The Feeding Zone

[Moose1am]

Just want to clarify a few facts. The Thermocline is a relatively thin area or slice of the lake. Like Bonefish said it's the area of the lake where there is a sharp temperature drop or change. This temperature change occurs within just a few feet in vertical distance. The surface waters are called the Epilimnion and this is where most life occurs in the hot summer months in eutrophic lakes. Not all lakes stratify or produce a thermocline. Oligotrophic type lakes ( Canadian Shield type lakes) don't have a lot of nutrients and they will not have oxygen depletion below any thermocline in some cases. Fish such as Lake trout are free to roam the depths of these lakes. But in an eutrophic type lake where a strong thermocline develops there will be some oxygen depletion below the thermocline. Detritus falls to the bottom of these lakes and bacteria consume this detritus and use up the oxygen in the lower hypolimnion layers of the lake. That's why you won't find many fish below the thermocline. Also you won't find many fish in the thermocline. The thermocline can be only a foot or two thick and it's where the oxycline is located in some case. The oxygen can drop to below 4 ppm in the thermocline and that's not enough oxygen to support fish live for very long. Therefore the fish will be found ABOVE the thermocline layer not in it.

Not all thermoclines develop at a specific depth. It all depends on the type of lake. Water clarity does have an effect as well. But the depth of the lake and the size of the lake has an impact on where the thermocline develops. Shallow lakes will have enough mixing to stop a thermocline from developing. While deeper lakes can develop two thermoclines that does not happen all the time or every year.

Bottom line and the thing to remember is that most of the fish will be found in the Epilimnion between the surface and the thermocline. This concentrates the fish in a smaller area of the lake during the hot summer months. After the lake turns over the hypolimnion is allowed to mix with the surface waters and oxygen is replenished though out the lake. After the surface water temperature drop to the same temperature as the thermocline temperature the lake will turn over. During turnover you may smell H2S gas as it's released from the bottom. This turnover can kill fishing for a few days but as the lake recovers fishing picks back up.

Also remember that in a large lake you can have different areas of the lake turning over at different times.

Also reservoirs that have a good flow running though them may not stratify or may only have limited stratification. It depends on the different in the water temperatures between the top and bottom, the amount of wind and the amount of water running though the reservoir.

BIGGW, this post is in your honor.

Bass have a prefered temperature range to live and feed. This temperature range is found in a layer of water called the metalimnion, or the thermocline. A lake's water temperature falls into horizontal layers of warm and cold water seperated by the thermocline. The thermocline is a thin layer of water between the warmer upper layer (epilimnion) and the colder lower layer (hypolimnion). The thermocline respresents the greatest temperature drop per 3 ft of depth and greatest increase in water density. This layer has a low level of turbulence and mixing so oxygen content and nutrients do not mix very well with the hypolimnion.

The thermocline starts in May and by June the water temps will be approaching 70 degrees. During the summer, there are three distinct water temps, 0-12', 12-22' and 22-45'. Bass become more predictable during this time and will stay in depths of 12-22', which is the level of the thermocline. During late autumn, the cold winds will start to drop the surface temps, and at a certain point, the cooler surface water will start to sink to the bottom, which is called the "turnover." At this point, the bottom layer rich in nutrients comes to the surface fertilizing the lake. After the turnover, water temps become more uniform from surface to approximately 25 ft. At this time, the fall bite starts. Bass will start to relate to structure and running shad in tributaries and coves off the main lale.

Lakes go through seasonal temperature changes. As the sun heats up the water in spring, the surface water becomes more dense and starts to settle above the colder water. The warmer surface water forms a thermal energy which opposes wind energy, thus preventing mixing of the warm and cold layers. Generally, the thermocline averages 7-10 ft and usually stays around 22 ft. Bass will be active in the upper regions of the thermocline, but the best fishing occurs where the thermocline starts to the first 5 ft above. From the thermocline upwards is where 99% of the bass stay. This occurs because bass are out of the bright sunlight, there is food, and the water temps are comfrontable. Mornings and evenings are good times to fish because the light is at more acute angles and is not as intense. Wind and water clarity also affect light penetration. This is why fishing windy, murky banks can be so productive.

Thermal stratification is an important regulator to the overall metabolism of a lake. Except in very clear lakes, photosynthesis doesn't occur below the thermocline. This is why you will find bass holding much deeper in clear water lakes. Clear water lakes can have mutliple thermoclines and will appear much deeper. In dingy lakes, the thermocline may be 6-8 ft where as on clear lakes it may be 30-35 ft.

During winter, uniform low temps are observed from top to bottom. The absence of vertical temp gradients allows mixing to occur throughout the water column. During this time, the lake water undergoes a temporary oxygenation process. Strong winds and feeder streams will add oxygen to the water. But as the water starts to warm in spring, the lack of plant life in the lower layer will cause oxygen levels to drop once more. As organic material decays, it will further decrease oxygen levels.

As water gets colder, it contracts becoming more dense. This process continues until it reaches a temp of approximately 39.2 F, and at this point if further cooled, it starts to expand, becomes lighter and floats to the surface. All the time the water column is mixing, pushing the warmer water to the surface. This process will continue until the water column reaches 39.2 F. At this time if the water is further cooled, it is no longer able to sink, and it starts to form ice. The ice forms a blanket slowing further the cooling, which provides stable temps for survial. The entire process of lakes cooling, heating and freezing is a gradual one so all living things can adapt slowly.

Now, back to fishing. The fish will tell you where the thermocline is located. If you can find where the thermocline touches a point or structure (hump) you are in business; this is a "sweet spot, an ideal place to fish. There is a 3 to 5 ft zone above the thermocline that can be "sweet." After the turnover the bass will suspend at all levels and at times there can be great surface action. On your electronics, use the sensitivity to located the thermocline. Increase the sensitivity to around 90%, and you should see a distint line. When you see a washed-out line, this is where there is a drastic difference in water temperature.

[Bonefish]

Thanks, Moose, for your added input.