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Largemouth Bass, Temperature, & Thermoclines

What water temperature can tell you about where to find bass.

Largemouth Bass, Temperature, & Thermoclines

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Water temperature drives ­behavior of largemouth bass. Warming water in the spring, sometimes in conjunction with day length, cues the spawning ritual. Temperature controls their metabolism and, thus, their need to feed to consume energy. In the fall, decreasing temperature may also trigger increased feeding. And their preferred temperature of 82°F to 84°F may influence their whereabouts in the summer. But, except for the spawn, the location of bass that tolerate water temperatures from near freezing to above 90°F is also strongly influenced by the distribution of their forage. Sorting out what affects bass distribution in the uncontrolled experiment called fishing is not easy, maybe not possible.

Largemouth Bass Thermal Biology

The optimum temperature for largemouth bass is 82°F to 84°F. This is the temperature at which their metabolic machinery is most efficient and food consumption, growth, digestion rate, swimming speed, and capacity available for exercise are greatest. Optimum temperature is also their preferred temperature, but largemouth bass also survive prolonged exposure to water temperatures as low as 40°F and high as 98°F.

Bass’ metabolism increases with temperature. The increase is logarithmic, meaning, for example, the increase in metabolism from 65°F to 75°F is much greater than the increase from 45°F to 55°F. Metabolism—the conversion of circulating energy compounds (sugars) to life-sustaining cellular energy molecules—requires oxygen. As temperature and metabolism increase, oxygen consumption—and the need for oxygen—also increases.

Although largemouth bass tolerate temperatures greater than their 82°F to 84°F optimum, little is known about the effects on feeding at temperatures above 84°F, a question of significant relevance to bass anglers, especially those fishing southern waters where summer water temperatures exceed 86°F. Studies in the 1960s and 1970s found declines in rate of weight gain by fry and subadult largemouth bass at temperatures above 84°F. This decline in growth rate is certainly a result of elevated metabolism, but to what extent it is also a consequence of decreased feeding remains unknown. This unanswered question is important to anglers but also very relevant to predicting the effects of a warming climate on largemouth bass and the forage that supports them.

A man standing in a boat holding a bass.
Fishing Lore: Maybe you have heard or read that in the summer largemouth bass in riverine reservoirs, like those on the Tennessee or Mississippi rivers, feed more actively when the water is flowing because the flow brings cool, oxygenated water that stimulates feeding. Not so. The current may trigger feeding, but flowing waters do not stratify because the continual mixing of the water prevents the development of layers of different density. Water flowing through the dam of an unstratified impoundment is the same temperature as the receiving reservoir.

Limnology 101: Lake Stratification

Lake stratification is a result of water temperature, but how it affects fish is as much about dissolved oxygen as it is temperature.

Lakes go through an annual cycle of cold to warm and back to cold. Starting in the early spring in the South or after ice out on northern lakes, the water is cold top to bottom. Spring winds generate water currents that mix the water. The circulation occurs throughout the water column, surface water mixing with bottom water. Water warming is primarily a result of solar energy (sunlight). As the days get longer and the sun is more overhead, more solar energy penetrates the water. The solar energy is converted to heat when it strikes the lake bottom and particles, like plankton and clay turbidity, suspended in the water. Water with more suspended particles can warm faster, and the upper part of the water column that receives more solar energy warms faster than deeper water. As the water warms, density decreases.

A graphic showing lake stratification.
The entire water column is the same temperature and density in the spring, fall, and, in ice-free waters, in the winter, and oxygen produced in the upper water circulates throughout the water column. When a lake is stratified in the summer, or in the winter in ice-covered lakes, oxygen produced in the epilimnion does not circulate into the hypolimnion.

At some point, with the rate of water warming increasing and during a period of relatively calm conditions and reduced water circulation, the upper portion of the water column will become warmer and—very importantly—less dense than the lower water. The warmer and less dense upper-water zone now literally floats on the cooler and more-dense lower-water zone. After the difference in temperature of the upper and lower water exceeds just a couple degrees, the difference in density is sufficient to prevent the upper and lower water layers from mixing when wind causes water currents. The lake is now thermally stratified. The term used by limnologists and fishery biologists for the upper-water layer is epilimnion; the lower-water layer is the hypolimnion. As time passes, solar energy continues to warm the epilimnion, further increasing the density difference between the epilimnion and hypolimnion and also increasing the amount of water circulation required to mix the two layers. The part of the water column where the epilimnion and hypolimnion meet is the thermocline or the metalimnion.

The thermocline is not a line, as the term might suggest, where the water temperature instantly changes from warm to cool, but rather a zone or layer of relatively rapid temperature change from the warmer epilimnion to the cooler hypolimnion. The thickness of the thermocline varies among lakes from several feet to more than 10 feet. The top of the thermocline may be as shallow as only 4 or 5 feet in shallow ponds or 60 feet or deeper in deep, clear lakes.

In the fall, the epilimnion cools as the air temperature cools and solar energy decreases. When the temperature of the epilimnion nears that of the hypolimnion, the thermocline disappears, the lake is no longer stratified, and the entire water column mixes.

Northern, ice-covered lakes also stratify in the winter. Water has a unique physical property: it is most dense at 4°C, about 39°F. When the water cools below 39°F, it becomes less dense and floats on the zone of max-density, 39°F water. This unique thermal property of water creates a bottom zone of 39°F water that provides fish with a thermal refuge from freezing temperatures.

Lake stratification has a much greater effect on bass distribution when dissolved oxygen is added to the equation. While some oxygen diffuses into the water at the air-water interface, most of the life-supporting oxygen is produced by photosynthesis by plants, and phytoplankton—algae suspended in the water—usually is the greatest producer of oxygen. As a generalization, one percent of surface illumination is sufficient for net oxygen production, the condition when oxygen production by plants exceeds the oxygen consumed by their respiration. The water column above the depth of 1 percent light transmission is called the photic zone.

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Fertile, productive bass lakes generally have high concentrations of phytoplankton that reduce light penetration. These lakes have a relatively shallow photic zone, commonly less than 20 feet. When the lake is mixing, oxygen produced in the photic zone is circulated throughout the water column. In a stratified lake, oxygen produced in the photic zone circulates throughout the epilimnion, but no oxygen is produced in the hypolimnion if the photic zone ends above the thermocline.

At the time of stratification, the entire water column contains oxygen. But as summer progresses, oxygen is consumed by living organisms and decomposition of all the dead organic matter that sinks into the hypolimnion. The thermocline prevents oxygen produced in the epilimnion from mixing into the hypolimnion. The hypolimnion becomes devoid of oxygen. And fish. Because oxygenated water from the epilimnion steadily mixes into the thermocline, the thermocline then becomes a zone where bass can find both cooler and oxygenated water.

Temperature Positions Bass

You learn to fish by catching fish—or learning how others catch fish—under different conditions. Years ago, while in graduate school in southern Illinois, I enjoyed good catches on a couple new reservoirs by dragging Texas-rigged plastic worms on long, ridge-like points. A Lowrance flasher was my underwater eye. No forward-looking sonar back then. Structure information came by counting how long it took the worm to hit bottom. In the summer, the strike zone was 12 to 15 feet deep. Was that where the thermocline hit the sloping ridge? Maybe. I didn’t have an underwater temperature meter, but around 15 to 20 feet is a typical depth for the thermocline in those lakes.

The second lesson occurred one beastly hot summer day on a small Mississippi impoundment. After several unproductive hours of trying a variety of patterns, I nosed my boat into a shallow slough with inflowing water that was about 4 degrees cooler. We caught a couple quick limits.

An analog thermometer showing a reading of 83 degrees.
Fishing Lore: How many times have you heard “the water warmed up 6 degrees today” or “the water temperature dropped 8 degrees overnight”? Unlikely. Even a small pond warms and cools rather slowly. The sun warms the water, with greatest warming occurring in the upper 1 or 2 feet, the depth where your temperature sensor is located. The surface water cools at night, which accounts for the afternoon-to-morning temperature drop. But what is the temperature near the bottom where bass live? Cool, more-dense water cannot float on warmer, less-dense water. The surface water temperature early in the morning is the best estimate of the temperature the bass are experiencing.

The ridge-thermocline lesson, if indeed the thermocline was a factor, is fairly easy to understand: bass were located where a typical bass-holding structure intersected with their preferred temperature. The turned-on bass in the cool-water slough is not so easily explained. The cooler water was a transient condition resulting from recent rains. How did the bass find the cooler water that was closer to their preferred temperature than the warmer water in the main lake?

Bass can do some pretty amazing things, like find pockets of oxygenated water in expanses of thick aquatic vegetation that become devoid of oxygen overnight. Or were they always there, enduring the hot water, and then became active when the water cooled? And in both lessons, were the bass responding to the temperature or the distribution or activity of the forage that may also have been responding to thermal conditions? Regardless of the reason, both patterns are worth trying when conditions are appropriate.

Yes, water temperature can help you catch bass. But here’s another lesson. I’ve caught bass in the cooling-water discharge from a power plant mid-morning on a summer day. I didn’t have a water temperature gauge, but the bass felt warm when we were unhooking them and well above their thermal limit. The bass probably were there gorging on the shad congregated in the outflowing water. Forage is important, too.

Thermocline Bass

Is fishing the thermocline in the summer a viable pattern? Absolutely, if you are targeting striped bass that have a relatively low upper thermal limit. These fish must swim in or near the thermocline to avoid the too-warm water above and too-low-oxygen water below. For the more heat-loving largemouth bass, biology suggests the thermocline is where largemouth should be in fisheries where the upper water is warmer than 84°F. But accomplished professional anglers don’t worry about the thermocline. New Market, Minnesota, B.A.S.S. 2021 Angler of the Year Seth Feider is no stranger to fishing deep, clear northern lakes. But on the professional tour he’s earned his honors and a lot of money rarely fishing deeper than 25 feet; he catches most of his fish in water less that 8 feet deep and far above the thermocline.

An angler in blue bibs holding two large largemouth bass in a boat.
Fishing Lore: The biological literature says bass feed very little below 50°F. The fishing literature says bass are lethargic below 50°F and only bite slow-moving lures. Neither are true. Bass are readily caught in water below 50°F as evidenced by huge weights in winter tournaments throughout the South in water temperatures in the 40s. Scientific studies found bass swimming speed declines about 60 percent from their optimal temperature of 84°F to 50°F, but they will chase and catch a lipless crankbait retrieved at moderate speed in 42°F water.

B.A.S.S. Elite angler Jeff Gustafson from Keewatin, Ontario, grew up fishing deep, clear Canadian Shield waters like Rainy Lake, but he, too, usually fishes shallow, “where the catchable fish are.” Feider agrees that you can find fish (with electronics) deep, but they are rarely catchable.

One reason fish may be in shallow water and above the thermocline is the temperature in the epilimnion (zone above the thermocline) may not be as warm as many anglers think. Yes, I’ve seen 90°F-plus on my temperature gauge on southern reservoirs and high 80s in Minnesota lakes in the summer. But those are, with rare exceptions, the afternoon temperatures 1 to 2 feet below the surface (the depth of my temperature sensor) on sunny days. Bass don’t live at the surface. Check your temperature gauge first thing in the morning if you want a good approximation of the epilimnion temperature, the zone that bass occupy.

Feider acknowledged that the thermocline will “set the bottom” on how deep to look for fish. How do you set the bottom of your search zone? Assuming you have modern sonar on your boat (if you don’t, you are probably better off fishing shallow, visible cover), you can adjust your sonar to detect the thermocline. But why bother? Let the fish tell you where the bottom of the search zone is. If you aren’t marking fish on your sonar—either forage fish or potential target fish—below a certain depth, don’t fish deeper than that depth.

B.A.S.S. Elite pro Cliff Pirch fishes the deep reservoirs near his Payson, Arizona, home. These lakes stratify in the summer, but Pirch worries little about thermoclines. Rather, he relies on the depth at which he sees fish on his Lowrance side-imaging and down-scan sonar. He then looks for topographical features—ledges, points, humps, and hard-bottom areas—at that depth to narrow his search for potential bass-holding structure.

James “Jimbo” Mathley (jimboon lanier.com) guides clients to giant spotted bass (now known to be Alabama bass) on Lake Sidney Lanier, Georgia. Fifteen years ago and prior to establishing his guide service on Lanier, Mathley often relied on the depth of the thermocline to locate largemouth-producing structure on deep, Virginia reservoirs. He exported his knowledge to Georgia. Lanier sometimes stratifies in hot summers, and the thermocline-structure pattern applies for Alabama bass as well as largemouth, but modern-day electronics have completely changed his game plan. Now he uses his electronics, especially Garmin Panoptix, to locate both forage fish and giant Alabama bass, a much more effective strategy for nomadic Alabama bass and their preferred blueback herring prey that roam throughout the pelagic (open-water) zone. The same approach works for largemouths.

Lake Stratification Profiles

A graphic showing lake stratification profiles.

These graphs show temperature and dissolved oxygen (DO) profiles at Ten Mile Lake, Minnesota, a deep, clear, oligo-mesotrophic lake with a photic zone depth of 40 to 50 feet. Stratification has just occurred by May 30 and the lake will probably mix in October. The epilimnion is the lake zone above the thermocline, and the hypolimnion is the zone below the thermocline. Note the high DO throughout the water column in May shortly after stratification but depletion of oxygen in the hypolimnion by mid-summer. Data provided by Dr. Bruce Carlson.

Putting Temperature Into Perspective

Water temperature is an important driver of bass physiology and behavior. But habitat and the distribution of forage also determine where bass are effective predators. The distribution of forage fish, for which biologists know less about temperature preference, is also influenced by temperature. But dissolved oxygen, which can be affected by thermal stratification, is a severe limiting factor—fish can’t live without it. In this way, the thermocline can provide important information about bass distribution.


In-Fisherman Field Editor Dr. Hal Schramm is former leader of the USGS Mississippi Cooperative Fish and Wildlife Research Unit. As a fishery scientist, avid angler, and educator, he brings unique insights to his articles on fish and fishing.




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