Let's Talk Thermoclines And Turnover 2024

J Francho reply : 

I've seen tons of posts referencing thermoclines, and while I know what they are, and how they affect my fishing, I'd like to see any information - including myths or misconceptions about them, or even turnover - included in the conversation.  Even a simple FAQ link or wiki page will do!

 

It's winter, so let's try and get a thread going that combines some science with experience.

 

I'll make a small point to get started.  A common misconception I hear is that there is no oxygen in the water below the thermocline.  Not true.  The proper statement would be there is very little oxygenation below the thermocline.  If there is abundant O2 use below the thermocline, then it can become oxygen poor.  However, in some lakes there is plenty of O2 to support fish. Think about lakes with trout and/or salmon.  They stay below the thermocline in summer.  The warm water will kill them, being cold water species. 

 

How does this relate to bass fishing?  I catch smallmouth from very deep, as many northerners will also report.  Below the thermocline?  Maybe.  I don't think the thermocline is the impassable barrier some make it out to be.  In some lakes, maybe, but certainly not all.  I think the main limiting factor may temperature, not O2.

 

Another tidbit: The thermocline is not a simple line between the two layers - the warmer epilimnium on top, the colder hypolimnium below - but a thin band of transition water that can be as thin as a few feet, or as thick as 15 feet.  So, it isn't always just a thin line on your graph.

 

Last bit on turnover: Storms or current change can cause localized turnover, where the three layers mix.  It doesn't mean the whole lake has turned over, but it can affect fishing. 

 

What do you have on the topic, or can add to what I posted?

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WRB reply : 

Don't have the time to dicuss this now, however searching the web under lake stratification, terms like epilimnion, metalimnion, hypolimnion will open the door to this discussion.

Things to consider are the differences between large and small natural lakes verses man made impoundments (reservoirs) and rivers.

Tom

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J Francho reply : 

Take your time, Tom.  I was hoping you'd jump on the discussion, as I know you are well versed both in book smarts and on the water smarts.

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shimmy reply : 

Wayne P. should pipe in. He spends a lot of time studying it and fishing the changing conditions.

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Nitrofreak reply : 

For those who do not understand what a thermocline is , pay attention to this thread .

 

 

I think the most misunderstood concept of the thermocline is that fish do not go below it due to the oxygen levels and can not survive , this can be misinterpreted due to the differences in lakes and water clarity , J.,  the trout you mentioned live below thermoclines in waters around my region , specifically the Brown Trout and pike ,  the cooler water is where they thrive just below the thermocline .

 

For me , when I was learning how important a thermocline is , understanding it was a different ballgame and quite honestly still is , but the advantages it provides in bass fishing is too great not to get an understanding of , for those who are about to read this thread that have no idea what a thermocline is , don't be afraid to ask questions !

 

In my own experience , using this as a guide redefines what part of the lake ( my favorite lake ) is the best to fish , basically I was able to use the thermocline as the lake bottom and aided in searching out structure just above and within this depth especially during the summer months .

 

What you learn from this thread especially when you read WRB's post and others , will open up a new light in the upcoming months and how the fish relate to this in different bodies of water . 

 

We are in for a treat !!!

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MacP reply : 

I could write a treatise on the subject. I got my degrees in fisheries biology, limnology and icthyology.  I'll see if I can scrounge up some links tonight. 

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DelcoSol reply : 

Could we start at the beginning for the thermocline idiots (me)? I do not really know much about it so I will be watching. Also maybe some pics of it on a fishfinder?  

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Crestliner2008 reply : 

One important aspect to discuss here is that I do not believe all lakes set a thermocline in summer. Take a reservoir for instance. With water moving through the main body (which it has to, if it's a reservoir), I find it difficult to believe that a thermocline could set up. Maybe I'm wrong. But if rivers don't thermocline, how would a reservoir have one?

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Todd2 reply : 

  On 12/17/2013 at 8:57 AM, Crestliner2008 said:

One important aspect to discuss here is that I do not believe all lakes set a thermocline in summer. Take a reservoir for instance. With water moving through the main body (which it has to, if it's a reservoir), I find it difficult to believe that a thermocline could set up. Maybe I'm wrong. But if rivers don't thermocline, how would a reservoir have one?

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WRB reply : 

For purposes of fishing we are discussing lake stratification layers of water with at least 4 degree change in water temperature with a few feet of depth change. The top layer of water in the summer is warmer, as the water gets deeper it cools, if the water get colder quickly, with a few feet 3' to 5' for example, a noticeable change called a thermocline layer. The thermocline layer can lower the dissolved oxygen (DO) in lakes where the depth of light is poor and prevents green aquatic growth. Green weeds produce DO, decaying organic matter like dying weeds absorb DO.

What this means is a lake can have clear water and a thermocline with weed growth below the thermocline depth and still have good levels of DO to support fish. Lakes with poor water clarity where the sun light cannot penetrate deeper than a thermocline layer may have poor levels of DO that doesn't support fish life.

Fish can't breath oxygen (02) it must be dissolved (DO) in the water, DO is measured in ppm or mg/L, most game fish need 3 to 12 mg/L, lower or higher they can't survive.

The water needs to be fairly stable with little current flow to stratify, otherwise it tends to mix preventing quick temperature changes. Most reserviors are built for water storage, flood control, navigation, power generation or a combination. Most dams discharge water from the bottom and this may not create enough current flow near the surface or near the thermocline layer, if one exists, to change water temperatures or disrupt the thermocline. In very clear water lakes with deep weed growth can have multiple thermocline layers. For example a thermocline can develop in the summer where the water temperature changes quickly at 15' to 18', lets say 80 degrees changes 75 to 72 degrees at 15' developing a thermocline and changes again at 35' to 40' from 65 to 60 degrees for a second thermocline. Bass can be at 40' or deeper if the DO levels support them.

Tom

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11justin22 reply : 

  On 12/17/2013 at 8:57 AM, Crestliner2008 said:

One important aspect to discuss here is that I do not believe all lakes set a thermocline in summer. Take a reservoir for instance. With water moving through the main body (which it has to, if it's a reservoir), I find it difficult to believe that a thermocline could set up. Maybe I'm wrong. But if rivers don't thermocline, how would a reservoir have one?

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kwhorne19 reply : 

what are some ways that you can tell on a cheap fish finder - i.e. I have a Humminbird Wide Potrait that isn't that great, it is black and white so what am I looking for?  Also, how can you tell if there is enough DO below the thermocline to support fish, are you just supposed to find structure, ledges, drops, etc and look for fish below...  Basically I need a "Thermocline for Dummies" book, you know one of those yellow one's....

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Snakehead Whisperer reply : 

  On 12/17/2013 at 10:30 AM, WRB said:

For purposes of fishing we are discussing lake stratification layers of water with at least 4 degree change in water temperature with a few feet of depth change. The top layer of water in the summer is warmer, as the water gets deeper it cools, if the water get colder quickly, with a few feet 3' to 5' for example, a noticeable change called a thermocline layer. The thermocline layer can lower the dissolved oxygen (DO) in lakes where the depth of light is poor and prevents green aquatic growth. Green weeds produce DO, decaying organic matter like dying weeds absorb DO.

What this means is a lake can have clear water and a thermocline with weed growth below the thermocline depth and still have good levels of DO to support fish. Lakes with poor water clarity where the sun light cannot penetrate deeper than a thermocline layer may have poor levels of DO that doesn't support fish life.

Fish can't breath oxygen (02) it must be dissolved (DO) in the water, DO is measured in ppm or mg/L, most game fish need 3 to 12 mg/L, lower or higher they can't survive.

The water needs to be fairly stable with little current flow to stratify, otherwise it tends to mix preventing quick temperature changes. Most reserviors are built for water storage, flood control, navigation, power generation or a combination. Most dams discharge water from the bottom and this may not create enough current flow near the surface or near the thermocline layer, if one exists, to change water temperatures or disrupt the thermocline. In very clear water lakes with deep weed growth can have multiple thermocline layers. For example a thermocline can develop in the summer where the water temperature changes quickly at 15' to 18', lets say 80 degrees changes 75 to 72 degrees at 15' developing a thermocline and changes again at 35' to 40' from 65 to 60 degrees for a second thermocline. Bass can be at 40' or deeper if the DO levels support them.

Tom

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WRB reply : 

Some smaller and medium size lakes use aeration systems to pump O2 (oxygen) down in deep water by the dam or deepest water area to off set the affects of DO loss due to decaying matter. Aeration system tends to mix the stratified layers in the deep water areas, breaking up thermoclines artificially.

High winds can also mix up water temperature by up welling the deeper water and mixing it with warmer upper layers, lots of factors in play. High wind often bends the thermocline where it's shallower on one side of the lake and deeper on the opposite side of the wind, thermocline depths vary to different locations.

What fisherman are really interested in is the life zone depth, often synominous with a thermocline layer depth.

With a good sonar unit you can set the gain manually to show the dense colder water verses the lighter heavier water, this shows up as a dark line or fuzzy darker zone, depending on how abrupt the temperature transition is. If you also watch for suspended fish they are often near the temperature break and this is the life zone you are looking for. You may not meter a thermocline, but should be able to meter fish at the same depth around the lake.

Setting the sonar gain by increasing power until the screen shows clutter, then backing off slightly to reduce the background clutter, but do not eliminate it. The gain should be high enough to show baitfish, some clutter and water temperature changes, this takes some practice.

Bass are warm water fish however warm comfortable water tends to be lower in DO levels, so bass go deeper into cool water about 65 to 70 degrees during the summer if they can because cool water holds high levels of DO and attracts baitfish. Shallower water with good green vegetation also cools the water and the vegetation produces DO during daylight, bass like to locate under weed mats for the same reason as deeper water; comfort and food.

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Wayne P. reply : 

Any sonar unit will display the thermocline if adjusted correctly. The thermocline has a 1 degree of temp change per 1 foot of depth change. That quick temp change makes the water denser and it reflects sonar pulses. Below the thermocline the water is denser too than it is above the thermocline. That density also reflects sonar pulses.

This screen shot shows the thermocline.  I use this in the summer though fall to pattern the bass. When I see this, I know that I don't have to search deeper than the top of the thermocline to find them.

 

 

This is a deeper lake and in this case it was in the first cool weather in the Fall with a lower surface temp, but the rest of the water column was still stratified. The fish are warm water species and mostly stay above the thermocline--thus the pattern is the same as the above photo:

 

 

An extension of the above showing more lake profile:

 

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KyakR reply : 

Wish I had electronics that good! Always amazed at Tom's insight, how he can apply what he knows!

My method of finding the thermocline is to cast my little depth/temperature gauge several times 'til I find it. But, Tom, from what you're saying it can vary hugely from place to place due to many factors! In a kayak my access is limited.  Sometimes I ask my region's fisheries biologist questions.........he usually focuses on the interplay of all the aquatic organisms in a lake, what temperature and oxygen levels they need, what they eat, where they can escape predation. He tells me that some prey fish actually use the thermocline as a place to hide.......that they actually go into the lethal zone to forage! It always amazes me how everything's connected in nature, how one thing changing can change everything else. He also mentioned pH, but I didn't quite get that one

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KyakR reply : 

Wow Wayne P!!! Thanks!

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MacP reply : 

So, I forgot that my Limnology books weren't at home, but at the office.

 

I'll try and scan the appropriate pages when I get a chance.

 

Here is an entire limnology course online, without the optional reading materials;  http://www.esf.edu/efb/schulz/Limnology/Limnology3.html

 

Here's the same thing from another school; http://academic.keystone.edu/jskinner/Limnology/

 

I'll scan those in and try and answer questions anyone has.

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Wayne P. reply : 

Bass will go into/below the thermocline for a short period.

 

This is a school of bass that I unknownly scared that took " refuge" in a structure I was checking out in/below the thermocline. a minute later they left that and I got their image again, then about a minute later they were on the surface and I caught some of them with a jerkbait and wacky rigged finesse worm. The top of the thermocline is at 15'.

 

 

 

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MacP reply : 

  On 12/17/2013 at 1:43 AM, J Francho said:

I've seen tons of posts referencing thermoclines, and while I know what they are, and how they affect my fishing, I'd like to see any information - including myths or misconceptions about them, or even turnover - included in the conversation.  Even a simple FAQ link or wiki page will do!

 

It's winter, so let's try and get a thread going that combines some science with experience.

 

I'll make a small point to get started.  A common misconception I hear is that there is no oxygen in the water below the thermocline.  Not true.  The proper statement would be there is very little oxygenation below the thermocline.  If there is abundant O2 use below the thermocline, then it can become oxygen poor.  However, in some lakes there is plenty of O2 to support fish. Think about lakes with trout and/or salmon.  They stay below the thermocline in summer.  The warm water will kill them, being cold water species. 

 

How does this relate to bass fishing?  I catch smallmouth from very deep, as many northerners will also report.  Below the thermocline?  Maybe.  I don't think the thermocline is the impassable barrier some make it out to be.  In some lakes, maybe, but certainly not all.  I think the main limiting factor may temperature, not O2.

 

Another tidbit: The thermocline is not a simple line between the two layers - the warmer epilimnium on top, the colder hypolimnium below - but a thin band of transition water that can be as thin as a few feet, or as thick as 15 feet.  So, it isn't always just a thin line on your graph.

 

Last bit on turnover: Storms or current change can cause localized turnover, where the three layers mix.  It doesn't mean the whole lake has turned over, but it can affect fishing. 

 

What do you have on the topic, or can add to what I posted?

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Nitrofreak reply : 

My Favorite lake is a Nuke Plant lake , I find the thermocline more easily in the upper part of my lake and mid part of my lake , down by dike 3 or the return there is little to no thermocline , although if I cross the lake ( roughly ) 1/2 mile in distance I can sometimes pick one up in the upper part of the opposite side , this is starting to make more and more sense to me , so what about turn over , should I expect to see areas that show thermoclines have turnover as well ?

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WRB reply : 

MacP a picture (illustration) is worth a thousand words, looking forward to seeing what you add to this thread.

The pH factor was a major issue during the early years of bass fishing and Dr. Loren Hill included a pH meter in his Color meter back in the 80's. As a baseline drinking water is neutral pH at 7 and what most bass learned was their lakes water was close to neutral pH 7, between 6.5-7.5 nearly everywhere. Lakes around industrial areas that suffered pollution from burning fuels with high levels of sulphur producing SO2 or nitric acid NHO3 has mostly disappeared by 2000, due to stronger laws.

Again a good sonar unit indicates where there is life zone, the pH becomes less important with today's bass anglers.

Tom

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MacP reply : 

Further reading (warning maths and graphs!)

 

http://jmp.sh/v/W6me4ORBfpxrvLMfKPnD

 

I don't know how to rotate PDFs, if you save the images you should be able to read the document easier.

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Dwight Hottle reply : 

On lake erie we get wind induced turnover from strong NE winds or NW winds. Since erie is shallow on the west (warmer water) & deep on the east (colder water) the wind causes a mixing effect when blowing from either direction strongly for several days. The end result is the fishing sucks for about a week after such an event. Strong NE winds will also cause bait fish kills in the spring. Natural current in lake erie runs from west to east as the lake drains into lake Ontario. Anybody else experience similar wind induced turnover?  

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RoLo reply : 

OXYCLINE

More important than the thermocline is the 'oxycline'. With regard to body temperature, humans need to maintain

a core temperature of 98.6 deg in order to survive, but fish have no body temperature to maintain. For this reason,

cold-blooded animals have a highly UNsophisticated temperature warning system. Although fish disposition

is involuntarily altered by water temperature, temperature change doesn't cause the discomfort we associate

with humans. When ice-fishing, fish laid on the ice quickly become frozen to the ice. But when thawed & released

they typically swim away (try that with a human). Radio-tracking has shown that in dissolved oxygen levels of 6 ppm

or more, bass won't budge from their favored cover regardless of water temperature. On the other hand,

when dissolved oxygen levels fall below 5 ppm, bass become increasingly stressed and will abandon

their favorite site. That said, oxycline studies are more compelling for deep water species like the salmonids

and lunker pike. Bass are inherently shallow-water fish, so dissolved oxygen generally isn't an issue

except during the summer. 

 

THERMOCLINE

Water not only seeks its own level, but gravity stacks water according to its density (water temperature).

The thermocline is a thin water stratum characterized by rapid temperature change, which floats on the hypolimnion.

I hasten to add however, 'all' waterbodies do not form a thermocline. On lakes with a thermocline, this bias ply strata

of mixed densities tends to resist churning and limit seiches (wedge-shaped epilimnion caused by wind).

Given the proper sonar settings, the thermocline may be seen on your depth sounder because of mixed water densities.

Unfortunately, that's never the case with the oxycline. Due to the unpliable nature of the thermocline,

water below the thermocline is essentially stagnant and anaerobic (without oxygen), which limnologists refer to

as the ‘anoxic zone’. As summer wears on, the mean water temperature of a waterbody rises, causing the thermocline

(metalimnion) to migrate 'downward' into the hypolimnion budget. At the same time, decaying plant matter progressively

reduces the lake's dissolved oxygen, causing the oxycline to migrate gradually 'upward' toward the surface.

By midsummer the oxycline may be several feet or even yards 'above' the thermocline. Therefore, even if fish

were disposed to seeking specific water temperatures (they are not), it would not be accessible.

 

Roger

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Catt reply : 

Thank you Roger!

I aint getting into this one again, do a site search

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MacP reply : 

 

 

They may not seek specific temperatures, but they do react differently at certain temperatures.  I'd also add that fish DO seek more comfortable waters at certain times of the year.  In the summer, the deeper you go, the cooler it gets.  In winter, if the water temperature gets low enough, water molecules arrange themselves differently than you would think.  Warmer water is less dense.

 

A link:

http://faculty.gvsu.edu/videticp/stratification.htm

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RoLo reply : 

  Quote

They may not seek specific temperatures, but they do react differently at certain temperatures. 

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WRB reply : 

To me water temperature is important to cold blooded fish like bass. Water temperature impacts their annual life cycle, spawning and natural distribution. For example Florida strain largemouth have a lower tolerance to water colder than 45 degrees and that limits distribution.

Look up Cold Shock and the stress that sudden water temperatures have on fish in general. Large die offs have occured as a direct result of sudden cold water thermal shock, it can be very stressful to warm water fish like bass.

To say bass will not seek cooler water during the summer if DO are OK and prey is available differs from my experience. Small bass may not relocate, some big adult bass will move when water temps approach 80 degrees or more and move deeper to the thermocline depth. Cooler water can absorb higher levels of DO then warmer water, plus pelagic fish tend to be deeper near the cooler water.

Keep in mind that cold water heavier until is 39.4 degrees, then it gets super saturated with DO and becomes lighter' floating toward the surface and freezes at 32 degrees.

Tom

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Nitrofreak reply : 

Thanks guys !!!

 

Learning a lot , Rolo , catt , looking for that conversation you two had about this I guess from a previous thread can you point me in the right direction please

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RoLo reply : 

  On 12/18/2013 at 5:42 AM, Nitrofreak said:

Thanks guys !!!

 

Learning a lot , Rolo , catt , looking for that conversation you two had about this I guess from a previous thread can you point me in the right direction please

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WRB reply : 

Rolo,

We have one lake in SoCal that reports the oxycline depths at various areas throughout the lake. Big Bear lake located at 7,000 feet altitude has both a good trout fishery and decent smallmouth/LMB population. The lake is a little over 7 miles long. the dam biult in 1890 and raised in 1903, old lake and not deep, about 35' aversge, 70' at the dam. BB freezes in the winter and the water may reach 70 degrees mid summer. This lake fits your thermocline-oxycline definition being 2 separate depths coming together late summer. BB also has a aeration system at the dam and tends to have strong westerly afternoon winds to help with aeration. Large weed beds in shallower areas.

Most of our bass lakes are highland class, deep structure lakes that are far different than Big Bear. I believe BB is similar to lakes in the northeast.

Tom

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ColdSVT reply : 

This thread needs to be a sticky...good good info!

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RoLo reply : 

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WRB reply : 

Turnover

Lakes that ice over may have a turnover when the ice melts and in the fall, lakes that stratify durning the umber may turnover in the late fall. A turnover means the upper layer of water quickly mixes with the lower bottom layers.

This occurs because the upper layer out weighs the lower layer and the layers switch places; the bottom water comes up to the top. When this happens bottom decaying debris come up along with a distinct odor of rotten eggs or sulfur smell and floating pieces of decaying stuff. The water then continues to mix for about 2 weeks until it is homogenous or the same temperature top to bottom.

Most bass lakes cool slowly in the fall and turnovers may not be abrupt, however a turnover occurs.

How does this impact bass fishing? The water temperature changes, the DO level change, the baitfish and other prey all relocate because their environment has changed. After about 2 weeks the lakes settles down and bass can be anywhere at all depths depending on the prey distribution, aquatic growth, wind and water temperature. Tough sledding during turnover periods.

Tom

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mod479 reply : 

 

It shows up on my Mark-5x with a bit of messing with the settings. As you can see, those fish are below the thermocline and feeding on a school of baitfish above it. The layer is about ~3-4' thick and down just above the 40 foot mark. 

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mudkart reply : 

Actual data showing both a thermocline and oxycline.

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J Francho reply : 

  On 12/18/2013 at 2:41 AM, Catt said:

I aint getting into this one again, do a site search

 

The goal here is to distill this info into one thread, not debate.  Though any discussion of points made is certainly welcome, especially if it can be backed with links or research.

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Nitrofreak reply : 

  On 12/18/2013 at 10:37 AM, WRB said:

Turnover

Lakes that ice over may have a turnover when the ice melts and in the fall, lakes that stratify durning the umber may turnover in the late fall. A turnover means the upper layer of water quickly mixes with the lower bottom layers.

This occurs because the upper layer out weighs the lower layer and the layers switch places; the bottom water comes up to the top. When this happens bottom decaying debris come up along with a distinct odor of rotten eggs or sulfur smell and floating pieces of decaying stuff. The water then continues to mix for about 2 weeks until it is homogenous or the same temperature top to bottom.

Most bass lakes cool slowly in the fall and turnovers may not be abrupt, however a turnover occurs.

How does this impact bass fishing? The water temperature changes, the DO level change, the baitfish and other prey all relocate because their environment has changed. After about 2 weeks the lakes settles down and bass can be anywhere at all depths depending on the prey distribution, aquatic growth, wind and water temperature. Tough sledding during turnover periods.

Tom

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RoLo reply : 

  On 12/19/2013 at 12:30 AM, Nitrofreak said:

It is my understanding that the fish , particularly bass , will for the most part suspend during this transition , do they just get comfortable when their environment changes as such ? , understandably there are many reasons bass will suspend , this turnover period is when it is most noticeable for me .

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DRH2O reply : 

  On 12/17/2013 at 11:50 PM, MacP said:

Further reading (warning maths and graphs!)

 

http://jmp.sh/v/W6me4ORBfpxrvLMfKPnD

 

I don't know how to rotate PDFs, if you save the images you should be able to read the document easier.

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WRB reply : 

Bass suspend about 90% of the time, it's the reason they have air bladders. When bass are resting, not swimming, they are suspending at various depths depending on the depth they are acclimated to.

During the turnover the bass will be at similar depths they were before the turnover, they slowly reaclimate to new depths where baitfish are located.

Are you calling bass suspended in the mid water column "suspended"? Bass suspend in the mid water column off shore to rest throughout the summer months, inactive and difficult to catch. After a turnover the bass may suspend off shore for a longer period of time and remain inactive until everything settles down. I know from personal experience the bite is difficult to figure out right after a lake turns over.

Tom

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Mainebass1984 reply : 

Once fall turnover occurs up here in New England the fish become very predictable. Its my favorite time of year to fish. The tend to congregate leading to some fast fishing. Its one of the best times of the year for big bass in New England.

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