The Thermocline and Fishing
Fishing the Thermocline:
Locating feeding fish is one of the most important skills a fisherman can develop. You can mark fish on your fish finder
and not entice them to strike your lure if they are not in their "feeding zone"!
Most species of fish have a preffered temperature of water that they will actively feed in. Find this area of temperture with
fish present and you will greatly increase your chances of catching fish. Remember, fish are cold blooded. In water too
cold, fish will be dormant, sluggish and will not feed. In water too warm, they will be seeking a more comfortable
environment. So, an understanding of how different temperatures of water break up will help.
Here are two main ways temperatures of water will break up:
On inland lakes, water temperatures tend to settle into horizonal layers of warm water and cold water that are seperated
by a moderating layer known as the "thermocline". The thermocline will be the most active "feeding zone".
On larger bodies of water like the oceans and Great Lakes, masses of water temperature are much larger and in a
constant state of change. The location of these large masses of temperature are highly affected by the changes in
weather conditions. These large masses of warm and cold water also have a moderating layer that is known as a
"temperature break". Because of the constant shifting of the warm and cold masses temperture breaks often appear as a
vertical layer. The area of, and immediately around, the temperature break is the most active "feeding zone".
If you're a bass fisherman, the thermocline is something that's very important
you must know and understand to be successful.
Simply put, the thermocline is a thin layer of water in a lake which is sandwiched between the upper layer of water (the
epilimnion) and the lower, colder layer of water (hypolimnion). During the summer months, 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.
Let's relate this to Lake Fork,Texas. 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, crankbaits,
spoons and jigs to name a few. Usually water depths between 12' to approximately 22' are the depths of choice. This is
the thermocline.
In 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 it 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 which occurs every year 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 such cases as 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, once again, 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, give or take a few feet. Fall fishing patterns have set in and
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, feeder streams feed the lake with 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 reasons are is that there is very little or no plant life beyond the
30 foot level. Unless the water is gin clear the suns rays cannot penetrate this far into Lake Fork 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, depending on the
weather we have, 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. Estimates of the shad die 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.
Take a look at the background on this web page. Once the water starts setting up in layers usually in late spring or early
summer depending on how much warm weather we have, the layers will look like the layers on this page. You will find
some fish in the top layer, the bottom part of the top layer and at varying depths within what is called the thermocline.
Most times you will be wasting your time if you fish any deeper than the thermocline. Many times your electronics can pick
up these varying layers of water. On quality, high end electronics you'll see a washed out line. This will represent where
there are drastic differences in the water temperatures.
Originally Posted by Glenn
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although good info., and worthy of logging into my memory bank somewhere, I think I'll just keep throwing a beetlespin or plug like he did. Carry on.... 
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