One of the most familiar, and often dreaded, changes is the fall turnover. It only occurs in lakes that have thermal stratification in the summer.
Thermal stratification is likely to occur in deeper farm ponds, and lakes and impoundments with little flow-through. In hydropower and navigation impoundments, like those on the Tennessee River and the Tenn-Tom Waterway, flowing water in the main lake or the navigation channel prevent stratification, but thermal stratification can still occur in large coves and embayments.
Thermal stratification occurs when warmer and less-dense water "floats" on the layer of cooler, denser water. The thermocline, a zone of water with rapidly changing temperature, separates the cool, lower water layer (the hypolimnion) from the warm, upper water layer (the epilimnion).
Stratification is established in the late spring when longer days, increasing solar energy and warm air warm the upper water faster than the heat can be circulated from top to bottom. Stratification occurs when the upper water warms and becomes less dense than the lower water.
As summer progresses, the epilimnion continues to warm, increasing the temperature and density differences between the epilimnion and the hypolimnion.
The process reverses in the fall when days shorten, solar energy declines and air temperatures cool. When the temperature of the epilimnion declines to that of the hypolimnion, the lake mixes. This is called destratification.
Mississippi's lakes and reservoirs are productive. Throughout the summer, the hypolimnion progressively loses oxygen because no oxygen is produce in the dark, deep water and dead plants and animals that sink into lower water decompose and use oxygen.
When destratification occurs, the low-oxygen water mixes with the well-oxygenated upper water, causing oxygen levels to plummet. In some lakes, you may see the water turn brownish and murky. You may also see partly decomposed organic matter - looks like peat moss - floating, and fish kills can occur.
The low oxygen levels can shut down the bite for a few days to a week.
Within a couple weeks of destratification, lakes typically develop an algae bloom. The algae bloom results from the nutrients released from decomposition of plant and animal matter near the bottom now being circulated into the upper water levels, where light is sufficient for photosynthesis. It's a fertilization effect.
Note that the algae bloom occurs several weeks after turnover. It may be a sign that turnover has occurred one or two weeks prior, but is not an indication that turnover is presently occurring.
Lakewide cool water and lack of thermal stratification means the fish can disperse through the entire water column. The cool, shallow water may be more attractive to your favorite sport fish, but so may be deep structure and cover that was off limits due to lack of oxygen in the summer.
Fish change, too
Fish live in their comfort zone of preferred temperatures and adequate dissolved oxygen when available, but they follow the forage.
That can mean different things for different game fish, depending on what they eat.
For fish like sunfish and smaller catfish that feed on invertebrates living on and in the bottom, the whole lake is now a buffet.
But there are changes in the invertebrates. Most of the aquatic insect larvae have grown, matured and metamorphosed to adults, and reproduced in the summer. The insect larvae in the fall are abundant but young, small stages that may not be as attractive to predators.
For fish that feed on fish - black bass, white and striped bass, crappie, larger catfish - the fish follow their mobile forage.
So the questions now become which forage do they eat and where do the forage go.
Shad are the dominant forage in lakes and reservoirs where they occur. The early-spawned young shad provide important forage for the young of many fish eaters, but they quickly outgrow their predators.
In the fall, the second-spawn shad are abundant and provide more forage for the young predators. Although these small, summer-spawned shad are easily detected in large surface-dimpling schools, especially when being marauded by predators, schools of larger shad attractive to larger game fish can be hard to find.
The movement patterns of larger shad are unstudied and poorly known by biologists. A few accomplished anglers I know are pretty good at predicting shad distribution, but they are often puzzled, too.
Only the shad really know, and they aren't talking.
Other fish - like sunfish, minnows, and even young crappie - can become attractive forage when the shad are too large, too small or scattered in open water. Anglers seeking larger game fish may improve their game by thinking outside the shad box, particularly in the fall.
Hard to do when you are surrounded by schools of predators bustin' shad.