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The Thermocline

Lake Fork Professional Guide J.W. Peterson What is a Thermocline and how does it effect fishing is a question we are ask a lot. Hopefully this will help you to understand what a Thermocline is and the effects it can have on your fishing at Lake Fork and other deep water lakes.

Simply put, the thermocline is a thin layer of water in a lake that is sandwiched between the upper layer of water (the epilimnion) and the lower, colder layer of water (hypolimnion). During the summer months the surface water is heated by the sun and the surface temp could be 80 degrees or more. This floats over a layer of colder, more denser water called the hypolimnion. Now, between these 2 layers you have a thin layer in which the water temp drops fair substantially. This will be the thermocline. The temp at this level may be high 60's and up in about the middle of spring.

The thermocline
 If you're a bass fisherman, the thermocline is something that's very important you must know and understand to be a successful fisherman

Let's relate this to Lake Fork. Normally the thermocline starts to set up in May. Fishermen have been able to enjoy catching spawning bass in depths of 2'-12' during the spawn. Bass have needed the warmer water temps to spawn (lower 60's and up). Moving on into the end of May, most bass have spawned at Lake Fork. This is a time of transition for the bass. As a fisherman, you are in a post spawn mode. Crankbaits, lizards, jigs and Carolina Rigs, to name a few have worked well for spawning bass and should still continue to produce through the summer. As we move into the end of May and into June, you can look for the post spawn to set in. This is generally the time the Thermocline will start to set up. When the water temps reach about 73 degrees, you can plan on predictable fishing. Plastic worms work well. The crankbaits, spoons and jigs to name a few, will also work well. Water depths between 12' to approximately 22' are the depths of choice. This is the thermocline.

In the full-blown summer, you will have 3 distinct water temperature changes (at these approximate depths), 0 to 12', 12' to 22', and 22' to 45'. The temperature may drop by 10 degrees at each depth. Many of you have probably heard of a lake "turning over" and this is exactly what it does. During late Autumn, (usually in October on Lake Fork) the cold winds blow as the fronts start coming through. This in turn drops the waters surface temperature. As the temperature cools, this surface water will sink to the bottom of the lake. So when cooler weather arrives the layer that was the warmest (the surface layer) displaces the lower level and the lake turns over. This movement occurs every year and allows the bottom layer to be exposed to the air allowing it to be used by living organisms. In shallow lakes with an average depth of 15 feet usually no thermocline will develop. Mother nature keeps all this in check. In the very shallow lakes you may find heavy cover to screen out some of the suns penetrating rays.

October is a month of water temperature changes for Lake Fork. Depending on our weather, you will find water temps becoming more uniform from the surface to about 25' in most areas of the lake. The temps may vary from 69 degrees (surface temp), to 66 degrees down to the 22' mark. When the Fall fishing patterns have set in, you can count on nice numbers of fish with predictable patterns.

Is the hypolimnion void of oxygen? At certain times of the year this may true but there are also certain times when it has more oxygen than the other layers of water. As you already know, the turnover on Lake Fork usually occurs in October. The water begins to cool. The shallow coves are among the first to cool and the bass will begin to relate to shallow structure, this is why Fall fishing is fantastic at Lake Fork. Generally speaking the thermocline averages 7 to 10 feet thick and is usually found at 22' of water. The bass will be caught in the upper regions of the thermocline (early October), but usually the best fishing occurs just above where the thermocline starts.

The main thing to remember is when stratification is evident the bass will be found in greater concentrations within the thermocline. Why is this? Two reasons. First the upper layer has too much light penetration to be comfortable for the bass and the hypolimnion is usually void of oxygen. This leaves the thermocline where the light is just right and the oxygen is comfortable for the bass. Remember bass can see ultraviolet rays and do not have eyelids, their pupils do not adjust as humans do. Also remember sunlight will diffuse differently depending on the time of day. Early morning and late afternoon the suns rays will be at more of an angle and not as intense. Wind will also affect the suns penetration into the water, as will the clarity of the water. Can you catch bass in the hypolimnion? Why do anglers catch bass in 40 feet or deeper water? During the late winter there is usually no stratification on Lake Fork or most other lakes. As a result, the water will undergo a temporary oxygenation process. Strong winds and the feeder streams feed the lake with the spring rains and plant growth begins. So under normal conditions the deeper end of a water body will be quite saturated with oxygen. Since the lower layer is much colder than the surface (averages 10 to 25 degrees difference) the deeper portions can retain the oxygen molecules sent its way during the pre-stratification. Why does the hypolimnion lose it's oxygen? The thermocline and the upper layer of water are continually replenishing their oxygen supply and the hypolimnion gradually loses it for several reasons. Probably the most important reason is there is very little or no plant life beyond the 30 foot level. Unless the water is completely clear, the suns rays cannot penetrate this far into the lake enough to grow any vegetation. The bottom 2 to 10 feet of a lake will also be where everything settles to decay thus eating up the oxygen. Fishermen will find Lake Fork usually has no thermocline until late spring or early summer and its this time you must understand what the thermocline is and what role it plays on fishing.

 Lake Fork and most all lakes will turnover in the fall. When this happens you can usually smell something that resembles the smell of rotten eggs and many times you will see particles of decaying matter in the water. This is the tale-tell sign that the lake has turned over. Some years the turnover will be more abrupt than others. Back in October 1994 Lake Fork experienced quite an abrupt turnover. Millions of shad were seen either dying or dead on the surface, and many fishermen reported the very strong smell of rotten eggs for a week or so The estimate of the shad that died off that year was over 12 million. Luckily though shad reproduce extremely fast and no noticeable decline in fishing occurred due to this turn over.
Good Fishing,
JW Peterson

Fishing Tip by Lake Fork Guide JW Peterson


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