From Maine to Florida and California to Kansas, no body of still water can escape fall turnover. The term is one most anglers are familiar with, though it’s questionable how well everyone really understands what it means, how it affects the fish, and, most importantly, how it affects our ability to catch them. How many fishermen perceive it is a rapid change that simply screws up the bite: the lake is suddenly off color, or rotting leaves plague a cove that was clean just a week ago, or the fish simply aren’t in the locations where they had been consistently. Depending on where you live and how the fall weather plays out, turnover will alter every lake or pond differently. But the decreasing lack of sunlight — a major player in the game — is unavoidable in every corner of the country.
We may not be able to perfectly predict when turnover will occur, or how long it will take for the conditions to normalize and kick off those killer fall fishing patterns, but by understanding exactly what it is and does to the fish we can at least get a better handle on where to cast when things get wonky, and gauge how long turnover will last. I couldn’t think of anyone better to explain it than my buddy, Shan O’Gorman. He’s a trained aquatic biologist and legend in the bass world who builds and manages private waters all over the U.S. He has a reputation for growing big bass fast, but to do so, O’Gorman must have intimate knowledge of how the whole ecosystem system works, including during that dreaded fall turnover.
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What Is Fall Turnover?
Turnover is quite complex, and I’m certain O’Gorman could write an entire book about it, but what makes him so popular on the podcast circuit is his ability to break down scientific processes into language recreational anglers can understand. In a nutshell, turnover boils down to oxygen levels. In summer, the warm water at the surface has the most oxygen, while deeper, colder water holds less oxygen. If these layers aren’t disrupted, you’ve got stability and probably a consistent fishing pattern. But the changing season loves to muck that up.
“Think about a cake with a chocolate layer on the bottom and a vanilla layer on top. Water naturally layers the same way based on temperature,” O’Gorman says. “The warmer layer will always be on top, and in summer, visualize that vanilla layer about one inch wide and chocolate layer about 10 inches wide. That thin vanilla layer is holding the most oxygen because the sun can penetrate it. That’s where all the plankton and microorganism at the beginning of the food chain will be. But as the sunlight wanes in fall and the temperature drops, that thin oxygenated strip will get [deeper].”
Though you might think that a larger layer of oxygenated water would be beneficial to anglers, during turnover it has the opposite effect. Once there’s no stark temperature differences in the layers to keep them separate, upper-level oxygenated water begins mixing with the deeper non-oxygenated water as if in a blender. Not only does this cause fish to move around, but it also supplies oxygen to bacteria and microbes near the bottom of the lake that have been starved all summer. One side effect of these tiny critters rapidly scarfing up the O2 is the quick change from green weeds and algae to brown, brittle weeds and “snot” algae. The bigger problem is these microorganisms consume oxygen so fast that it can de-oxygenate a body of water to the point of a fish kill.
“If you’re dealing with a small body of water and a rapid turnover, the fishing can get very tough for a while because oxygen-starved fish aren’t usually hungry,” O’Gorman says. “If you put a plastic back over your head, would you still want to eat?”
The Warning Signs
In a perfect scenario, turnover happens gradually. Unfortunately, we can’t always bank on a transition where summer gently cools more and more each week until weather patterns settle into winter mode. If you’re lucky enough to experience a gradual fall transition, however, turnover might affect your fishing very little. But, more commonly, we’re thrown a curveball, and sometimes it can trigger turnover almost overnight.
“It’s very hard to pattern weather and use that to predict turnover,” O’Gorman says. “But one thing you really need to keep an eye on during the early fall are rain events. If you’ve got a warm surface layer of water that’s been cooling slowly and then is suddenly mixed with a lot of cold rainwater, that can trigger turnover very fast.”
O’Gorman also pointed out that snow can trigger a rapid turnover for ice fishermen, which helps explain why early, black ice often produces the best fishing. If the sun can penetrate the ice, oxygen will be produced in the upper water layer. Once that ice is covered in snow, however, oxygen production will cease. Therefore, months like February can be very tough on the ice, especially on smaller bodies of water, because as the weeks press on without sunlight penetrating the water, the oxygen level just continues to fall.
The good news is that turnover conditions are always temporary. Whether they affect your fishing a lot or just a little bit, homeostasis will return to the lake. How long this takes depends on the stability of the weather regardless of temperature. In other words, if it gets cold quickly and stays cold, things will normalize much faster than up-and-down cold snaps followed by prolonged periods of abnormally high fall temperatures.
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