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The Thermocline's Effect on Fishing

Marc Martyn - 8/18/2008
Fishing for trout in a lake and not knowing what or where the thermocline is can make the chances of being successful slim. It could be compared to looking for Huckleberries in Odessa, WA. You are just not in the correct temperature range.

A typical lake in the summer time consists of three stratified layers. The upper or warmer layer is known as the Epilimnion. The bottom or colder layer is called the Hypolimnion. The Thermocline is the layer that is between both the bottom and top layers. Depending on the overall depth of the lake, the number of feeder streams and underground springs there are, the thicknesses of these layers will vary. Wind and wave action will also determine the depth and thicknesses of these layers.



Cartoon by David Ford


On a hot August day with the surface temperature of the lake at about 74 degrees, you are not likely to find trout in depths of 10 feet or less. Fish are cold blooded animals. They use more oxygen at higher temperatures because their metabolic rate increases. At 41 degrees Fahrenheit, a trout uses about 50-60mg of oxygen/hr. At 77 degrees, they may need five to six times that amount. The lethal temperature of a Rainbow trout is 77 degrees F.

Knowing this, a person may assume that the trout would be down on the bottom where it is cool and the oxygen would be plentiful. This is not always true. In lakes with a depth of say 100 ft., sunlight would be dim and there would not be enough for plants to survive and go through the process of photosythesis, thus producing oxygen. If you have ever pulled up an anchor from these depths, you have probably notice a black oozy material on the anchor. This ooz is made up of decaying plant matter that has settled to the bottom. Trout usually avoid this lower layer of the lake. The decaying matter through bacterial action uses up a great deal of oxygen. Therefore, even though the water is cooler, it is lower in dissolved oxygen.

With the upper and lower layers of the lake not meeting the needs of the trout, this leaves the last layer of the lake, the Thermocline. It is the level of the lake where there is a rapid temperature drop. Trout will usually cruise just above this depth. They will often find the temperature that is comfortable for them and where there is more food and oxygen.

Water becomes more dense as the temperature drops. It reaches it’s maximum density at 39.2 degrees Fahrenheit. Water is heaviest at that temperature. Therefore, in the middle of winter, water at 39 degrees will be on the bottom and the water above will be colder and lighter. When spring arrives, the sun warms the surface water to 39.2 degrees and then it begins to sink. It is a this point the lake is said to have “turned over”.

So, how does a person find where the Thermocline is? There are a couple of ways to determine at what depth it is. Todays higher end fish finders with color displays are by far the easiest way to determine at what depth it is. The manufacturers of these sonar units have fine tuned them to measure the density of the water. Just this year I purchased a Humminbird 585C fish finder with a color display. It clearly shows where the thermocline(s) is. The unit displays the denser (colder) water with a series blue pixels. The trout will most likely be suspended just above this level. They will find the right mixture of the deeper cold water and the shallower warm water.

Another less expensive, but more time consuming method of finding the thermocline is with a Vexilar Deptherm thermometer. It is a thermometer inside of a plastic tube which collects and holds water. To measure the temperatures at different depths, use a metered line marked with a permanent marker and a loop in the line every five feet. Lower the thermometer to different depths and take readings. I use an inexpensive ice fishing reel with Dacron fly line backing. When the temperature difference is sudden, you know that you have found the thermocline.



At this point, you may be asking what this has to do with fly fishing. Well, actually quite a bit. Say for instance, you take a surface temperature of 74 degrees in a cove where you are fishing and it is 8 feet deep. The comfort range for most trout is between 60-70 degrees. If the thermocline for the lake is at 23 ft and the temperature at that depth is 62 degrees, that cove will most likely not have any fish lying in it. Dry fly fishing in that cove will produce muscles in your arm, but not many fish.

My friend Steve and I were out at Amber Lake fishing one warm summer day. The surface temperature was 72 degrees and a thermocline showing up on my fish finder at 23 ft. We both marked fish at about 21 ft.. Steve anchored, measured out 21 ft of fly line with leader and proceeded to fish with a chironomid. A couple of hours later he had landed about 17 fish. He was fishing just above the thermocline.

There are times that there will be more than one thermocline. I have noticed this on a couple of lakes. I am not sure as to why there would be more than one. I can only assume that wind and wave action are stirring the water column. The wind blowing across the surface of the lake must have a rotating effect on the water column. As the colder water is drawn to the surface, it will begin to sink once again towards the bottom.



In the above photo, you can see that at about 11 ft. there is a small thin thermocline. On that day, I was getting some markings of fish just at about 10 ft.
The second thermocline was at approximately 23 ft. and went all the way to the bottom which is shown as the dark red band. The majority of the fish I marked were just above this level at 21 ft. I took a temperature at 21 ft. and got a reading of 63 degrees F. This temperature falls right in the middle of a Rainbows comfort range. The bottom temperature at 37 ft. was 54 degrees. Rainbows can tolerate that temperature, but it is not comfortable for them and plant life is minimal.
The 23 ft. level thermocline stayed very consistant in depth as I moved towards shore. The the photo below, at 23.8 ft you can see that the thermocline is very thin (blue line above the red bottom) and close to the bottom. The surface temperature was 70.8 degrees and the bottom temperature was 63 degrees. This is an excellent level to fish at. The day I was taking these readings, a friend was fishing at exactly that depth, not far from where I was when I took this picture. Using a black leech with sinking line, he dropped his line just above the bottom and proceeded to move very slowly and jigging his line. Don (Spoolguy) had a 40+ fish day!



One might think that the surface temperature of a lake during the summer stays the same in a 24 hour period, but it doesn’t. I have seen the surface temperature rise 5 degrees over the length of a day. The coldest part of a night is said to be just before sunrise. Therefore, fish will come to the surface when the water is cooler and feed. How many times have you talked with a person on the lake and he has told you, “You should have seen them rising at 5:00 a.m. this morning!” With the early morning hatch and the cooler water, fish will leave that area just above the thermocline until the sun gets warm and the water temperature starts to climb. They will then move back down to the comfort zone above the thermocline.

I have seen several well meaning souls on a hot summer day that are anchored in 12 ft of water fishing a chironomid with a strike indicator. I politely ask them if they are getting any fish. Almost always they reply with a disgruntled “Not very good. I have been here for 3 hours and haven’t had a strike.” I then kindly suggest that they move into deeper water because I am reading fish at 23 ft. Some thank me and pull up anchor and move. Some don’t, thinking that just maybe there will be some fish there. I go on my way thinking that since his fishing trip that day was non productive, maybe he will try Huckleberry picking in Odessa next weekend.