Turnover occurs in autumn when the stratified water layers created during the summer start to mix due to cooler temperatures. Prior to the turnover, the surface water is warm and light, while the lower layers are cooler and heavier. The top and bottom layers contain less oxygen than the middle section, so fish tend to hold in the oxygen-rich middle.
"If the water temperature is in the 70s, and all of a sudden you have four or five nights when the air temperature is in the 30s and it drops the upper layer 10 to 15 degrees, the lower layer will come up and the colder water on top will sink (creating the turnover)," says Wirth.
When the turnover occurs, the water quality in the affected area suffers and so do the fish.
"It's an oxygen-depletion scenario, so it's just like when you're starting to choke," Wirth says. "You're more worried about breathing than eating. The fish try to transition through it and tend to scatter and suspend in the water column a lot, which makes it hard to catch them."
Anglers can check for telltale signs to find out if the area they are fishing has turned.
"The water has a greenish-gray look," Wirth says. "It almost looks dead; you don't see anything in it. There will also be a (rotten egg) smell, and you'll start to see a lot of bubbles."
When you find an area that fits this description, you should heed Wirth's next bit of advice.