"Lowrance HDS Fishing The Thermocline"

Lowrance logo

HDS Ultimate Upgrade Sales Event 
HDS Series

"Lowrance HDS Fish Finder, Thermocline & Structure Superstar Meters"

Fishing the Thermocline

(Water Layers Which Are Defined By Temperature)

The thermocline is the transition layer between the mixed layer at the surface and the deep water layer. The definitions of these layers are based on temperature.

The mixed layer is near the surface where the temperature is roughly that of surface water. In the thermocline, the temperature decreases rapidly from the mixed layer temperature to the much colder deep water temperature.

The mixed layer and the deep water layer are relatively uniform in temperature, while the thermocline represents the transition zone between the two.

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 preferred temperature of water that they will actively feed in. Find this area of temperature 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 horizontal layers of warm water and cold water that are separated 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 temperature 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.

High Definition Mapping  screenshot

Fantastic New High-Definition Insight Mapping

Only Lowrance offers this stunning new world of built-in cartography choices that go to greater depths, and heights, in dimensional GPS navigation reference and guidance.

NEW Insight USA™ - A Charting Revolution in Detail and Resolution
Insight delivers unprecedented inland mapping and coastal charting detail with lightning-fast screen display updates. Amazing contour shading lends life-like perspectives to bottom structure as well as topography.

Expanded Basemap Marine Detail
HDS models with Enhanced U.S. Basemap offer the same geographic coverage area as Insight, with over 100,000 bodies of water and nearly 5,000 enhanced U.S. lakes with high detail lake shorelines and contours.

HDS Mapping Options Go All-Platinum
Your trusted favorites in Lowrance 2D mapping choices, like Fishing Hot Spots® PRO and LakeMaster® ProMaps are fully functional with HDS. Lowrance HDS expands your world of high-detail, high-definition 3D charting options with total compatibility for all 2009 (and earlier) Navionics® chartcards.

Lowrance HDS Support For Navionics® Now Available As A Software Update.

  • Platinum+ and Platinum – Revealing 3D views of bottom structure, and incredibly detailed satellite imagery, for the ultimate enhancement to navigation and fishing.
  • HotMaps Platinum – Fantastically detailed 3D lake surveys with fishing points of interest give marked advantages for the serious angler and tournament competitor.
  • Exclusive TurboView™ Advantage – This exclusive HDS built-in graphics engine delivers seamless zooms and 2D-3D chart displays, real-time 3D map nav data overlay, striking depth views in 2D and 3D, plus high-speed panning/zooming with detailed aerial views for all Platinum chartcards, and seamless zoom for Navionics® Gold.
  • Plays Navionics® Favorites – HDS is also fully compatible with your library of Navionics® Gold, Silver, Classic and HotMaps chartcards.

HDS Insight mapping choices include:

  • NEW Lake Insight™ U.S. inland mapping (for HDS-5) includes nearly 5,000 freshwater lake maps with life-like perspective of bottom structure and topography. Coverage includes the Great Lakes and more than 500 Fishing Hot Spots® Lakes – a $400 value!
  • NEW Nautic Insight™ (for HDS-5) offers a superior like-like view of coastal bottom structure with detailed coastal and offshore depth contours, plus nav aids, for the entire U.S. coastline.
  • NEW Insight USA™ (for HDS-7, -8 and -10 models) combines inland and coastal coverage ares of Lake Insight™ and Nautic Insight™.A Charting Revolution in Detail and Resolution
Enhanced Base Map Insight Map
Amistad TX 1mile Base Map
Amistad TX 1mi Base
Amistad Insight
Amistad TX 1mi Insight
Bull Shoals Base
Bull Shoals AR 0.5mi Base
Bull Shoals Insight
Bull Shoals AR 0.5mi Insight
Lake Michigan Base Map
Lake Michigan 20mi Base
Lake Michigan Insight
Lake Michigan 20mi Insight

View the HDS Product Line

"Lowrance HDS Fish Finder, Thermocline & Structure Superstar Meters"

Lowrance HDS Fish Finder Links Below

LOWRANCE Marine High-Definition Meters

LOWRANCE HDS System Revolution In Sonar HD Meters, Maps, And Fish Finders 100,00 Bodies of Water, 5,000 US Lakes with Structure.

Lowrance Fish Finder Manuals

"Thermocline: Definition. A Transition Layer Between Deep And Surface Water"

The 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.

Depth at which the rate of decrease of temperature with increase of depth is the largest. In general the sea water temperature decreases from the surface to the deepest levels, except in high latitudes where the configuration can be more complex. There exists in most ocean areas (apart from polar and sub-polar oceans) a zone where the rate of decrease of temperature is much larger compared with that above and below, hence the definition. Depending on the geographical location, the thermocline depth ranges from about 50m to 1000m. A simplified view is to consider the thermocline as the separation zone between the mixed-layer above, much influenced by atmospheric fluxes, and the deep ocean. In the tropics, the thermocline can be quite shallow on average, as in the eastern Pacific (50m), or deeper as in the western part (160-200m). In extra-tropical regions a permanent (or main) thermocline is found between 200m and 1000m. However the thermocline depth varies seasonally, especially in the mid-latitude regions where a secondary and much shallower thermocline (above 50m) occurs in summer. In high latitudes, a thermocline may appear only seasonally. Thermocline can also vary from one year to the next, as in the tropical Pacific where thermocline vertical displacements play a fundamental role during ENSO. As the pycnocline, the thermocline is a prominent feature of the ocean which conditions many physical, chemical and biological processes occurring in the oceanic upper layers. In many situations, the thermocline can be identified with the pycnocline when the vertical contrasts of salinity are small.

Click Here To Return To Link Page