Kat-a-Log

[katfish]

Thermocline effects on Flathead Catfish.

A thermocline (sometimes metalimnion) is a thin but distinct layer in a large body of fluid (e.g. water, such as an ocean or lake, or air, such as an atmosphere), in which temperature changes more rapidly with depth than it does in the layers above or below. In the ocean, the thermocline may be thought of as an invisible blanket, which separates the upper mixed layer from the calm deep water below. Depending largely on season, latitude and turbulent mixing by wind, thermoclines may be a semi-permanent feature of the body of water in which they occur, or they may form temporarily in response to phenomena such as the radiative heating/cooling of surface water during the day/night. Factors that affect the depth and thickness of a thermocline include seasonal weather variations, latitude, and local environmental conditions, such as tides and currents.

For our purposes, the thermocline in lakes is a temporary condition that occurs from mid summer till late fall. The importance of the thermocline as it relates to fishing is that cooler water below the thermocline does not have sufficient dissolved oxygen to support fish. This is even of more important as it relates to flathead catfish. As water cools in fall it becomes denser and sinks in the water column. It then forces the water below the thermolcine upwards. The resulting “mixing” of water results in a slick on the water.
Fishermen referring to a lake turning over are speaking of this event.

Flathead are typically sedentary fish that normally only move from their security cover to feed. Except spawning and annual migration they tend to lay under some cover and rest. They stay tight to bottom and could not survive below a thermocline.

Flathead are also piscatorial and will normally want to be close to the other fish they prey on. There is no advantage for the preyfish to be below the thermocline so they will all be above the thermocline break to survive.

I suppose there are two ways to look at water stratification. One thought is that a large part of lakes are made unusable each year due to stratification. This would reduce the total amount of living space and thus keep populations smaller.

The other way is to understand the stratification and use that knowledge to understand where flathead catfish and their prey would relocate. Since we have no control of the weather I sugggest we use the information to our advantage.

Just a note here about baits. Flathead may depend heavily on shad for food throughout the year. But shad suspend happily over thermocline areas that hold no flathead. This may mean they depend more on small carp and bluegill once a lake has stratified.

There are several ways to determine the depth of the thermocline. The most accurate would be to drop an oxygen sensor to various depths until the dissolved oxygen drops suddenly. Another way is to drop a thermometer to various depths until there is a rapid drop in temperature. The simplest and quickest way is to use a depthfinder. Adjust sensitivity to highest levels for easiest detection. The water below the thermocline is denser and normally shows as darker below the thermocline. On old paper graphs and flashers we used to call this the gray line.



Once the depth of the thermocline is determined, we can eliminate all of the lake with depths that exceed that as potential flathead water. The meaning is do not expect any fish to be below the thermocline. Depending on structure this may eliminate from 50-80% of the lake. Now we can begin dissecting the remaining portion of water by comparing it into the best flathead structure.
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Catchabiggun,
Robby

[ducky]

Good read! Grandpa was out the other day and checked the thermocline he said it was at 14 feet, so water between 14 and 4 should be key right now, I am a shallow fishermen no doubt about that in May and June I like water between 2 and 6 feet, but right now i been targeting 6 to 12 feet.
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Brock

[CC]

Nice post. Ill have to check it out this weekend on my lowrance.