BOB MAINDELLE: Anticipating the autumn turnover | Outdoor Sports

Although the weather has moderated compared to the lengthy string of 100-degree days we endured this summer, the waters of our local reservoirs, Belton and Stillhouse Hollow lakes, are still very much thermally stratified.

Fishing with clients this week was much the same as it has been all summer. We searched deep, open water to find areas holding threadfin shad and then scrutinized side-imaging technology to find small “wolfpacks” of fish holding near this bait.

Typically, the gamefish were found holding on the deeper side of sloped areas as compared to the baitfish they were holding near.

On a number of occasions, gamefish were found holding in a horizontal band roughly 4- to 5-feet thick. On Stillhouse Hollow, this band occurred routinely at 34 to 39 feet deep. This behavior comes about as a result of thermal stratification — the layer of water by density due to decreasing temperatures from the surface down to the bottom.

To illustrate this, consider the temperature readings I took using a FishHawk TD device Wednesday from the surface down to 60 feet:

0 feet 84.0F

5 feet 84.0F

10 feet 83.8F

15 feet 83.6F

20 feet 83.4F

25 feet 83.4F

30 feet 83.1F

35 feet 82.6F

40 feet 82.1F

45 feet 75.7F

50 feet 68.5F

55 feet 64.9F

60 feet 62.9F

Note the rapid temperature drop between 40 and 55 feet. This is due to thermal stratification.

It was not surprising, then, that most of the fish we landed on that day were taken from those same depths.

Stratification begins in late May when increasing daylength and increasing temperatures warm the water nearest the surface making it less dense than the heavier, colder water near the bottom. All summer long this warm, lower density water floats atop the cooler water.

Generally speaking, fish tend to congregate at the lower level of the upper, warmer, lower density layer of water where they find the coolest water they can which still has sufficient levels of dissolved oxygen to support life. The deeper, cooler, higher density layer of water near bottom is even colder, but typically has insufficient oxygen levels for fish life.

This scenario will persist until we enjoy a sustained fall cooldown in our weather. Once the air temperature, both day and night, falls below the surface temperature of our lakes (which now stands around 83F), that surface water will become more dense and begin to sink downward toward the lower, colder layer of water near bottom.

As sustained cooling continues, that entire upper layer of water will cool to a temperature lower than the water below it and will sink down into that lower layer, thus displacing it. This process is called turnover.

Once turnover occurs, the water will be nearly uniform in temperature from top to bottom and will also be sufficiently oxygenated from top to bottom so that fish can inhabit any depth at that time.

With the extended forecast for the forthcoming 10-14 days showing little promise of a cooldown, it seems turnover is still quite some time away.

In years past, we have experienced turnover as early as late September and as late as late October.

Over the span of just a few days following turnover, fish will begin to make much more extensive use of bottom structures and topographic features at greater depths than they have since stratification began in late May and early June.

Additionally, following turnover, the water temperature will continue to fall through the 70s during October and early November, through the 60s in November and December, and so on.

As the water temperature transitions from the 70s to the 60s, it falls in many warmwater species’ preferred temperature range. Such is the case for largemouth bass, smallmouth bass, crappie, white bass, hybrid striped bass and others.

During the time the water temperature remains in fishes’ preferred temperature range, they tend to feed frequently and heavily and for longer periods of time than at times when the water temperature is warmer or cooler.

Although many anglers leave fishing behind in the fall as kids’ school activities, hunting season and cold weather detract from the fishing, in my opinion the fishing during the period immediately following turnover is every bit as productive as during the spring warmup, only without the crowds.