Bob Maindelle Guide Lines June 20

This down-imaging sonar screen shot shows the thermocline as a horizontal band. The themocline separates the warm epilimnion surface waters from the cool hypolimnion beneath. Water below the thermocline lacks sufficient dissolved oxygen to support fish life.

Each summer our local reservoirs begin to stratify with the warmest water near the surface and with the coldest water near the bottom. This stratification occurs because the warmer water becomes, the less dense it becomes, thus the warmer water actually floats atop the cooler water just as oil floats atop water.

On Belton and Stillhouse Hollow lakes, our surface water often hits 86-87F in the summer, while near the bottom in deep water (over approximately 60 feet), the water temperature can remain in the 50s.

Many mistakenly believe fish will seek out the deepest, coolest water they can find to avoid the summer heat. This is erroneous. Although many species of fish prefer water cooler than that found near the surface, there is a limit to how deep these fish can go in the summer months.

To understand why this is, one must understand a bit about the three strata found from the surface down to the bottom.

The topmost strata, or layer, of water is the warm, light, low density water referred to as the epilimnion.

Most summers this layer is about 35-40 feet thick.

The bottom strata of water is the cold, heavy, high density water referred to as the hypolimnion. This layer begins where the epilimnion ends and extends to the bottom of the lake.

Separating these two layers is a layer of water known as the thermocline. This thermocline is typically 5-8 feet thick and prevents mixing of the upper layer (epilimnion) and the lower layer (hypolimnion).

During the summer months, fish are all found from near the thermocline and upward. This is because that cold water below the thermocline in the hypolimnion does not contain sufficient dissolved oxygen to support fish life.

So, the upshot of this is that all summertime fishing efforts must be focused on those depths from the thermocline and shallower.

This year, although the water is already stratifying, fish are still being found and caught from waters over 40 feet in depth because water is flowing through both Belton and Stillhouse as the Army Corps of Engineers continues to release water through both lakes’ dams in order to shed recently accumulated flood water.

This flow creates mixing, which delays the formation of the thermocline.

Once the thermocline forms, fish tend to suspend above the thermocline and spread out in small, splintered schools horizontally versus schooling in large congregations on the bottom as they did during the cooler months of October through May.

Horizontal presentations like deep trolling with Jet-Divers, Dipseys, Hell-Pet rigs, and downriggers really shines in the summer months. Done correctly, these presentations allow baits to be presented in a horizontal plane directly above the thermocline where suspended fish are most abundant.

The thermocline may be detected in at least two ways. First, well-tuned down-looking sonar, both traditional and down-imaging, can detect the thermocline if the sensitivity (also called gain), is turned up fairly high.

FishHawk makes a depth-temperature gauge called the FishHawk TD, which, when allowed to descend to the lake’s bottom, will record the temperature at 5-foot intervals. By inspecting the readings and paying attention to the most rapid change of temperature monitored, the thermocline can likewise be identified.

Not all fish in a given body of water will suspend above the thermocline, but no fish will be found beneath the thermocline.

Cover-loving, ambush feeders like largemouth and smallmouth bass, crappie and sunfish will still orient around cover in water as deep or shallower than the thermocline, whereas pelagic species like white bass, striped bass and hybrid striped bass will tend to be in open water as deep or shallower than the thermocline.

I monitor the temperature profile from the surface down to at least 60 feet on both Belton and Stillhouse all summer. I post these readings on my blog at Just look for the “Lake Reports and Photos” tab. So, if you lack well-tuned sonar or a FishHawk TD, you can keep up with the impact of the summer heat and how I contend with it there.

Long story short, if you continue doing during the summer what has been producing fish for you through the cool months, you will likely note a sharp decline in your success as our area lakes begin to stratify and fish reorient as a result of this annual occurrence.

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