Every baker knows what a turnover is. No football fan is confused about what has happened when there is a turnover on the field. But ask an angler what “fall turnover” or “lake turnover” means, and you are likely to get a confused response. Chances are that any angler who can accurately define the term will catch more fish in September and October than those who can’t.
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For fishing purposes, “turnover” means the exchange of surface and bottom water in a lake or pond. This mixing happens twice a year. Differences between the two periods contribute to anglers’ confusion about the process.
During the summer, the sun heats water near the surface of lakes. Because warm water is lighter than cool water, it remains at the top of the lake while cool water sinks to the bottom. Summer weather tends to be calm, and without wind to mix top and bottom water, the layering grows increasingly pronounced. Eventually a condition known as “stratification” develops with a well-defined warm layer of water riding over a cool one.
Oxygen-producing plants live near the surface, leaving water near the bottom of the lake cool but oxygen-poor. The decay of plants and animals that fall to the bottom of lakes heighten this condition.
Fish prefer cool water, but they need oxygen to survive. Consequently, they tend to stay near the “thermocline,” as the border between warm, oxygen-rich surface water and cool, oxygen-poor bottom water is known. Anglers who recognize this concentrate their efforts near the thermocline. Some fish may hang suspended near the thermocline above deep water at this time, but some of the best fishing spots are where sloping banks or underwater humps intersect the boundary between cold and warm water.
This can actually improve fishing success, as fish are easier to locate. However, anglers will have less success fishing on the bottom or near the top than they did before stratification set in.
With the arrival of fall and cooler air temperatures, water at the surface of lakes cools, becoming almost as dense as bottom water. Strong autumn winds move surface water around, which promotes mixing with deeper water. This is “fall turnover.”
As mixing continues, lake water becomes more uniform in temperature and oxygen level, allowing fish to move around freely. They can be difficult to find as long as these conditions prevail. This continues until the weather turns cold, chilling the lake’s surface.
Water is a strange substance in some ways. One is the fact that it grows denser as its temperature decreases, but only until it reaches 39 degrees Fahrenheit. Past that point, it actually grows less dense. By the time it forms crystals at 32 degrees, it is considerably lighter than liquid water. That explains why ice cubes float in a glass of water.
This oddity causes 39-degree surface water to sink to the bottom of lakes as winter deepens, forcing colder, less-dense water to the top, where it freezes. Ice-over prevents winter winds from stirring the water and a mild winter stratification develops, with the densest water sinking to the bottom.
With the spring thaw, strong winds again are free to move water around, and a spring turnover occurs. Once again, temperature and oxygen levels are more nearly equal throughout the lake and fish locations are less predictable. Gradual warming as spring passes into summer brings on stratification – and more predictable fish location.
For a brochure on this subject, write to MDC, “Pond Turnover,” PO Box 180, Jefferson City, MO 65102-0180.
