As spring temperatures rise, natural lakes across northern Indiana begin to divide themselves into three distinct thermal layers. How deep each layer becomes and how much oxygen each layer holds will ultimately affect fish and fishing throughout the summer. Anglers who understand this concept, called lake stratification, can increase their fishing success. “The density of water varies based on its temperature,” said Jed Pearson, DNR Division of Fish and Wildlife fisheries biologist. “As the surface water warms in the spring, its density decreases. By summer, it eventually floats on a layer of deep, colder water.”

Biologists call the warm top layer the epilimnion and the cold bottom layer the hypolimnion. In between is a transitional layer called the thermocline. Most lakes stratify, but the thickness of each layer can vary from lake to lake. Because water is such a great insulator, the thermocline acts as a barrier and prevents the epilimnion from mixing with the hypolimnion.

“Most of our natural lakes stratify during May,” Pearson said. “After that, the differences between the surface and bottom temperatures increase even more dramatically and stay that way for the rest of the summer.”

The temperature difference is usually more pronounced in deep lakes.

“It’s common to see summer surface temperatures over 80 degrees and bottom temperatures still in the low 40s in lakes that are over 60 feet deep,” Pearson said. “In shallow lakes, the temperature gradient is less sharp. Bottom temperatures might stay around 55 degrees throughout the summer.”

Because fish are cold-blooded, they react to water temperatures. Different species have different temperature preferences.

Popular sport fish in Indiana natural lakes, such bluegills, largemouth bass and crappies, avoid temperatures that exceed 80 degrees. Other species, such as trout and ciscoes, can’t tolerate temperatures much above 70 degrees. Walleyes, northern pike, and muskies are usually found somewhere between.

“When the water temperature gets too warm, it increases the energy requirements of fish and puts extra stress on them. Fish look for colder water when the surface layer gets too hot,” Pearson said. But don’t think fish simply go deep to find cold water in the summer.

“The bottom layer is cold but it usually has very little oxygen,” Pearson said. “Plants that produce oxygen in shallow water seldom grow in the hypolimnion because sunlight can’t penetrate deep enough. So the amount of oxygen near the bottom depends on how much is present in the spring before the lake stratifies and how fast it gets used up by decaying organic matter.”

Fish cannot stay where they cannot breathe. As a result, the deep holes in a lake may be devoid of oxygen and therefore devoid of fish.

Most fish need an oxygen concentration of at least 3 parts per million (ppm) to survive. Summer oxygen levels within the hypolimnion of most Indiana natural lakes are usually less than 1 ppm.

So where do most fish hang out in summer once a lake stratifies?

“If we are talking about bass and bluegills, they usually concentrate more at the top of the thermocline,” Pearson said. “In most of our lakes, that’s around 10 to 12 feet deep. In muddy lakes, they might be a little shallower. In clear lakes, they might be a little deeper.”

The Waynedale News Staff
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