Understand Natural Lakes

Properly classify natural lakes to better understand a fish's physical universe.

Back in the 1970s, Al Lindner, Jeff Zernov, Bill Binkelman and I began working on a system of classifying natural lakes. Our goal was to come up with a universal fishing vocabulary that would allow better communication between anglers. We began with scientific terminology. Limnologists (scientists who study lakes) classified glacially-made natural lakes in three broad categories: oligotrophic, mesotrophic and eutrophic. In more common terms, we also referred to these categories as being geologically young and infertile, middle-aged and fertile, or old and very fertile, respectively.

Understanding Age
If the concept of lakes being young, middle-aged or old seems strange, think of it this way: It’s not so much the actual amount of time a lake has existed, but rather its condition—how much time it has remaining in its life cycle. A geologically young lake is chiefly a rock basin, with relatively little sand or sediment on the bottom. Theoretically, it has thousands of years left in its life cycle, through which it will experience all sorts of erosion, siltation and other environmental influences that fill it in. At the opposite end of the age spectrum, an old lake nearing the end of its life cycle typically looks more like a swamp than a lake—shallow, silted and extremely fertile.

In between these two extremes are all sorts of waters in differing stages of transition—middle-age lakes. Lake classification has a lot to do with location. Young lakes typically lie at the northern end of the natural lake range, primarily in Canada, where expanding glaciers originally scraped the landscape down to bedrock, pushing the dislodged materials southward. The midsection of the natural lake belt lies across the far northern U.S. (roughly Minnesota to New England), where glacial movement created lakes within the dislodged materials pushed ahead of glaciers. This accumulation of earth and stones that was deposited by a glacier is called “moraine.” Along the southern end of the natural lake range, most waters lie within materials carried by meltwater and deposited in the glacial outwash plain formed when glaciers melted. As ice continued to recede, meltwater filled the network of gouges and holes exposed by glacial retreat, forming thousands of lakes with a range of depth and bottom content characteristics.



Lakes In Transition
When we originally classified natural lakes in three stages of age and fertility, we further broke each of them into several intermediate classifications. Basically, each stage had an early, middle and late subdivision, for a total of nine designations.

However, after nearly 50 years of experience, we’ve developed a six-stage breakdown that’s easier to understand.
Oligotrophic: young and infertile
Late-stage oligotrophic: the transition period from infertile to fertile
Mesotrophic: middle-age and fertile
Late-stage mesotrophic: the transition period from fertile to very fertile
Eutrophic: very fertile
Late-stage eutrophic: extremely fertile, approaching final stages of being able to support fish

The Aging Process
As mentioned earlier, as lakes pass through the natural aging process (called eutrophication), a lake grows older not so much in time as in condition. The initial stages of change may take thousands of years. But the final steps may happen quickly, especially with the addition of human factors. Natural changes tend to occur so slowly that they are almost imperceptible. But where humans become involved, lakes experience accelerated siltation, increased fertility due to runoff , shoreline degradation due to construction and dredging, and so on.

Major changes may occur in as little as several decades. As a result, the total environment of a lake—its structural condition, food chains, vegetation types and levels, and dominant fish species—are subject to rapid change. In recent years, true wild cards have evolved with the introduction of invasive species like zebra mussels, which strain fertility from the water, reducing the amount of food available to support aquatic life.


In just a few years, for example, the water clarity of Lake Erie went from as little as several feet during summer algae blooms, to clearer and less fertile than at any time in modern history. In effect, the zebra mussels have reversed the natural aging process. The danger is, if the process proceeds too far, the plankton and fish populations could be subject to not only stress, but potential collapse. Only time will tell how nature copes with this.

Multiple Personalities
Like most people, anglers prefer simple explanations and easy solutions. And while it would be nice to say all lakes fall squarely into the previous designations, nature, of course, has other ideas. Large lakes, in particular, often feature multiple basins—some deep, some shallow and some in between. They have bays—some weedy, some not. Although there will obviously be some interaction with fish swimming between these diverse individual sections, each operates like its own minienvironment.

In a nutshell, a large lake could have deep, infertile, oligotrophic sections; moderately deep, fertile, mesotrophic portions; and numerous bays that fall more into the very fertile, eutrophic category. Two important points arise with this concept. First, you need to consider which lake section you are fishing, and plan your actions accordingly. Second, realize that fish in each section behave and react according to their local habitat.

A huge fishery like Lake of the Woods, which sprawls across the Minnesota/Ontario/Manitoba borders, has several deep, clear sections where lake trout abound. Yet, as a cold-water species, they cannot survive in the vast middle-age portions of the lake, where cool-water fish like walleyes, pike, muskies and smallmouth bass thrive. Nor do these species do well in fertile back bays where largemouths live. The good news is most lakes are considerably smaller and simpler to fish. However, they still tend to have some localized patterns in different portions of the lake. Keep this in mind during your fishing forays.

To Each Its Own
Our lake classification system is invaluable for several reasons. For one, it helps us understand where, why and how fish are found, and what they’re likely to be doing. It also helps us communicate these concepts with other anglers, allowing us to share ideas and make sense out of otherwise conflicting observations.

All that being said, however, each individual lake is somewhat unique, almost like nature broke the mold as each body of water was formed. It may be similar to other lakes with comparable depth, structure and fertility, but it nevertheless has its own distinctive combination of elements that make it one-of-a-kind.

Even lakes that at first appear nearly identical to one another may support different species, and the locations where you catch fish may differ as well. Some lakes routinely produce trophy-class fish, while others are known for producing numbers of “eaters.”

On the human side, some lakes are managed with slot limits and other regulations that manipulate the average size of fish, while others are subject to significant general harvest. You may have also wondered why one lake resembles pea soup, while a nearby lake is ultra-clear. Or, why ice-out on one lake is always a week earlier than on another right across the road. These differences result from nature’s complex web of interrelated physical and chemical properties within a lake, such as water temperature, fertility and oxygen levels, all of which change throughout the seasons.

While each of these properties tells you a little about a lake, it’s really the combined effect of all these factors that really counts. That’s the habitat in which fish must live, and the better you understand what it offers them, the better you’ll understand their behavior, and the more and bigger fish you are likely to catch.


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