I’ve had a lot of people ask me about thermoclines lately so I will share here on discovercatfishing.com what I know about them and how I use thermoclines to my advantage to target lake and reservoir catfish.
Being able to locate active feeding catfish on lakes and reservoirs is an important skill that a catfisherman should develop. You can mark fish all day long on your depth finder and still not persuade them to take your bait if they are not in their feeding zone.
Thermoclines are best defined in lakes where there is little if any running water. The thermocline is a thin layer of water in a lake which is sandwiched between the upper layer of warmer water and the lower, colder layer of water. During the summer months, surface water is heated by the sun and the surface temp could be 85 degrees or more. This floats over a layer denser colder water. Now between these 2 layers you have a thin layer in which the water temp drops considerably, this will be the thermocline. The temp at this level may be as low as the high 60’s to low 70’s in the summer months. Lakes go through seasonal water temperature changes. There are times when the layers mix and then separate especially in the summer your lake will develop 3 distinct water layers and temperature changes, usually occurring at these guestimated depths, depending on the body of water your fishing water clarity, ect. 0 to 12′, 12′ to 22′, and 22′ to 45′. The temperature may drop by as much as 8 to 10 degrees at each depth.
You will find some fish in the top layer, the bottom part of the top layer and at varying depths within and just below what is called the thermocline. Many times your electronics can pick up these varying layers of water. You’ll see a distinct faded line running horizontally across your depth finder. This will represent where there are drastic differences in the water temperatures are. However keep in mind that most times you will be wasting your time if you fish much deeper than the thermocline. Reason being is that the oxygen levels are low due to rotting debris or matter sinking to the bottom and eating up all the oxygen in the decaying process. I can almost assure you that 98 percent of the fisheries population prefers to be above the bottom of this zone, but if you do see fish deep below the thermocline I can assure you that they will not be very active. The one exception to this would be in a lake where the water is very clear. In a clear lake, photosynthesis can occur below the thermocline. When you have phytoplankton growth the oxygen levels will be higher because of photosynthesis, catfish could seek out deeper water. I like to fish from about five foot below the thermocline to up above it about five feet at no matter what depth I find the thermocline. Keep in mind though the thermo cline can and will change from day to day, wind can and temperature can change the thermocline.
If you don’t know for sure where the thermocline is, let the bait fish tell you. If there is a thermocline present, bait will be above it. You will see a distinct zone where the majority of fish are holding on your depth finder. This is how I decide on the depth I’ll fish. If I’m seeing a bunch of fish at 15 to 18 feet, I’m going to target structure or even open water at this depth. If I see a lot of shad and bait fish that are present above this line, the catfish will be close by. So locate some deeper structure along the thermocline, and you’ll more than likely find a sweet spot. This can take a minute or two to find but when you do hit one of these spots, you’ll be glad you spent the time. Also keep im mind that the catfish will also relate to bait fish in open water as if they were structure.
Watch your depth finder as you travel down or across the lake, look for fish to appear along the thermocline, this the best way to locate catfish.
What a lot of folks don’t realize is that there may be schools of catfish along a thermocline in the middle of a lake. Although the water is 50-60 feet deep, the catfish will be suspended on the thermocline over that deep water. I’ve had great success placing my baits in 15-25 ft deep, drift fishing over that deep open water in and around the thermocline.
Keep in mind though, Just because you spot fish around the thermocline in the middle of the lake doesn’t always mean that these fish are catfish. But it will still give you some indication as to what depth you should target. About the only way you can know for sure what types of fish are occupying the thermocline is to put your baits out and see what happens, if you don’t catch a catfish within at least the first half hour, then more than likely the fish your seeing on your depth finder are not catfish. Once I spot the fish, I’ll start putting out my baits. If the fish are 20 feet deep, I’ll stagger my depths, one in 20 feet of water, one in 16 feet of water and the other bait in 24 feet of water, targeting all three feeding zones associated with the thermoclines, increasing my chances of hookin up with a catfish, whether they’re feeding below the thermocline, on the thermocline or just above it.
Its been my experience that at night the catfish will more than likely feed up above the thermocline. The big cats will come up as much as 6 to 8 feet from the surface, while the smaller catfish may feed as high up as 2 to 3 feet from the surface and even on the surface. The cats will move up into more-shallow water all summer long from dark until just after daylight. So, if you’re going to be catfishing at night, you may want to put your lines a little more shallow than you will in the daytime. Just remember that the deeper lines that are still above the thermocline will usually produce bigger fish.
In the fall, the thermocline will sink and dissipate. As the water starts to cool in the fall, it becomes denser and begins to sink to mix with the water in deeper layers associated with the thermocline. The bottom water travels to the top and the top circulates to the bottom. This is normally called a turnover and all the layers are now mixed together. This is probably the worst time to try to catch fish is during this period. The water will begin to smell foul and look like thick coffee.