Science Of The Fall Turnover
When the thermocline disappears and the water at the bottom of the lake water is forced up, it can send fish into a tailspin.
Bassmaster Elite Series angler Mark Menendez, of Paducah, Ky., has earned more than $1.2 million over his two-decade career as a pro fisherman, but many may not realize Menendez also holds a B.A. in Fisheries Biology.
In a column on Bassmaster.com, Menendez uses his biology background to break down the science behind fall turnover and what it does to the fish—both prey and predators.
“Most large reservoirs have a thermocline in the summer. Above it, there is oxygen, good water. Below it, little or no oxygen and bad water,” Menendez said. “The turnover pushes that good water down into the bad water. Turnover is caused when the upper water cools, becoming denser and heavier, and forcing its way into and through the lower water.”
When the thermocline disappears and water from the bottom of the lake is forced up, it leaves mix of smelly, black water at the surface with a lot of partially rotted leaves and other types of floating debris.
“Things like a hard, quick cold snap or a lot of wind and cold rain — a half-inch or more — will drop the upper layer almost before you know it. I have seen turnovers develop in Kentucky in just 24-36 hours. Very few things happen that fast in nature. It causes issues for everything in the lake. This process can last for as much as four weeks, although typically is over in just two weeks,” Menendez said.
“From the prey’s point of view, a turnover does two things. First, it forces them to drop down to try to find water with enough oxygen to allow them to survive. They search both horizontally and vertically. They scatter everywhere. It can be somewhere between tough and impossible to find them.
“While that is happening, the pressure of the heavier water over them also pushes the fish down. That adds to the problem of finding them. At times, they will drop down to 50, 60 or even 70 feet. Finding scattered baitfish at those depths is no easy task.”
The predators — bass for our purposes — react much the same way. One difference, though, is that they are easier to find when scattered and really deep because they’re bigger.
“You can find single fish with your electronics. That might not be ideal, but it is better than not finding any!” Menendez said.
“Finding them is one thing. Making these bass bite is another. They do not feed much in that nasty water. They simply try to stay alive, and do that by finding better water. If they find it, bass will bite in one or two places.
“The first, and most useful to the majority of anglers, is at very shallow depths. The wind and photosynthesis will put some oxygen in the first foot or two of water. Bass that normally would not hold that shallow will move in anyway.
“The other place you will find some is in very deep water. In some reservoirs, the water around the 70-foot range will hold oxygen. The turnover does not always reach down there. Like in the shallows, bass that normally would not hold that deep would do so in order to survive.
“A third place that I’ll mention in passing is anywhere there’s a pretty good runoff into the lake. Creeks, ravines and the like will sometimes have running, tumbling water that puts oxygen into the water. Every species of fish in the lake will be attracted to the area where that water enters the lake.
“Beware of mud in those places. There is nothing tougher on a bass, or on a bass angler, than cold, muddy water.” Menendez said.
That is the simple, scientific view of a fall turnover. You’ll get plenty of advice from other anglers on how to fish it and what techniques and lures are best, but I hope you now understand what a turnover is and what effect it has on the ecosystem.
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