As the seasons pass, fish adjust to natural changes in lakes, reservoirs and ponds. Fish movement is keyed to two factors: dissolved oxygen and water temperature. Throughout the year, fish seek the zone in a lake that comes closest to satisfying both of these needs.
To understand how lakes change, it’s important to know what happens to water at different temperatures. Water becomes lighter when warmed and heavier when cooled. But water has a unique property. When it cools below 39°F, it becomes lighter. This ensures that a lake’s bottom water stays warmer than the surface during winter.
Because of this property of water, most lakes form three separate layers in summer. The upper layer, called the epilimnion, is warmer, lighter water that is easily circulated by the wind. As it mixes, it renews its oxygen supply. Meanwhile, the cold, heavy bottom layer, or hypolimnion, becomes stagnant and may lose its oxygen. Separating the two layers is the thermocline, a zone where the temperature drops very fast. In very shallow lakes, however, these layers may not form because the entire body of water is mixed by the wind.
These diagrams show the annual cycle of a moderately fertile lake in a northern climate. Seasonal fish movement is different in other types of lakes. For example, infertile lakes do not lose oxygen in the depths, so fish are not forced into the shallows in summer or winter.
In extremely fertile lakes, low oxygen levels restrict fish to the shallows most of the year, with the exception of spring and fall turnover periods, when they may be found anywhere.