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Fish are cold blooded animals with their body temperatures controlled by their environments. Temperature directly affects their activities, and each fish species has an ideal temperature range in which they are the most active. (See Figure 1) Since fish can detect sense, recognize, perceive, and discern changes as slight as 0.1 degrees F. A small change in temperature does not go unnoticed.

Likewise, aquatic insects are cold blooded creatures. Temperature influences insects’ activities such as emergence and migration. Because aquatic insects are a fish’s primary food source their availability directly influences the fishes feeding activities. A waters greatest thermal source is solar radiation, which is directly absorbed. The surrounding air mass likewise affects a waters temperature by conduction at the surface. Heat is also transferred from the waters bottom sediments and adjacent rocky shorelines, rocky. This last source has only a minor influence on water temperature; the first two factors play primary rules.

Water losses heat from evaporation, outlet flows, and air conduction. Wind action distributes heat in a body of water, by causing waves which the water mixes.

A large water mass is slow to change its temperature as it is efficient at its energy. Consequently, large lakes are slow to warm up in the spring; likewise, they are slow to cool down in the fall. It requires a tremendous amount of energy measured in kilocalories to influence a temperature change in a large body of water. This is why water temperatures always lag behind the air temperature changes.

Lakes stratify or divide into three layers defined by both their temperature and oxygen contents. Waters oxygen content and temperature are inversely proportionate. That is the cooler the water the more oxygen it will hold; conversely, the warmer the water the less oxygen it will hold. In general water is at its greatest weight density when it is at 39.2 F. As a result the deepest water will be the densest and closest to this 39.2 F. As water warms from this 39.2 F it will becomes proportionally less dense and stratifies towards the surface. Water cooler than 39.2 F also becomes less dense and will be found at the surface depths. Waters melting/freezing point is at about 32 F and this is found at the surface because it is less dense than the 39.2 F water. Ice is a solid phase which due to its crystalline structure is even less dense than its liquid phase. That is why ice develops on a lakes surface.

During warm weather times, a lake stratifies into epilimnion, thermocline, and hypolimnion layers.

The epilimnion is the uppermost layer which is both well oxygenated and uniform temperature. The

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thermocline is a mid layer containing a drastic temperature reduction change and decreased oxygen content. The hypolimnion is the bottom layer characterized by both the lowest oxygen content and the coldest water approaching 39.2 F. These bottom two layers are oxygen poor because they are remote from the oxygen producing surface and aquatic plant areas. These layers increase densities cause a barrier which seals them from obtaining additional oxygen supplies as a result thermocline and hypolimnion layers are oxygen poor.

In the fall water temperatures approach 32 F throughout, the lake mixes and these stratification layers are lost. At this time all of the lakes water reaches the same temperature and density. This blending distributes the oxygen evenly through out the lake. Wind and wave action also helps this mixing. After ice out in the spring this wave action again mixes and causes the loss of winter thermal stratification. These periods are often referred to as spring and fall turnover times. Since the turnover mixes the oxygen evenly fish can be found scattered throughout a lake. During summer and winter stratification times, fish can only live in restricted areas where oxygen is plentiful and temperature is comfortable. Winter conditions have the coldest 32 F water on the surface and the warmest 39.2 F water on the lakes bottom. A lakes ice lid cover prohibits mixing by wind/wave action and locks in the stratification layers. Under the ice oxygen production comes from photosynthesis caused by light penetration reaching the aquatic plants. A heavy snow and ice cover shields this needed light penetration and curtails oxygen production by plant photosynthesis. The top hypolimnion water layer has the most oxygen, aquatic life, and fish. The middle thermocline layer is absent.

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The bottom hypolimnion layer is oxygen poor and devoid of life. Winter fish kill is caused by a complete blocking of sunlight penetration causing the aquatic

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plants to die. Decaying plants use oxygen and deplete the lakes oxygen causing fish death from suffocation. Aquatic animals further deplete oxygen by respiration. During hot weather fish are found close to the bottom layer of the eplimnion. They opt for the coolness of the thermocline but still require the oxygen richness of the epilimnion. Commonly, fish daily migrate to the shallows to feed and return to the comfort of the epilimnion-thermocline junction. Resting fish are most comfortable at this junction.

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During the fall turnover, a lake blends these three layers into one uniform body of water with stable oxygen and temperatures throughout. This causes the fish to become highly active and very hungry. Fall fish can become scattered due to these widespread ideal conditions.

Water temperatures initiate insect hatches a cooler than normal year delays seasonal insect migrations. The calendar timing of insect hatch can be altered by variations in local temperatures. For example, a cold front can delay the stonefly hatch on a river spoiling a planned vacation. In addition unseasonable warm temperatures may cause this same hatch to

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occur earlier than usual. Aquatic insects demand well oxygenated water for emergence. Unseasonably warm temperatures may deplete the waters oxygen content and retard insect emergence.

A streams current thoroughly blends its water, resulting in fairly uniform temperature and oxygen contents. Small temperature variations may still happen. Since fish can detect a 0.1 F difference, they may seek out more favorable water temperatures during

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extreme weather conditions. At these times fish search out submerged springs and some cooler tributaries. For example, during extremely warm weather in Yellowstone Park, the Firehole River’s trout will school below the confluence a cooler tributary streams.

I believe that fish become activated by a sudden swing in temperature approaching the fish’s ideal range. Especially after a prolonged spell of unfavorable warm or cold temperatures, this sudden change toward ideal temperatures highly activates the fish. This results in the fish going on a sudden active feeding spree. For example a February Chinook-thaw provokes a water temperature swing from stable thirties to the mid-forties F. The fish answer, by going on a feeding frenzy. The peculiarity of this event is that the forty degree temperatures are far below the trout’s preferred range of 55-60 F. The trout behave like college students from Minnesota on spring break in Texas. Likewise the dog days of summer are cooled by a sudden storm front causing a favorable cooler temperature swing. Again the fish respond by actively feeding. This junction is where the most rapid decline in temperature occurs

Two conditions resulting in extremely inactive fish are extreme cold (melting point) and heat (greater than 70 F for trout. During these times fish are so inactive it is as if they have disappeared. Perhaps they have migrated elsewhere.

The fishermen needs to know, during hot temperatures for lake fishing, monitor the water temperature to find the depth of the epilimnion/thermocline junction. During high light conditions target fish the deepest portion of the epilimnion layer because the fish will concentrate there to rest. During low light conditions fish the upper layers of the epilimnion because fish migrate there to feed. In both cases locate areas where these depths coincide with bottom structures such as submerged weed beds.

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Preferred Temperatures
Species Preferred Temperature Range
Brown Trout 60-65 F
Lake Trout 48-52 F
Rainbow Trout 55-60 F
Chinook Salmon 48-55 F
Coho Salmon 48-55 F
Largemouth Bass 68-78 F
Smallmouth Bass 67-71 F
Bluegill 75-80 F
Crappie 70-75 F
Northern Pike 60-70 F
Walleye 65-70 F
Yellow Perch 65-72 F
Striped Bass 60-70 F
White Bass 65-75 F

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