Fishing The Thermocline: Depth, Causes, Fishing Tips

I was cruising along the lake fishing last weekend and I was carefully watching my electronics. No I wasn’t looking for underwater bass and heavy structure(well maybe a little).

What I was mainly looking for was the underwater thermocline. If you’ve been fishing for a while I’m sure you’ve heard somebody mention thermocline.

You might even have a basic understanding of what thermocline actually is. So what actually is thermocline and how do you use it to find bass?

What is Thermocline?

We’re all fishermen, so most of us don’t really care about the science behind what causes Thermocline. Simply speaking thermocline is the horizontal layer of water where there’s a steep change in water temperature.

Most deep bodies of water, lakes, rivers and oceans have a fairly predictable thermal structure. They have a well mixed warm upper layer of water separated from the colder bottom portion.

These bodies of water that exhibit this quick change in temperature are said to be stratified(arranged close together in layers). The point where the warm upper layer meets the colder bottom layer is called the thermocline.

Why Does Thermocline Matter in Fishing?

Thermocline depth is a key piece of information when looking for baitfish. Everybody knows that once you find the bait you’ll find the bass.

How Do We Detect Thermocline?

Unless you’re a scientist, you’ll probably be detecting the thermocline with your fish finder. Use your sonar to look for large concentrations of life and differences in water density.

Sometimes the thermocline will be clearly evident on your sonar. If big balls of bait are always hanging out 20 ft underwater that’s probably your thermocline. Other times you’ll need to ramp up the sensitivity on your fish finder.

Identifying Thermocline Throughout The Seasons

Since the thermocline develops as a result of changing water temperatures it’s going to look different throughout the seasons. There won’t be as much thermocline in early spring as mid summer.


Throughout the spring you probably won’t recognize much of a thermocline. The weather will be fairly stable, not really warming up the surface.

When there’s not much of a difference in water temperature throughout a lake we call them homothermal. Basically just means there’s a similar water temperature throughout the entire water column.


As summer starts to roll around and weather starts to warm up a bit you’ll start to notice the thermocline starts to develop. Throughout the hot day solar radiation warms up the surface water.

As the surface starts to warm at a rapid rate. Colder denser water starts to settle towards the bottom beneath the warm surface. Over time, surface water continues to heat causing a huge discrepancy between the top and bottom.

Thermocline will be at it’s strongest during the hottest months of the year(Mid-Late Summer).

How Does Thermocline Affect Fishing?

Why should you care if there’s cold water at the bottom of the lake? It all has to do with oxygen production in the lake.

Phytoplankton and other microscopic plants need heat and light to survive. As we learned in elementary school plants produce all of the oxygen we need to survive. Phytoplankton produces underwater oxygen that dissolves into the water, making it habitable for fish.

Since warm upper layer supports almost all of the phytoplankton and vegetation in the lake, that’s where where most of the oxygen is going to be produced. Below the thermocline where there’s limited light aquatic plants can’t grow so there’s no oxygen.

Since the bottom of the lake is isolated from the warm surface and nothing can grow, by mid summer there’s very little oxygen. This cold oxygen-less wasteland is not suitable for life.

Where is The Thermocline?

The depth where thermocline exists is going to be different from one lake to the next. You can even have a big difference between two lakes in similar climates.

It all depends on the temperature and water clarity. In especially clear lakes where light penetrates deep into the water the thermocline will be deep.

Very clear lakes can support aquatic life at depths greater than 40 feet during the summer. Whereas dirty water, might only be able to support life 20 feet down. Even lakes separated by only a couple miles can have a very different thermocline depth.

Use The Thermocline to Find Bass

You know bass are unable to live below the thermocline. However, the rest of the lake is capable of supporting life. This means not all bass are going to be found around the thermocline.

Throughout the summer not all bass are going to be found near the thermocline. Having larger bodies bass require a lot of oxygen to survive. So they’re going to go wherever they can find the best habitat.

Small prey species on the other hand require less oxygen so they flock to the safety of deeper water. Since the thermocline represents the deepest habitable water in the lake with dissolvable oxygen that’s where they’re going to go.

As we all know, bass are especially lazy during the heat of summer. The best time to attack the thermocline is early morning and before dusk when bass are actively feeding.

Lakes Without Structure

Looking for structure is one of the easiest ways to find bass. What about lakes that don’t have a lot of structure? Unable to hold to structure bass bass become less predictable moving throughout the water column.

When they aren’t holding to cover they’re using more energy which requires constant feeding. So in lakes with less available cover they’ll likely follow the bait. Considering most bait schools near the thermocline that’s where you want to fish.

Thermocline During The Fall

As fall approaches and the temperatures start to drop the sun loses its intensity. Day length starts to decrease and surface water starts to cool. Throughout the cold night lakes start to lose heat to the environment.

As the surface water cools, density starts to rise and water starts to mix throughout. Oxygenated surface water starts to move down the water column making the dissolved oxygen and temperature constant throughout the water column.

Thermocline During The Winter

The thermocline is a little different during the cold winter months. Instead of surface water warming it actually starts to cool. This cool water starts to drop to the bottom.

Another thermocline forms, but this time it’s a little bit different. The warm thermocline gets sandwiched between the cold bottom and rapidly cooling surface.

Water temperature might only be a few degrees warmer in the thermocline, but that’s enough to attract lethargic bass. When cold fish will do just about anything to stay just a little bit warmer.

There won’t be as much of a difference as there was during the summer, but you can really clean up fishing these warm spots.

So What Does All This Mean For Fisherman?

So what does this mean to all of us anglers out there? Understanding how to use the thermocline is just another way to analyze fishing conditions. It will help you eliminate water and establish a starting point.

During the summer you know baitfish hold near the thermocline, which makes it an excellent way to pattern bass. While the sun is high you can turn to structure and deep water where bass are going to be more concentrated.

The winter you know bass like to hold near the warm thermocline. They’re going to have just a little more energy and be more willing to feed.

Where Should You Look?

Most of the time I’m going to look for thermocline nearby natural fish highways. Look for long points, and flats nearby steep ledges. Anywhere that allows bass to quickly move up and down with the bait.

Look for those deep bluffs and points where fish naturally congregate. More often than not they’ll be stacked in this one area and stay there all winter long.

The next time you’re out on the lake why don’t you try to identify the thermocline? It shouldn’t take more than a minute with a fish finder. Thermocline might be small, but when you find it you might get a ton of bites.

Recent Content