# thermocline



## ballast (Sep 15, 2011)

Does it or does it not show up on a fishfinder? I think it does.


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## Huz-yak (Jun 3, 2011)

I have a cheap humminbird 160 and it shows on mine no problem. Just gotta turn up the power or sensitivity. Looks like a flat dark area near the bottom. You can tell when you go over deep areas or channels but the bottom looks flat


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## Mr. A (Apr 23, 2012)

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.)

~^~^~^~^~^
| Mr. A
|
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## sherman51 (Apr 12, 2011)

it usually shows up as a dark line on most fishfinders.
sherman


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## PolymerStew (Feb 17, 2009)

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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## Bad Bub (Jan 17, 2006)

PolymerStew said:


> 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.
> 
> ...


Great presentation!!!

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## MadMac (May 2, 2005)

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 (Jun 3, 2011)

MadMac said:


> 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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## louisvillefisherman (Jun 9, 2012)

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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## MadMac (May 2, 2005)

Huz-yak said:


> 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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## sonar (Mar 20, 2005)

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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## ya13ya03 (Sep 21, 2010)

Great stuff guys. I too is a learning.


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## Huz-yak (Jun 3, 2011)

MadMac said:


> 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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## jimthepolack (Mar 24, 2013)

MadMac said:


> 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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## Huz-yak (Jun 3, 2011)

jimthepolack said:


> 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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## bdawg (Apr 14, 2009)

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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## MadMac (May 2, 2005)

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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## Daduru (Apr 14, 2004)

http://www.bassresource.com/fishing/thermocline.html
This is an interesting resource. I will say that some of our clear upground reservoirs I have had massive luck in the middle of summer slaying bass with bottom bouncers in 30+ feet of water. So bass can be found below the thermocline, but it requires 15+ of water clarity in my experience.


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## MadMac (May 2, 2005)

If your catching fish on the bottom in 30+' there is no thermocline. No fish can survive without oxygen.


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## Daduru (Apr 14, 2004)

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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## MadMac (May 2, 2005)

That's interesting. Never read that before. Never knew we had reservoirs that clear in Ohio either. I'm used to the ones around here and especially West Branch.


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## Daduru (Apr 14, 2004)

There are a few in the western part of greater cleveland. Upground, with no trees near by to drop leaves in, and rocky bottoms.


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## sonar (Mar 20, 2005)

I know that Wellington Up-ground Res.,,is next in clarity,,to the old Marbelhead Limestone Quarrys,,They are the most clear water I know of,, You can see a penny flutter past 20' at Wel.,, & way deeper at Mar.,, + the Quarry is the COLDEST water I EVER did dive into!!! But you can see fish swimming below you,,on the bottom as you pass over them, at both sites.. There must be oxygen down there.... ----->>>>>sonar<<<<<-----


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## Eliminator (Aug 26, 2006)

I read somewhere that fish kills do happen due to the thermolcline phenomena if I remember correctly, and the fish actually get trapped within that layer of water and sorta go into shock and suffacate.


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## sonar (Mar 20, 2005)

I've read that 'Strong Wind"!!,, across the surface of a body of open water can remove most to all of the oxygen from that effected area/lake.. With the depletion of oxygen,,sucked out as a result of an EXTREMELY-fast moving wind across the surface ,,there can be a 'Kill" that may occur.. As I read,,this doesn't happen all that often around here...BUT,,it has happened,,& it will happen again.. That's the natural cycle of Nature.. ----->>>>>sonar<<<<<-----


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## Huz-yak (Jun 3, 2011)

http://www.arkstriper.com/thermocline.pdf

The wind can help to start the turnover in the fall which causes the stagnant, depleted water on the bottom to mix with the top. I do not think the wind itself can remove the oxygen, but quite the opposite.


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## Bassbme (Mar 11, 2012)

Huz-yak said:


> I do not think the wind itself can remove the oxygen, but quite the opposite.


Yes....... wind and wave action oxygenates water ..... they don't deplete it.


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## SteelEyes (Jul 1, 2013)

Bassbme said:


> Yes....... wind and wave action oxygenates water ..... they don't deplete it.


That's my understanding as well, but would note that with a thermocline you have temperature stratification in the water. So the waves are only oxygenating the upper layer, the layer below the thermocline isn't getting oxygen from wave action. 
That said that doesn't mean that dead zones (anoxic) are a given below the thermocline, but are possible if decomosition of dead organisms consumes all the available oxygen, which is slow to be replenished.
The second post in the link gives a good breakdown of how it works on Lake Erie.

http://ohioseagrant.osu.edu/discuss/index.php/topic,142.msg597.html


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## sonar (Mar 20, 2005)

Fact is Huz & Bass,,is that the wind does do as you said,,mixes the water & oxygen as you believe,, BUT as I said,, EXTREME HIGH winds,,with the right 
Baro-pressures,,& winds can deplete the oxygen,,Sucks it right out..& fish & aquatics will die.. in an instant!! ----->>>>>sonar<<<<<-----


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## SirDubblin (Jul 19, 2013)

Great stuff... glad I found a post on this, I've been reading up on this topic over the past few days now that I picked up a fish finder. Seems to me the key to catching fishing the summer is finding the thermacline and fishing at (or just above) that depth... especially with the heat wave we just had.


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## Huz-yak (Jun 3, 2011)

With all due respect, Sonar, that is complete nonsense. There would need to be an impossible deficit of oxygen in the atmosphere before it could absorb oxygen from the water. I think the only situation would be when turnover is close and the wind forces the process to start


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## sonar (Mar 20, 2005)

"Turnover of the thermal stratification in lakes can cause oxygen depletion when the anoxic (low oxygen) bottom water and decaying organic matter mix with the oxygen rich upper water. In shallow lakes, this can be caused by high winds, heavy cold rain, or severe hailstorms during a period of prolonged hot weather. When oxygen depletion occurs, large fish and those fish species with the highest oxygen requirements die first."Pa. Boat & Fisheries... One Quote,,One possibility,, of "Oxygen Depletion.. I'm NOT saying this IS the "RULE",,only an element of occasional occurrence,, that has in the past been responsible for fish kill in our area.... ----->>>>>sonar<<<<<-----


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## The Ghost (Jul 3, 2013)

In that case, the wind isn't depleting the oxygen. Its breaking up the stratification and exposing fish to _already_ oxygen depleted water.


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## sonar (Mar 20, 2005)

This is ALL I got,,on this subject,, Oxygen is separate as itself,, like is water,,and as I understand it from the report I read years ago,,& it was a very warm Summer like what we have now,,that the HIGH winds passing over a body of water,,would form ,,for lack of better terms,,a vacuum like effect that would along with the right Barometric Pressure condition,,& higher air & water temps.,,can separate & pull the oxygen out of the water as it passes over it.... According to the Report that was in the News.. I DID NOT think of this phenomenon on my own,, I read it In a local State Report..& thought that it was quite an amazing occurrence!! WHAT,, H-y??  ----->>>>>sonar<<<<<------


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