Where is the thermocline?

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Tomcat
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Where is the thermocline?

Post by Tomcat »

I’ve been attempting to better understand precisely what and where the thermocline is because of its importance to successful fishing.

As you know, as one descends in water, temperatures usually drop. Less light is able to penetrate, so there is less conversion of light to heat as depth increases. Generally, as water cools it becomes more dense and sinks down while the warmer less dense water rises. This process results in lake stratification into three layers:

1. The epilimnion layer – the top warmer layer of the lake.
2. The metalimnion layer – the middle layer that is characterized by rapid temperature changes; and
3. The hypolimnion layer - the bottom cold layer.

Some internet references use the terms metalimnion and thermocline synonymously while others claimed that the thermocline is the layer where the temperature decreases most rapidly, one degree centigrade or more with each meter of depth. Others yet claimed that the thermocline is the complete stratum of water between the top warmer layer and bottom colder layer. Which is it?

Here is a set of actual water temperatures versus depth recorded during a Fall Walleye Index Netting study.
Water Depth--------Water Temp--------Water Depth
Meters----------------° C------------------Feet

0.5--------------------18-------------------1.6
1----------------------18--------------------3.3
2----------------------18--------------------6.6
3----------------------18--------------------9.8
4----------------------18--------------------13.1
5----------------------18--------------------16.4
6----------------------18--------------------19.7
7----------------------18--------------------23
8----------------------18--------------------26.2
9----------------------18--------------------29.5
10---------------------15--------------------32.8
11---------------------12--------------------36.1
12---------------------10--------------------39.4
13---------------------9---------------------42.7
14---------------------8---------------------45.9
16---------------------7---------------------52.5
18---------------------6.5-------------------59.1
20---------------------6---------------------65.6
24---------------------5---------------------78.7
28---------------------5---------------------92
32---------------------5---------------------105
36---------------------5---------------------118
40---------------------5---------------------131

Is the thermocline the water layer from 9 meters to 16 meters or is it more precisely the water layer between 9 meters and 11 meters because that’s where the largest temperature decline per meter took place (i.e. 6 degrees C in 2 meters)?

Secondly, in the same lake at the same time of year as the above water temps were recorded, my sonar screen displayed the typical thermocline foot print (dark line across the screen with shaded area beneath). The dark line was at approx 25 meters (80 feet) and the shaded area layer beneath that dark line was about 3 meters (10 feet) thick. I believe that I should be able to see a thermocline at 9 meters (30 feet). Could it be that I don't have my sonar sensitivity set high enough to see the higher thermocline, or is it that my sonar unit only indicates a thermocline when the water density reaches a specific level (e.g. the density of the coldest water in the lake (4 or 5 degrees C))?

I’d appreciate any input. Thanks in advance.
Last edited by Tomcat on Sun May 28, 2006 2:14 pm, edited 2 times in total.
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McQ
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Post by McQ »

I've never been able to make the leap to metric conversion so maybe you could do the changes for me.

My advice would be to start from the bottom up rather than top down to locate and position the thermocline.

Base cold water temperature will be 39 degrees, as you move upwards from that measurement to the point where you experience a temperature differential of 3 degrees/5' you will most likely be at the right level.

Oxygen depletion below the thermocline is quite rapid so look for your fish to be staged just above (within 5' of the layer), especially so, where the thermocline meets structure (points & humps). A helpfull idea is to understand the habits of your quarry: eg - trout follow migratory routes much like we drive on highways. Necked down areas created by points & saddles between visible structural elements indicate a continuance of the same contour underwater. When your sonar reading confirms this fact then you should be in the strike zone. Large bays with a pronounced break line are also good target areas especially if the contour indicates a feeding flat.

Bait fish (schools of cisco) will adjust to the thermocline first - trout follow the food source.

Adjusting your sensitivity level to capture the thermocline line is subject to the variables of each individual unit - too much gain will increase surface clutter so my thoughts are to use your zoom feature to isolate to the higher percentage area.

Taking the time to dip a thermometer first to find the coldest water should always be the first step. Your electronics can then be best utilized once this known factor is set and I'm sure others with more experience will be able to make this factor clearer.
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LostFisherman
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Thermoclines

Post by LostFisherman »

Hi Tomcat

I run a Lowrance Paper Graph and a Matrix37 on my boat when I fish and based on your numbers I would say your thermocline would be between 10.0 and 14.0 meters. Normally when you get the dark line on your screen in deeper water and then head for a shoreline the dark line lets say is at 20 foot. When you get near a botom that is at 20 foot the line will disappear or blend in. The one thing you may want to keep in mind that on some Canadian lakes that you may have more than 1 thermocline. You may also have one much deeper down. Thermoclines are the most oxygen rich waters and the prefered tempurature for most fish up there so they tend to go to it in the summer. From experience I know that Lake Trout sitting way under the Thermocline are ussually not feeding. The ones that are just sitting under the bait fish IN the thermocline are the most aggrasive ones. I hope this helps you some. I always think of the thermocline as a point where the tempurature starts making a radical drop per foot down and ends where it starts to level off again.
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Post by Tomcat »

Thanks for the replies. I edited my first post in this thread to include depths in feet in order to facilitate consideration of the question. I also opted to use temperatures recorded at various depths for another year's FWIN study so as to include deeper depths (down to 40 meters (131 feet)). Doing so may help explain why I see the typical thermocline footprint at 25 meters (80 feet) instead of at 9 meters (30 feet).
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Post by Wallyboss »

I went scubadiving at Black Donald Lake close to Calabogie years ago and the Thermocline there started at 31' and it was about 4 feet deep in 45 FOW. It is really cool cause if you look right where it starts or finishes it looks like the haze over pavement on a hot sunny day.
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Tomcat
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Post by Tomcat »

Hi Dan:

Thanks for the input on my question. Regarding your statement that the thermocline layer looked like a haze over pavement on a hot sunny day, I came across the following explanation in an In-fisherman article:

"Plankton veil -- Nearly every time I watched the camera probe descend into the thermocline (in a variety of sites) a discernible veil of what was later determined to be zooplankton appeared. Most often, this cloud of organisms hovered most densely near the upper range of the thermocline. As described earlier, it looked on screen like a thick layer of fog or muddy water when viewed from slightly above its level. This gathering of plankton exhibited a definite depth (or water temperature) boundary. "

The same article also made the following statements about where the thermocline is:

"Strictly speaking, although many individuals use the terms metalimnion and thermocline interchangeably, they actually refer to two different layers of water, one within the other. The metalimnion refers to the thinner middle layer of water where temperature drops rapidly. The thermocline, which lies within the metalimnion, is defined as the area where water temperatures drop by at least one-half degree per foot."

Understanding the "plankton veil" sures makes it easier to understand why the baitfish are in the same area.

Jay
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Post by Todd B. »

Tomcat,

With the exception of Spring and Fall turnover, the zone below the thermocline is oxygen poor and typically a dead zone. Fish that do not tolerate warm water will often be found around structure that is just above the thermocline. 4C is generally the temperature immediately below the thermocline. The easiest method to find the thermocline is with a hi-low thermometer to determine the approximate depth. Then you can use your depth finder, etc to fish structure at that depth.
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