Let's Talk Thermoclines And Turnover

[J Francho]

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?

[WRB]

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

[J Francho]

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.

[shimmy]

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

[Nitrofreak]

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 !!!

[MacP]

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. 

[DelcoSol]

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?  

[Crestliner2008]

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?

[Todd2]

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?

[WRB]

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

[11justin22]

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?

[kwhorne19]

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....

[Snakehead Whisperer]

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

[WRB]

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.

[Wayne P.]

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:

 

[KyakR]

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

[KyakR]

Wow Wayne P!!! Thanks!

[MacP]

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.

[Wayne P.]

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'.

 

 

 

[MacP]

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?

[Nitrofreak]

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 ?

[WRB]

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

[MacP]

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.

[Dwight Hottle]

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?  

[RoLo]

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