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
Fork and other deep water lakes.
If you're a bass fisherman, the
thermocline is something that's very important
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 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
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
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
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
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.
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
Until next time,
Good Fishing & Best
J.W., JIM, LaVonne & DANNY
If you would like to book a fishing
J&J Guide Service: Call: 1-903-383-3282 or
Visit our website at:www.lakeforkproguides.com