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The Thermocline.

23393 Views 20 Replies 8 Participants Last post by  chunkster
The link between fish behavior and water stratification

If you're an impoundment striper fisherman you must know and understand the thermocline to be successful. The thermocline can be found by measuring the water temperature at various depths with a recording thermometer.

The thermocline is a thin layer of water in a lake which is sandwiched between the upper layer of water (the epilimnion) and the lower, colder layer of water (hypolimnion). During the summer months, surface water is heated by the sun and the surface temp could be 80 degrees or more. This floats over a layer of colder more denser water called the hypolimnion. Now, between these 2 layers you have a thin layer in which the water temp drops fairly substantially. This will be the thermocline. The temp at this level may be high 60's and up in about the middle of spring.
This is the home and feeding area for the larger fish. The thermocline layer also has more oxygen and an available food supply.
The hypolimnion layer is the colder water on a typical lake, but is almost devoid of oxygen because of plant and animal decomposition. If the surface temperature is 75 degrees, then the hypolimnion will usually be anywhere from 39 degrees to 55 degrees.

In the summer the stripers cant take the high surface temperatures of the upper layer and the underoxygenated properties of the lower stratum. So hence the necessity of finding the lakes thermocline.
Each fall a lake will "turn over" the cooler top layers will displace the bottom layers and recirculate. The shallow coves are among the first to cool and the bass will begin to relate to shallow structure.
Also fish dont have eyleids and light refracts differntly at dawn and dusk so they are then more likly to surface and feed.

In shallow lakes with an average depth of 15 feet usually no thermocline will develop.


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Does that also apply to the thermoclines on my fishfinder when I'm in Saltwater...???
BB said:
Does that also apply to the thermoclines on my fishfinder when I'm in Saltwater...???
OK BB The answer required a little research. I always like to learn something. We can start with these two.
Fresh water stratification

The Ocean
Thanks Jim...I also like to learn new stuff...

...always good to add to the mental library of fishing knowledge...
I especailly found the part about the moon/tide phases and beach characteristics, the most useful...

Its amazing about how science can have such a large affect on your fishing...

...My science teacher is a amazing of the main reasons is that he understands every bit about weather, the moon, the tides, barometric pressure etc...
...I can't imagine how greatly all that knowledge could help improve my fishing...

These two paragraphs about the oceans thermocline pretty much sum it up.

Waters warmer than 10?C (50 degrees Farenheit) which dominate the sea surface do not extend much below 500 m in the ocean; the warm waters provide just a veneer of buoyant warmth over a basically very cold dense ocean. The sharp drop off in temperature with depth, characteristic of the ocean between 40?N to 40?S is called the thermocline. The waters below the thermocline exhibit much reduced temperature decrease with depth. In the salinity field the surface tropical and subtropical ocean is salty, with the deeper waters somewhat fresher. The rapid decrease of salinity with depth, accompanying the thermocline, is called the halocline. The deep Atlantic is relatively saline. This water is derived from the sinking of cooling of saline surface water in the northern North Atlantic. This is the North Atlantic Deep Water. In contrast the deep Pacific is relatively fresh, as it experiences no deep convection of cooled salty surface water, its surface layer is too fresh and thus buoyant to sink into the deep ocean, at reasonably cool subpolar SST. The deep Atlantic is also warmer than the deep Pacific, as the saline North Atlantic Deep water is sufficiently dense water to inject relatively warm water into the deep layer. Towards the sea floor temperatures reach below 0?C (32 degrees F.) marking the presence of Antarctic Bottom Water derived from the shores of Antarctica. Below the thermocline is a low salinity layer derived from the Antarctic Circumpolar Current. This water mass, made relatively fresh by excess precipitation of the circum-Antarctic belt is called the Antarctic Intermediate water. A low salinity intermediate of more limited extent forms in the North Pacific.

A parcel of sea water achieves its temperature and salinity at the sea surface in response to sea-air heat and freshwater exchange. Its surface derived T/S values change within the ocean interior only by mixing with other water parcels. Hence sea water spreading from the surface into the ocean volume can be to trace by Temperature Salinity properties. That 75% of the ocean volume falls with a narrow range of temperature and salinity indicates that only a small part of the sea surface contributes to the characteristics of the deep ocean.

Oceanographers often use a temperature/salinity (T/S) diagram to determine the origin of the sea water properties
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That's excellent stuff - I'm new here but I suspect a few others besides myself fish fresh water for more than just stripers and that's useful for many fisheries. Those Quicktime movies make it so much more simple to understand. Thanks :D :D
Putting it in jus hatched terms....

it sounds like your saying that the Thermocline should be around the mid to high 60's, is this the temp that Stripers prefer? Right now in my area its cold and rainy, real overcast, so this means I should be surface fishing right? Get used to the name here, I will have a ton of quistions....thanks guys.
Around here...the magic number is 50 degrees, once it hits 50...its on!!!
Know the preferred temperature range of your favorite species and you'll catch more of them.

Just like humans, fish have a defined temperature range in which they feel most comfortable. But unlike humans, that range varies - often considerably - from species to species. Therefore, the angler who knows the exact comfort range of the fish he's looking for has a definite edge.

Since fish cannot add or remove layers of clothing to stay comfortable, their only option is to move to another area when they find the temperature too warm or too cool. fish are essentially cold-blooded creatures (although some tunas are slightly warm-blooded), which means they can't regulate their body temperature internally. This makes them extremely susceptible to rapid fluctuations in water temperature. For example, plummeting temperatures can stun or even kill large numbers of fish before they're able to move to another area. Even subtle changes in temperature will affect the behavior of fish. In some cases, a degree or two can alter the feeding habits of a species without causing them to leave the area.

Get a Thermometer!

A quick glance at the thermometer will let you know if you should bother targeting a particular species. Striped bass, for example, typically head for deeper water when the shallows are too hot or cold for them. So why beat your brains out trying to catch stripers near shore in midsummer when the water temperature is over 75 degrees? Better to be looking for bluefish, which are happy as clams until the water passes the mid-80-degree mark.

Of course, there are some exceptions to this rule. For example, a high abundance of food will often keep game fish in an area that's a few degrees outside their preferred range.

Optimum Temperatures
Fish are, of course, happiest when they're in water of their optimum temperature range. This is especially true of many inshore species, such as striped bass, bluefish, bonefish and tarpon. Tarpon, for example, can often be found in deep channels when the water temperature is barely 70 degrees, and will only move onto the flats to feed when the water is within their optimum temperature range.

Offshore Temperatures
Modern satellite technology has enabled us to determine offshore sea-surface temperatures accurately and in excellent detail. There are even several companies that provide this information in chart form for subscribers.
When a large body of warmer or cooler water moves into an area where those temperatures are not normally encountered, it often brings "exotic" species with it. This is why many tropical and subtropical game fish were caught from California to northern Washington during the 1997 El Ni?o. During this same period, overly warm water off some parts of Central and South America pushed those same species out of areas where they are normally abundant.

But if you wait for the next El Ni?o or La Ni?a to cause unusual fish movement, you'll be missing out on a lot of action. On the East Coast, large fingers or eddies of warm, tropical water often break off from the Gulf Stream and move into cooler water, bringing with them warm-water game fish. Billfish appear off New England every year because of this phenomenon.

On a more day-to-day level, noticing the change of a degree or two that delineates the edge of a current can pay big dividends. Such current edges, or rips, tend to concentrate baitfish, and fishing on the side of the rip where the temperature is best for the species you're seeking can make all the difference in the world. This is why no savvy offshore skipper would ever leave the dock without some sort of water temperature gauge.

Sudden Changes
A rapid decrease in water temperature can chase fish away or bring them in. Even a strong breeze that lowers the water temperature a few degrees can affect the presence or behavior of fish.

If the temperature of the water drops too fast, fish will sometimes leave the area, even if the temperature remains within their range of tolerance. Why? They simplydon't like the sudden change.

In some cases, the water temperature can plummet ten degrees or more overnight due to the passage of a cold front. Even if it's calm and sunny the next day, the ocean may seem like a biological desert. That's how important water temperature can be.
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one aspect of water's thermo dynamics is often over-looked by anglers, that can be, and often is, the 'holy grail' of fishfinding..

that is the somewhat false thermocline a river mouth has during the ebb and flow of the tides... the colder ocean waters that lay in the bottom layer of the river bottom quite often a ways up-river (often cleaner..see: key to holy grail) most times is much cleaner.. ever fish an outgoing tide that looks like pea soup?... ever wonder where the fish went when this occurs? most times one needs to find CLEANER water to find fish. A lot of times that is in the trenches of the bottom... fish will lie just below this 'false thermocline' just because it's clean enough to breath and a bunch of food drifts thru just over their heads. they'll dart in and out of the muddy stuff to chow but won't hang in it for more then a few seconds before they retreat back to the clean layer... most times it's tough to get a sonar reading past the goop to see the clean layer at the bottom or the fish lying there for that matter.. but it's there and so are the fish ... when you have this situation, don't call it quits... find a trench near where you were catching them before the ebb turned to goop.. and fish it deep with a bit of chartruese or black colored jigs.. also jigs with a spinner like "stump jumpers" can be the only thing they'll touch.. been there.. done that..
don't tell anyone.. it's one of my deep dark secrets.. :wink:
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finally out of the thermocline! hope all is okay with you jigman!
alls cool, now but I been in the trenches the last few months... dude I came in just to see that avatar and you changed it to jed!.. WTF! :D ol' Jed just doesn't do it for me... the can lady was better :roll:
sorry jethro, will change it back in the morning, jd must be upset, he never replied about my goof

Here you go Zimno
The Beverly Hillbillies
Update from the Lamont Earth Observatory

oThe water column in the ocean can be divided into the surface layer, pycnocline, and deep layer
oThe surface layer is about 100 m thick, comprises about 2% of the ocean volume and is the most variable part of the ocean because it is in contact with the atmosphere
§ The surface layer is less dense because of lower salinity or higher temperature
o The pycnocline is transitional between the surface and deep layers and comprises 18% of the ocean basin
§ In the low latitudes the pycnocline coincides with the thermocline, but in the mid-latitudes it is the halocline
§ Tropical and subtropical oceans are permanently layered with warm, less dense surface water separated from the cold, dense deep water by a thermocline,a layer in which water temperature and density change rapidly
Tropical and subtropical oceans are permanently layered with warm, less dense surface water separated from the cold, dense deep water by a thermocline,a layer in which water temperature and density change rapidly
Temperate regions have a seasonal thermocline and polar regions have none
· Explain development and breakdown of seasonal thermocline
o Salinity displays a latitudinal relationship related to precipitation and evaporation
§ Highest ocean salinity is between 20-30o N and S or the equator
§ Low salinity at the equator and poleward of 30o results because evaporation decreases and precipitation increases
§ In some places surface water and deep water are separated by a halocline, a zone of rapid change in salinity
§ Water stratification (layering) within the ocean is more pronounced between 40oN and 40oS
o Density of sea water is a function of temperature, salinity and pressure
§ Density increases as temperature decreases
§ Salinity increases as pressure increases
§ Pressure increases regularly with depth, but temperature and salinity are more variable
§ Higher salinity water can rest above lower salinity water if the higher salinity water is sufficiently warm and the lower salinity water sufficiently cold
§ Pycnocline is a layer within the water column where water density changes rapidly with depth
o The water column in the ocean can be divided into the surface layer, pycnocline, and deep layer
o The surface layer is about 100 m thick, comprises about 2% of the ocean volume and is the most variable part of the ocean because it is in contact with the atmosphere
§ The surface layer is less dense because of lower salinity or higher temperature
o The pycnocline is transitional between the surface and deep layers and comprises 18% of the ocean basin
§ In the low latitudes the pycnocline coincides with the thermocline, but in the mid-latitudes it is the halocline
o The deep layer represents 80% of the ocean volume
§ Water in the deep layer originates at the surface in high latitudes (polar regions) where it cools, becomes dense, sinks to the sea floor and flows outward across the ocean basin

Chem 1103: Lecture 3: The Properties of Sea Water: Chap. 5

· Four types of oceanography
ogeological, chemical, physical, biological
· Basic Chemical Notions

o Atoms are the smallest units which display all of the properties of the material
o Molecules are chemically-combined compounds formed by two or more atoms H2O NaCl CO2
o Atoms are composed of:
§ Nucleus - the center of the atom consisting of positively charged particles called protons and neutrally charged particles called neutrons
§ Electrons - negatively charged particles which orbit the nucleus in discrete electron shells
o Periodic chart of elements
§ each has atomic number and that is the number of protons
o Electrically stable atoms have the same number of electrons as protons
o Ions are atoms with either more or fewer electrons than protons and are therefore electrically charged H+ Na+ Cl- OH-
§ Shells and atoms want to have shells filled
§ First shell 2; second shell 8
o Periodic chart
§ Noble gases have filled shells don't react with much
§ Gases O2 and N2
§ Carbon not a gas - graphite and diamond - solids
o Isotopes are atoms which contain the same number of protons but different numbers of neutrons and therefore have different weights -1H 2H 3H 16O 18O 12C 14C
· Basic Physical Notions
o Heat results from the vibrations of atoms (kinetic energy) and can be measured with a thermometer
§ In solids the atoms or molecules vibrate weakly and are rigidly held in place by electrical bonds - example kids in high chairs or on bus with seat belts
§ In liquids the atoms or molecules vibrate more rapidly, move farther apart, and are free to move relative to each other - example kids who have to hold hands and stay in a room
§ In gases the atoms or molecules are highly energetic, move far apart, and are largely independent - example kids on the loose with the doors open
§ Evaporation (vaporization) is the transition from liquid to gas; condensation is the reverse
· Molecules with enough energy escape from liquid into gas
· Cold removes some energy and gas forms into liquid
§ Temperature controls density. As temperature increases, atoms or molecules move farther apart and density decreases because there is less mass (fewer atoms) in the same volume
· Warm air and warm water both rise and cold air and cold water sink
· The water molecule
o The water molecule is unique in structure and properties
§ H2O is the chemical formula for water
§ Unique properties of water include:
· Exists naturally as solid, liquid, and gas
· Higher melting and boiling point than other hydrogen compounds
· High heat capacity, amount of heat needed to raise the temperature of one gram of water 1oC
· Greater solvent power than any other substance
§ Water molecules are asymmetrical in shape with the two hydrogen molecules at one end separated by 105o when in the gaseous or liquid phase and 109.5o when ice
§ Asymmetry of a water molecule and distribution of electrons result in a dipole structure with the oxygen end of the molecule negatively charged and the hydrogen end of the molecule positively charged
§ Dipole structure of water molecule produces an electrostatic bond (hydrogen bond) between water molecule which cluster together in a hexagonal (six-sided) pattern
§ Ice floats in water because all of the molecules in ice are held in hexagons and the center of the hexagon is open space, making ice 8% less dense than water
§ Hydrogen bonding is responsible for many of the unique properties of water because more energy is required to break the hydrogen bonds and separate the water molecules
· High melting and boiling points due to more energy needed to break the Hydrogen bonds
§ Water dissolves salts by surrounding the atoms in the salt molecule and neutralizing the ionic bond holding the molecule together. Dissolved salts from cations (positively charged ions) Na+ and anions (negatively charged ions) Cl-
§ Fresh water reaches its maximum density at 3.98oC
· Below this temperature increasing numbers of water molecules form hexagonal polymers and decrease the density of the water
· Above this temperature, water molecules are increasingly energetic and move farther apart, thereby decreasing density
§ The process of water surrounding an ion is called hydration
o Sea water consists of water with various materials dissolved within it
§ The solvent is the material doing the dissolving and in sea water it is the water
§ The solute is the material being dissolved
§ Saturated solution can hold no more salt
§ Unsaturated solution can hold more
§ Supersaturated solution has more than it can hold
§ Salinity is the total amount of salts dissolved in the water
§ Salinity is measured in parts of salt per thousand parts of water and is expressed as ppt (parts per thousand0 or abbreviated o/oo
§ Now salinity really as psu (practical salinity units)
§ Average salinity of the ocean is about 35 o/oo or 3.5%
o Many chemicals in seawater
o Five groups
§ Major constituents
§ Nuturients
§ Gases
§ Trace elements
§ Organic compounds
o Major constituents are anions and cations - show table
§ 99% of all the salt ions in the sea are sodium (Na+), chlorine (Cl-), sulfate (SO4-2), Magnesium (Mg+2) calcium (Ca+2) and Potassium (K+)
§ sodium and chlorine alone comprise ~86% of the salt in the sea
§ the major constituents of salinity display little variation over time and are a conservative in sea water
o Nutrients are chemicals essential for life - like fertilizer
§ Major nutrients in the sea are compounds of nitrogen, phosphorus, and silicon
§ Because of usage, nutrients are scarce at the surface and their concentration is measured in parts per million (ppm)
§ Concentration of nutrients vary greatly over time and because of this they are considered a nonconservative in seawater
o Gases
§ In order of decreasing abundance, the major gases in the sea are nitrogen, oxygen, carbon dioxide, an the noble gases, argon (Ar), neon (Ne), and helium (He)
§ Nitrogen and the noble gases are considered to be inert because they are chemically non-reactive
§ Oxygen and carbon dioxide are not conservative since they are involved in photosynthesis and respiration by plants and animals
· Photosynthesis - CO2 in O2 out - plants
· Respiration - O2 in CO2 out - animals
§ Oxygen and carbon dioxide vary greatly in space and time due to these processes
o Trace elements
§ Manganese Mn, lead Pb, mercury Hg, gold Au, iodine I, iron Fe
§ [SIZE2]Occur in minute quantities and are usually measured in parts per million (ppm) or parts per billion (ppb)[/size]
§ Even in small quantities they are important in either promoting life or killing it
o Organic compounds
§ At this level, organic compounds are large complex molecules with carbon, except graphite and diamond - also not gases - CO2 but gasoline, oils, carbohydrates, fats, proteins, hormones, and vitamins
§ Marine organic compounds occur in low concentrations
§ Produced by organisms in metabolic reactions or through decay.

· Salinity
o Salinity is the total mass, expressed in grams, of all substances dissolved in one kilogram of sea water when all carbonate has been converted to oxide, all bromide and iodine has been replaced by chlorine, and all organic compounds have been oxidized at a temperature of 480oC
§ Principle of constant proportion states that the absolute amount of salt in sea water varies, but the relative proportions of the ions is constant
· Relative percentage of each salt is constant - ex. with % and $
· Because of this principle it is necessary to test for only one salt ion to determine the total amount of salt present
· Use chlorine because it is abundant and easy
§ Chlorinity is the amount of halogens (chlorinity, bromine, iodine, and fluorine) in the sea water and is expressed as grams/kilogram or o/oo
§ Salinity is equal to 1.8065 time chlorinity
§ Salinometers determine chlorinity from the electrical conductivity produced by the dissolved salts and get salinity from that
o Salinity in the ocean is in a steady-state condition because the amount of salt added to the ocean (input from source) equals the amount removed (output into sinks)
§ Steady state - condition of equilibrium, no net change, but things can be changing
§ Conservation of mass - what goes in, comes out
§ Salt sources include weathering of rocks on land and the reaction of lava with sea water
· Weathering mainly involves the chemical reaction between rock and acidic rainwater, produced by the interaction of carbon dioxide and rainwater forming carbonic acid
· Carbonic acid will be discussed later
· For now, acid eats the rock and weathers it
· Example, mineral orthoclase, a common feldspar of granite, decomposes to kaolinite in acidic solution
· 2KAlSi3O8 + 2H+ + H2O ® 2K+ + Al2Si2O5(OH) 4 + 4SiO2
· orthoclause - KAlSi3O8
· kaolinite - Al2Si2O5(OH) 4
· Dissolved silica - SiO2
§ Salt sinks include the following
· Evaporation removes only water molecules
o Remaining water becomes increasingly saline, eventually producing a salty brine
o If enough water evaporates, the brine becomes supersaturated and salt deposits begin to precipitate forming evaporite minerals
· Wind blown spray carries minute droplets of saltwater inland
· Adsorption of ions onto clays and some authigenic minerals
· Shell formation by organisms
§ Lack of similarity between relative composition of river water and the ocean is explained by residence time, average length of time that an ion remains in solution in the ocean
· Ions with long residence times tend to accumulate in the sea, whereas those with short residence times are removed
· Rapid mixing and long residence times explain constant composition of sea water
o Addition of salt modifies the properties of water
§ Pure water freezes at 0oC, adding salt lowers the freezing point because salt ions interfere with the formation of the hexagonal structure of ice
§ -1.91oC for 35 psu
§ Density of water increases as salinity increases
§ Vapor pressure is the pressure exerted by the gaseous phase on the liquid phase of a material. It is proportional to the amount of material in the gaseous phase.
· Vapor pressure decreases as salinity increases because salt ions reduce the evaporation of water molecules
· Chemical and Physical Structure of the Oceans
o Ocean surface temperature strongly correlates with latitude because insolation, the amount of sunlight striking Earth's surface is directly related to latitude
§ Ocean isotherms, lines of equal temperature, generally trend east-west except where deflected by currents
· Ocean currents carry warm water poleward on the western side of the ocean basins and cooler water equatorward on the eastern side of the ocean
§ Insolation and ocean-surface temperature vary with the season
§ Ocean surface temperature is highest in the tropics (25oC) and decreases poleward
· El Nino
o Upwelling of deep, nutrient-rich water supports large populations of phytoplankton and fish
o The waters off the coast of Peru normal is an area of upwelling, supporting one of the world's largest fisheries
o Every three to seven years, warm surface waters in the Pacific displace the cold, nutrient-rich water on Peru's shelf in a phenomenon called El Niño
El Nino results in a major change in fauna on the shelf and a great reduction in fishes, which can lead to mass starvation in fishes
o El Nino starts with a shift in the wind in the Pacific, which changes the Ekman transport, reducing the pile up of water on the west side of the Pacific. The water moves east and shuts off the upwelling.
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I know this takes things in a different direction, but inland hybrids seem to be unaffected by the thermocline. Inland stripers, on the other hand, are considered very oxygen sensative. A few years ago--not that many--there was a large kill-off of stripers in Tennessee on, I think, Norris Lake. It was so bad that they laid long oxygen pipes through the lake so, if needed, they could pump oxygen into the water. But through it all the hybrids seem unimpressed with oxygen levels.

A couple of years ago (2004) at Brookville Lake in Indiana, I went to fish stripers and near the ramp there were two of them thrashing about, obviously in death throes. They appeared healthy, just dying. When I got to my favorite spot near the dam there were twenty or thirty of them floating. I wrote to the DNR and the official reply was oxygen depravation.
Thanks slow hadnt realized this was the hybrid forum. Should be striper forum I guess. (I'll copy it to the Striper Room)
Good point. Hybrids are bred to withstand the extreme temperatures and can withstand much lower 02 levels.
I've read that The pure breds will literally stack on top of each other to find cooler water.
The lower layer is the least oxygenated as well with the detritus and decaying organic matter.
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