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It is no secret to anglers that certain species of fish can be found in specific areas of lakes depending on the species, the time of year, or even the time of day. That is why anglers may choose to fish for walleye by jigging off the bottom in deeper water, cast for northern pike among bulrush, or use top-water plugs for bass near fallen trees along the shore. These simple strategies are employed because certain species of fish will associate with particular types of habitat. And while biologists often debate the basic functional definition of habitat, one notion is universally accepted: habitat quality and quantity affect both fish abundance and the ability of anglers to catch fish. However, it is still largely unknown how much and what type of habitat is needed to keep a healthy balance.

In lakes, the majority of structural habitat is concentrated in the nearshore area, or littoral zone. The littoral zone, by definition, is the shallow region of the lake where aquatic plants may be found. These plants provide shelter and more living space for all types of aquatic organisms. Littoral zones are also the portion of the lake where fallen trees provide overhead cover for fish and homes for smaller creatures such as insects or algae. The fallen trees and aquatic plants exist above the lakebed, which is made of anything from “muck” to sand and/or gravel to large boulders. This variation in bottom substrates also provides a certain structural habitat for fish and other aquatic organisms. And finally, the steepness or slope of the littoral zone is also important when considering structural habitat in this near-shore region of lakes.

It is the littoral zones of lakes where nearly all lake-resident freshwater fish spawn. Moreover, fish and other aquatic organisms relate to different types of structural habitats during certain stages of their life cycle. For instance, young smallmouth and rock bass will assemble near larger bottom substrates, such as cobble and boulders. Bluegills and other panfish are usually found swimming among water plants, while emerald shiners may school along littoral zones having sand bottoms with very little structure. Green frogs undergoing final stages of metamorphosis are easy to spot on lily pads in calm bays.

Alterations of these habitat types can have an impact on the organisms that live there. For better or worse, structural habitat in littoral zones can be modified with relative ease. The depletion of this habitat can occur as human development along shorelines increases. Greater numbers of homes and cabins along lakeshores may lead to removal of shoreline trees and other vegetation, as well as aquatic plants in the littoral zone. To counteract this dilemma, structural habitat is sometimes added in the form of fish cribs and spawning reefs along with harvested trees placed in shallow water along the shore. This form of habitat mitigation is relatively easy to do, and we feel good about “improving” structural habitat in lakes. But how does this affect the organisms that live there?

Honestly, we don’t know. We lack biologically based habitat assessment methods for these nearshore areas. Ultimately, we are not familiar with the type or amount of structural habitat we have already lost or continue to lose, nor do we have a good understanding of how much habitat is needed to sustain most species of fish. So how do we learn more about the relationships between structural habitat and aquatic organisms? The first step is to better understand the habitat that exists in littoral zones of lakes. The objective of current research at the Wisconsin Cooperative Fishery Research Unit, at UW-Stevens Point, is to provide a framework for displaying and measuring the complexity of this nearshore habitat. To tackle this objective, we measured depth, bottom type, and the abundance of aquatic plants and downed trees in eleven northern Wisconsin lakes. Combining the data in a four-dimensional graph, researchers can see a clear picture of the littoral zone habitat revealed as a “fingerprint” unique to each individual lake. Watery fingerprints reveal clear patterns of littoral zone habitat; in essence, the habitat personality of the lake. Essentially, each graph provides an opportunity to learn about a particular lake’s “habitat identity”.

Research continues as we work to determine the potential, limitations and compatibility of a lake’s habitat. As the concept of creating and using littoral zone fingerprints gets developed and refined, there will be many potential uses by biologists, landowners, and lake visitors. These fingerprints will help to locate rare habitats, show lake habitat diversity, or determine which species of fish and other semi-aquatic organisms can benefit from particular habitats. In turn, this will aid people in working with the particular habitat “capital” that their lake holds. Efforts to protect, restore, and enhance habitats will clearly be guided by measuring these fingerprints.

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We often get phone calls and emails from Lake Tides readers with a variety of questions about lake districts. Do you have a question about lake districts that you would like to see answered in Lake Tides? Send it to uwexlakes@uwsp.edu so we can include it in a future issue.

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