Thermocline Impact on Grand Lake Fishing

This has been a pretty good year for Bass Fishing here at Grand Lake. June through September is the time to do some great top water fishing early and late in the day. Its also the time of year to catch some giant Bass in deep water. While many fishermen don't want to be bothered by the summer heat, it can be the very best time to cash in on catching a trophy Bass. It still takes a little luck, but knowing what to look for and exactly what time to fish is the key to being successful. There are several reasons one may be able to catch trophy Bass in the Summer months, and the fact that the fishing pressure is very low compared to spring time fishing is probably the main one. Not having as many boats on the lake allows the fish to feed in more of a natural pattern and on a day to day basis they are a lot easier to find and predict. From June until late fall, most of these big Bass are caught in water that can range from 20-30 feet. In order to be successful, understanding what a Thermocline is and how it behaves is very important.

What is a thermocline? How does it affect fishing?

Hopefully, the following will help you to understand what a thermocline is and the effects it can have on your fishing at Grand Lake and other deep water lakes.

THERMOCLINES

If you're a Bass fisherman, the thermocline is something you must know and understand to be a successful fisherman.

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 temperature could be 80 degrees or more. This floats over a layer of colder, more dense water called the hypolimnion. Now, between these two layers you have a thin layer in which the water temperature drops substantially. This generally means that a one degree centigrade temperature difference occurs in the space of one meter.  This will be the thermocline. The temperature at this level generally reaches the high 60's and up in about the middle of spring. Let's relate this to Grand Lake. Normally the thermocline starts to set up in May. Fishermen have been able to enjoy catching spawning bass in relatively more shallow depths (2' to 12') during the spawn. Bass have needed the warmer water temperatures to spawn (lower 60's and up). Moving on into the end of May, most Bass have spawned at Grand Lake. 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 temperatures reach about 73 degrees, you can plan on predictable fishing. Plastic worms work well. The crankbaits, spoons and jigs will also work well. Moderate water depths (often between 12' to approximately 22') are the depths of choice.

In the full-blown summer, you will have three 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 (these depths change based on many factors and they aren’t hard and fast rules.  Wind speeds and ambient temperature readings have a large effect on when a thermocline sets up and at what depth it is located. High winds can push a thermocline down deeper in the water column or in some instances delay the onset of the thermocline because it mixes the water).

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 Grand Lake) the cold winds blow as the fronts start coming through. This in turn drops the water’s 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 or less, usually (but there are exceptions) 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 sun’s penetrating rays.

October is a month of water temperature changes for Grand Lake. Depending on the weather, you will find water temperatures becoming more uniform from the surface to about 25' in most areas of the lake. The temperatures 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 be true, but there are also certain times when it has more oxygen than the other layers of water. As you already know, the fall turnover on Grand Lake usually occurs in October. The water begins to cool. The shallow coves are among the first to cool and the Bass will begin to relocate to shallow structure. This is why fall fishing is fantastic at Grand Lake. The Bass will be caught in the upper regions of the lake (early October), but usually the best fishing occurs just above where the thermocline starts. The main thing to remember is that 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’ eyes do. Also remember that sunlight will diffuse differently, depending on the time of day. In the early morning and late afternoon the sun’s rays will be at more of an angle and not as intense. Wind will also affect the sun’s 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 Grand Lake 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. 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 sun’s 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 Grand Lake usually has no thermocline until late spring or early summer and it is at this time that you must understand what the thermocline is and what role it plays on fishing.

Grand Lake, and most all lakes in Oklahoma, will turn over 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 turn over will be more abrupt than others.

It looks like this is another great Summer for Bass Fishing at Grand Lake and we hope you found this article helpful with your fishing.