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The thermocline is the layer of water between the top water and the dead zone where fish have the best mix of O2 and temperature. So, even if it won't show up you can have a real good idea of where its at by knowing what depth you consistently stop making fish at. One thing to remember is that larger fish will hang out at the bottom of the thermocline more so than the smaller ones.

(i just did some reading on this and I posted the "laymans" reading.)

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I can often see the thermocline on my fishfinder.

Here's a look at some pictures I took at Nimisila in July 2011. The thermocline is visible on my sonar as the dark band at about 14 ft.


I made a device to measure the temperature at various depths by modifying one of those indoor/outdoor digital thermometers. It was one with a 10ft wire for the outdoor probe. I spliced in a spool of speaker wire to give more cable length and then marked the depths on the wire with tape.

Here's a plot of the temperature at depths I gathered with the probe over the deep area by the south end of the lake. You can see that the temperature is dropping quite rapidly in the same range as the dark band on the sonar. So the surface water is in the low 80's, but below the thermocline it's in the high 50's.


And here's a plot of the change in temperature per foot increase in depth. You can see that the temperature is changing most rapidly in the 13-14ft depth range.


The temperature probe. A bit crude, but it seemed to work alright.
 

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I can often see the thermocline on my fishfinder.

Here's a look at some pictures I took at Nimisila in July 2011. The thermocline is visible on my sonar as the dark band at about 14 ft.


I made a device to measure the temperature at various depths by modifying one of those indoor/outdoor digital thermometers. It was one with a 10ft wire for the outdoor probe. I spliced in a spool of speaker wire to give more cable length and then marked the depths on the wire with tape.

Here's a plot of the temperature at depths I gathered with the probe over the deep area by the south end of the lake. You can see that the temperature is dropping quite rapidly in the same range as the dark band on the sonar. So the surface water is in the low 80's, but below the thermocline it's in the high 50's.


And here's a plot of the change in temperature per foot increase in depth. You can see that the temperature is changing most rapidly in the 13-14ft depth range.


The temperature probe. A bit crude, but it seemed to work alright.
Great presentation!!!

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Huz-yak, what you are seeing is photo plankton. In the evening you will see it rise to the surface as it gets dark. They actually capture a bubble of air and take it back down with them. Thats why they show up on your graph so well.
 

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Huz-yak, what you are seeing is photo plankton. In the evening you will see it rise to the surface as it gets dark. They actually capture a bubble of air and take it back down with them. Thats why they show up on your graph so well.
http://www.bassresource.com/bass-fi...ybody-use-switchfire-mode-on-your-humminbird/

In this link, the first picture is usually what I see in late July/August... only in black and white. I can still make out the actual bottom and the depth reads correctly, it simply looks like a false bottom on a low res b&w screen. Do you still think that this could be plankton? I am not being a [email protected]$$, I honestly don't know.
 

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Great work! I am no expert, but it sure looks like a typicial thermocline signature to me (I see them on my low end bird) and your temperature/depth data coincides with the visual evidence.

Either way, the information was very useful. I never suspected the temperature was so low at the bottom when the surface was so warm. Incredible.
 

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http://www.bassresource.com/bass-fi...ybody-use-switchfire-mode-on-your-humminbird/

In this link, the first picture is usually what I see in late July/August... only in black and white. I can still make out the actual bottom and the depth reads correctly, it simply looks like a false bottom on a low res b&w screen. Do you still think that this could be plankton? I am not being a [email protected]$$, I honestly don't know.
Those pics look like thermocline. I misunderstood what you were saying. The photoplankton will form a layer from the bottom up. As evening approaches this layer will actually rise from bottom to the top.
 

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Come on JIG,,This is your SPECIALITY!! I know you have a bit of experience to share Buddy... I too want to hear a bit more on this topic too.....& as MadMac said- that plankton actually cycle daily,, through the same image area that the thermocline is at & will hi-lite the image being represented on your eletronics,,in combination & give a more definite image on the screen,,& will then become a wider band of "echo marks" that further puzzles us as to why the thermo line was a narrow mark/line,,is now a less definite & wider mark/line??.. & I will add this,,that anyone that has jumped in a lake with water depths better than 20' with surface temps at 75degs' or better,,will know what thermocline is .... It's that wall of COLD water that you run into at about 8'-10' down,, & that's just the cooler water that is above the actual thermocline!! The Cold stuff is even deeper...... Like the info brought out here on this post Guys,,We're still learnin'!! ----->>>>>sonar<<<<<-----
 

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Those pics look like thermocline. I misunderstood what you were saying. The photoplankton will form a layer from the bottom up. As evening approaches this layer will actually rise from bottom to the top.
No problem Mac. I started using sonar just 2 hears ago and i really just use it to find depth and bait. I know what bait looks like bc i went out a few times at 4 AM and shined a flashlight into the water till the shad boiled and watched the screen. I noticed the thermocline forming in late July when i would cross familiar areas and noticed the dark area that always seemed to be at 15' - 20'. I assumed this was the thermocline.... But your phytoplankton comment has me wondering. Another thing I see a lot, and I hope somone comments, is a thick dark line/area/streak near the bottom that has the same signature as bait. I noticed it much more this spring than last. Anyone think this is bait holding the bottom like they do in a creek?


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Huz-yak, what you are seeing is photo plankton. In the evening you will see it rise to the surface as it gets dark. They actually capture a bubble of air and take it back down with them. Thats why they show up on your graph so well.
That is plankton. The thermocline should be a THIN line across the screen. There is NO thermocline in the spring. I have a Lowrance X136 on the bow, and I have a Humminbird 798ci si combo on the back. They both show the same. If you turn it up too far, then you'll get plankton. If you can't find it. look for where the bait is suspending The thermocline is just under that. Think about when you jumped off a dock, or dove down. Suddenly it's cold. That's the thermocline.
 

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That is plankton. The thermocline should be a THIN line across the screen. There is NO thermocline in the spring. I have a Lowrance X136 on the bow, and I have a Humminbird 798ci si combo on the back. They both show the same. If you turn it up too far, then you'll get plankton. If you can't find it. look for where the bait is suspending The thermocline is just under that. Think about when you jumped off a dock, or dove down. Suddenly it's cold. That's the thermocline.
I must have mine cranked up too high. Also explains why I can never catch all those HUGE fish I see... I pulled mine out of the box and never played with it. I think I will try messing with the sensitivity next time I go out. I'll snap a few pics of the screen and post them to see what you seasoned guys think.
 

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Is the thermocline shallower in shallow lakes? Just curious cause I was fishing Wingfoot last weekend, which is less than 20' deep everywhere, and was only getting bites when drifting up or down the slopes leading to the islands or in other shallower areas. I was fishing the bottom with a sinker and hook. I'm thinking that maybe I should have been fishing a few feet above the bottom in the deeper parts of the lake? I don't have a fish finder yet.
 

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Keep in mind when you are on a reservoir there is current from the inflow to the dam out flow. If there is enough current it can prevent or disturb the thermocline. A true thermocline prevents oxygen underneath it so the fish can't stay there very long. Wind and lake orientation can also effect it.
 

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Did you read the link? It claims in very clear waters photosynthesis can take place below thermoclines. As far as I know there is no correlation between lack of oxygen and thermoclines. It's just usually it's so deep there is a lack of oxygen.
 
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