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How temperatures of lake layers concentrate fish

There is an old saying: “Ninety percent of the fish can be found in 10 percent of the water.”

Well, there is a lot to the pearls of angler wisdom, especially when fishing large natural lakes early in the year. 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.

Fishermen who understand the concept, called lake stratification, can increase their fishing success.

“The density of water varies based on its temperature,” said Jed Pearson, Department of Natural Resources (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’s natural lakes, such as bluegills, largemouth bass and crappies, avoid temperatures exceeding 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 in-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.

This farm news was published in the April 23, 2008 issue of the Farm World, serving Indiana, Ohio, Illinois, Kentucky, Michigan and Tennessee.

“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.”

Creel surveys being conducted at Robinson and Pike lakes
Angler surveys to monitor fishing success at Robinson and Pike lakes in Kosciusko County have begun and will run through September.

A DNR clerk will be stationed at the boat ramp of each lake, and will periodically interview anglers to gather information regarding their fishing trips. Anglers will be asked a series of questions regarding their experiences. Any fish harvested will be measured.

“These surveys provide us with detailed information about the fishery and how it’s being utilized,” said Rod Edgell, DNR assistant fisheries biologist “They are essential in evaluating the success of management decisions.”

Pike Lake is a 228-acre natural lake located in the city of Warsaw and is stocked with approximately 11,400 two-inch walleye annually. In addition to having a good walleye population, Pike Lake is also known for its abundance of channel catfish.

The survey at Pike Lake will provide biologists with information on how popular the two species are among anglers, and if the stocking of walleye is justified.

Robinson Lake is located on the Whitley-Kosciusko counties line near Pierceton, and is one of a handful of Indiana natural lakes to have an 18-inch minimum size limit and a two-fish bag limit on largemouth bass. DNR biologists have been monitoring the effects of the harvest restrictions at Robinson Lake since the regulation was implemented in 1996, and the results have been positive.

“Angler pressure on this lake has more than doubled since 1996, but the harvest restrictions have done a great job of protecting the quality of the fishery,” said Ed Braun, DNR fisheries biologist.
The angler survey at Robinson Lake, in combination with a bass population estimate, will provide state fisheries biologists with a measure of just how beneficial the special regulations have been to the fishery.

Participation is completely voluntary; however, the success of the angler surveys is dependent upon the cooperation of local anglers. The DNR is urging everyone to participate.

The views and opinions expressed in this column are those of the author and not necessarily those of Farm World. Readers with questions or comments may contact Jack Spaulding by e-mail at jackspaulding@hughes.net or by writing to him in care of this publication.

4/23/2008