the settings are correct since, due to the lack of available oxygen in this area of the lake, the water will have a slightly different chemistry and potential water clarity than the aerated water above it.

What does a Thermocline Look Like on a Fish Finder

On color displays, you will notice the thermocline area due to a combination of water temperature and density.

Different fish finders will have different ways of displaying where the thermocline water begins, but in general, it will be a very noticeable thick horizontal area on both colored displays as well as a grayscale imaging view regardless of the thermocline depth. 

On color displays, you will notice the thermocline area due to a combination of water temperature and density. A colored display will normally show it as a mixed band of orange and red shades to reflect the warmer water and coolest water combining, with a mixture of greens and blues above and below it showing the temperature and oxygen changes.

On a grayscale display, the thermocline will be a very dark line, while a gray toned area will be seen above and below it to reflect the water changes. Bass shadows and shapes can still be seen above the band, and even rarely below it, which gives you a good indication of that dark area being the thermocline.

Depending on your specific settings and sonar sensitivity, the thermocline displayed in your local lakes may be more grainy than solid. It can also be in different thicknesses but will depend on the overall depth of the lakes or reservoirs you are scanning. If you can find baitfish along with the deeper layer of the lake, chances are good you have found the thermocline as well.

Final Thoughts

Knowing where the thermocline is in your local lake can help improve your success in catching trophies during the summer months. Since the thermocline is formed in deeper water, and is mostly uninhabitable by fish, having a good idea of the location and depth of the thermocline at your favorite fishing spot will help you know where to cast your bait.

Casting it into the thermocline will reduce your chances of getting fish while casting it just above the thermocline can almost guarantee a fair amount of bites. Any good fish-finding electronics can use sonar signals to help you find the thermocline relatively easily.

If you know a thermocline is present in the deep water of your lake or reservoir, but your fish finding gear is not showing it, adjusting the sensitivity on your sonar setup can make all the difference.

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Jeff Knapp is an expert fisherman, guide and outdoor writer whose work is widely published across a range of sites including Tackle Village. Jeff is based in Pennsylvania and loves exploring the waterways of that state in pursuit of smallmouth bass, largemouth, panfish and trout.
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