Each summer stratification of the waters in our two local reservoirs, Belton and Stillhouse Hollow, takes place. This stratification significantly impacts fish location and, therefore, fishing.
Although what happens involves some physics and some chemistry, I am going to use the example of a layer cake to help simply illustrate what takes place.
Consider a layer cake, with a thick upper layer of cake, a thick lower layer of cake, and a thin layer of filling in between these two cake layers. This is a good visual of how the water in our lakes stacks up during the summer. This stacking is called stratification.
The thick upper layer of cake represents the warm, upper layer of water which is heated daily by the sun, and which is blown about and churned by the wind and waves. This warm, upper layer is lighter (it has a lower density) than the cooler, heavier, higher density water layers beneath it.
This warm, upper layer is called the epilimnion. On Belton and Stillhouse this layer extends from the surface, down to around 34 feet or so.
The thick bottom layer of cake represents the cool, bottom layer of water which is more distant from the sun’s heating and is not nearly so impacted by the mixing caused by wind and waves. This cool, bottom layer is heavier (it has a higher density) than the warmer, lighter, lower density water layers above it.
This cool, bottom layer is called the hypolimnion. On Belton and Stillhouse this layer extends from about 40 feet deep down to the lake’s bottom.
Just as oil spilled on the water floats on the water’s surface because the oil is less dense, so the warmer water of the epilimnion floats on top of the cooler water of the hypolimnion because the warmer water is less dense than the more dense cool water beneath it.
The thin layer of “filling” in between the upper layer of cake and the bottom layer of cake represents a band of water in which a rapid temperature drop occurs with increasing depth.
This layer in between the epilimnion and the hypolimnion is called the thermocline. On Belton and Stillhouse this layer is found from the bottom of the epilimnion (at about 34 feet deep) to the top of the
hypolimnion (at about 40 feet deep). Hence, the thermocline is typically about 6 feet thick or so.
Many anglers incorrectly assume that fishing “deep” is the best summertime alternative because fish are trying to escape the heat and seek the comfort of cool water.
In reality, the water found beneath the thermocline is not only cool, it has very low dissolved oxygen content such that it is not able to support fish life. Additionally, because fish are cold-blooded and their body temperature is controlled by their surroundings, they do not sense heat and cold as we humans (warm-blooded creatures) do.
Local anglers may be able to relate to the following two anecdotal pieces of evidence of the existence of this low dissolved oxygen level below the thermocline.
First, during the summer months, if water is flowing through the gates at Belton’s dam, a sulphuric or rotten-egg smell is quite noticeable while driving over the dam. This odor is emitted by anaerobic decomposition of organic matter in the cool water of the hypolimnion. The bacteria eating this organic matter give off sulphuric compounds. When water is released through the dam, that water is being released from below the thermocline.
Next, if you have ever used live bait, like shad, sunfish or minnows, during the summer and let them down below the thermocline, they will die quite quickly. This is because there is not enough oxygen in the water for them to sustain life.
Above the thermocline, living aquatic forms of vegetation (hydrilla, plankton, algae, etc.) give off oxygen during the process of photosynthesis. Waves and wind mix the water and keep the epilimnion sufficiently oxygenated to support fish life.
Some fish species do tend to hold, in a suspended manner, just above the thermocline. At this location, the water is as cool as it will be before becoming anaerobic (lacking oxygen). Threadfin shad are particularly prone to position themselves right above the thermocline.
With all of this considered, the main thing for anglers to sit up and take notice of is the depth at which the thermocline is set up and to confine angling efforts entirely above that level. This can be measured with a device like the Fish Hawk TD, which takes temperature readings every 5 feet as it is lowered toward the bottom.
The thermocline can also be detected visually by adjusting and observing sonar. Sonar will reveal a distinct, horizontal band which appears more opaque or cloudy than the more clean, clear epilimnion above it.
As autumn approaches, the air begins to cool, thus cooling the upper layer of water (the epilimnion).
Eventually, this upper layer is cooled to the point where it becomes cooler than the hypolimnion beneath it. As a result, this water of the epilimnion sinks down into the thermocline and hypolimnion, thus causing a massive mixing of all three layers. This is called turnover.
When turnover occurs, the temperature distinction of the summer is erased and the water temperature becomes fairly uniform from top to bottom and remains this way all through the cool months until the following summer’s heating causes the cycle to repeat.
When turnover occurs, the dissolved oxygen levels also becomes nearly uniform (and sufficient to support fish life) from top to bottom, regardless of depth. This, too, will remain the case all through the cool months until the following summer’s heating causes the cycle to repeat.
Hence, fish are often caught from the greatest depths of the year during the winter months.
Turnover typically takes place in early October here in Central Texas. Stratification by temperature leading to the development of the thermocline typically begins in the last days of May and into the first two weeks of June. Stratification typically takes place first in the lakes’ upper reaches and migrates downstream near the dams later on.