Fishing The Thermocline

What’s a Thermocline?

A thermocline is a thin but distinct layer of water in a body of water in which temperature changes rapidly more than it does in the layer above or below. When water temperatures rise in early summer, the top layer, the hottest surface water doesn’t mix very far down. The top layer has good oxygen, but high temperature.  The middle zone where the most temperature drop occurs is the thermocline. Below the thermocline is a deep, cold layer which has a more gentle temperature drop but is devoid of oxygen.Because this bottom layer of water is depleted of oxygen and thus bait fish like gizzard shad and threadfin shad and the other fish simply cannot survive below the thermocline. The area below the thermocline is often referred to as the Dead Zone.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 and oxygen levels high because of photosynthesis, bass could seek out deeper water. In very clear lakes there can be a second or double thermocline. Temperature is the real key in this case, since bass don’t generally prefer the colder temps at these depths I still fish the upper thermocline, which is generally suitable for most fish.

 

When and where does a Thermoclineoccur?

Some lakes never have a thermocline. However, many of our lakes do develop a thermocline during the summer. In this area of the country the thermoclines generally begins to develop in June and can be prevalent through October with their most dramatic effects being felt from July-September. Several factors affect when the thermocline sets upand at what depth and thickness they’ll occur. These factors include seasonal weather variations, latitude and local environmental conditions, such as current and water clarity.Strong wind and heavy rains can also change the thermocline. I don’t worry too much about these unless there are strong winds for a several days and enough rain to cause a lake to start releasing lots of water and generating current.The depth of the thermocline usually follows clarity. The clearer the water, the deeper the thermocline. In dingier lakes, it may be only 6-10 feet. In clearer ones like Table Rock it may be as deep as 30-35 feet.  In the fall, the thermocline will sink and disappear. As the water starts to cool in the end of September and on into October, it becomes more dense. As it sinks, it mixes with the water in deeper layers. The bottom comes to the top. This is called fall turnover. Now, the lake is completely mixed. This mixing is why you’ll find bass suspended a great deal of the time in the fall. The water temps and oxygen content are practically the same at every depth. Fall turnover is Mother Nature’s way of re-fertilizing the lake. Nutrients from the bottom are brought back to the top.

 

How do I find the Thermocline?

The way I find the thermocline is to turn on my depth finder (regular 2D sonar) and set the sensitivity on manual. I then run the sensitivity up to about 90 percent. As I cruise out into the lake, it will show a distinct line.See images below:

LOW SENSITIVITY                                                       HIGH SENSITIVITY

 

 

 

If you don’t know for sure where the thermocline is, let the bass and bait tell you. If there is a thermocline present, bait will be above it. You will see a 3-4 foot zone where the majority of fish are holding.

 

How do I fish the Thermocline?

In bass fishing, you need only concern your efforts from the thermocline upward. 99% of life will be above the bottom of the thermocline. In the summer, this is how I first approach my decision on what depth to fish. If I’m seeing the thermocline at 15 to 18 feet, I’m going to target structure especially during the mid-day at that depth. If a lot of shad are present, I promise you there’s bass right there close.  Depending on depths here are a few of my most productive presentations: crankbaits, weighted swimbaits, big plastics (worms and brush hogs), football Jigs, wobble heads, Texas rig, Carolina rig, shaky heads, and drop shot.Sometimes it can be hard to get these fish to bite, but I would rather spend some time trying different angles and presentations on deeper structure where I know they are living than fishing shallow water where I haven’t had a bite in the last three hours.I usually fish from the top of the thermocline and up about 5-6 feet. This is the zone I often target during the summer. Don’t get me wrong, I fish shallow in the summer. Especially on lakes with current, grass, and docks. I also fish shallow in the summer during low light conditions like early or late in the day or at night. But in most cases if you want to have tournament winning sacks fishing will be most productive around the thermocline depths during the dog days of summer.

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