Thermal Stratification

Most lake trout seem to have a preference for water of 50° F. (10° C), however some subspecies tolerate colder temperatures. Although lake trout occasionally venture into waters of higher temperature, they do so for only short periods of time.

Water has the remarkable ability to become most dense at a temperature of 39.2° F. (4° C). Water of different densities has difficulty mixing, setting up barriers or layers of water instead. Temperature moderation takes place over a long period of time, protecting fish in the southern limits from excessively high temperatures. This same feature also protects the fish in the northern shallow lakes from freezing into a block of ice. As you can see from the diagram, the most dense mass of water remains at the bottom of the lake while the colder, less dense water floats above, followed by a layer of ice. If it were not for this unique feature, freezing would take place from bottom to top, probably killing off the lake's inhabitants. This layering of water into definite temperature zones is called thermal stratification. (Figure 5-1.)

In the southern limits of their range, lake trout inhabit deep cold lakes with a generous supply of oxygen and the ability to stratify. In these lakes water segregates into three distinct zones: the epilimnion -- a highly oxygenated warm-water zone found at the surface; the hypolimnion -- a very cold and poorly oxygenated zone found at the bottom; and the thermocline -- the zone which lies in between. This middle zone is an area of rapid temperature change, in most cases over half a degree Fahrenheit per foot of water depth. It is adequately supplied with oxygen, and of the three stratas offers more of the necessary requirements for the survival of the lake trout. During the warm summer months, lake trout are found in abundance either at, or near, this thermal layer of water.

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