Thermoclines

For many years I knew thermoclines existed in water, having experienced them in swimming pools as well as larger bodies of water. I could even spot them on dams when the water was calm enough to make out the different surface textures. However one important factor eluded me and that was the understanding of how and why this phenomenon existed and more importantly how I could use them to my advantage when seeking the ever elusive Large Mouth Bass. Hopefully this article will give you the knowledge to understand them and equip you with the tools necessary to take advantage of them, increasing your chances and your bag limits.

Your results will improve, more and better fish will be caught and a potentially tough day on the water will open new opportunities.

I am sure most of us have been in water and felt a drastic change in temperature, sometimes it can be down right creepy and one can conjure up images of under water currents or worse ready to sweep you away. This is a thermocline and exists in almost all bodies of water at certain times of year if not all year; in short a thermocline is a layer within a body of water or air where the temperature changes rapidly.

Now we need to understand why this happens. Water has limited visibility and as such almost all sunlight and its heat is absorbed by the surface layer (the epilimnion). Wind and the moon causes waves which circulate this layer giving it a somewhat uniform temperature, the thickness of this layer will vary from dam to dam depending on the dams depth and underwater structure and can very from inches to many feet. Below this surface layer a totally different world exists in a much cooler layer (the metaliminon).

This beautiful 3.76kg bass specimen was taken along the edge of a thermocline against a deep drop off by good friend Robert Brown.

Any bass fisherman should understand the importance of a change in water temperature especially in summer when we are looking for colder water which is rich in oxygen but before we can take advantage of this newly acquired knowledge one first has to be able to identify where and how deep these thermoclines run.

During the warmer month's surface thermoclines of a couple of degrees can be spotted on the surface, these appear as area of contrasting smooth and rippled water surfaces and are most noticeable in the afternoon when the sun starts its descent to the western horizon. Initially I thought these were wind channels affecting only certain portions of the water surface; however after research I have since discovered that this is a result of the colder layer of water below interacting with the warmer thermoclines. As the warmer surface thermocline thins in depth the underlying colder layer has more effect as it breaks down the surface layer this gives the appearance of a rippled effect which is in fact the heat dissipating.

The method I use to find the thermocline with my electronics is to turn the sensitivity of the sonar to manual and set it to between 80% and 90% then I cruise as if looking for underwater structure. The thermocline will show up on your sonar as a black line normally with grey streaks below it. Submarine pilots use to use the thermocline as a disguise as the sonar from ships searching for them from above would be echoed from the thermocline almost cloaking the submarine lurking just below the thermocline.

The thermocline is clearly visible in this picture, the flatter water being the cooler. The reeds on the left would be a potential sweet spot.

When surface temperatures reach the high twenties in summer, which is the case for most of our local fishing destinations it becomes very important to find the cooler pockets of water. Most people I know will look for water inlets, vegetation that provides oxygen to the water or they will look for deep structure, all of which are good strategies but now apply the above knowledge and you will find that your results will improve, more and better fish will be caught and a potentially tough day on the water will open new opportunities.

It is hard enough to find suitable structure in most of our local waters and known structure is often like high street during rush hour especially if you are competing against a whole field of boats. However if you have taken the time to do your home work, on the day of the competition if you find the thermocline and are able to match it with structure in the dam the chances are you will find the mother load.

There are many factors to consider when approaching water and trying to read its patterns and unlock its secrets, factors such as water temperature, water colour, depth, colour choice of lures and many more all of which have scientific explanations and will be discussed in detail in further episodes. Thermoclines are just one part of the jigsaw puzzle that we as fisherman try to piece together and I hope this article has assisted you in placing this one important piece of the puzzle. Remember, if you never give up you can never fail.

Now to make sense of all the science and apply it to our fishing techniques and strategies. The warmer thermoclines will be rich in micro organisms which attract plankton, this plankton attracts smaller bait fish and basically forms a basket of food moving around the dam which the bass follow or wait for. Once you have determined the depth of the thermocline you need to find structure at that depth and fish just below it. Sometimes you will find that fish are present in certain areas for most of the day but only come onto the bite at certain times, this could be mainly due to the moving and changing thermoclines. It also explains why sometimes we find fish suspending most often above various forms of structure but sometimes what would appear as the middle of nowhere. Clearly you can now see this is not the case.

Reproduced with the permission of the author
pomerob