Fishing the thermocline on Delavan Lake

One word oddly found in the vocabularies of both anglers and hydrologists is thermocline. It's a ten-cent word that means a distinct layer of a lake above which the water is a different temperature than below it. Any swimmer or diver in the summer will feel the temperature shift cooler as they go deeper. Thermocline is the band between the warm and the cool.

During the summer, warm water, which is less dense, will sit on top of colder, denser, deeper water. The sun heats the top layer, the thermocline doesn't move deeper or shallower in calm weather. A storm will create more mixing of the warm and colder waters, but the thermocline is mostly stable in our summer before the Fourth of July.

For the fall and winter, surface water temperatures drop as nighttime cooling dominates over daytime sunlight. Around October, a point is reached where the density of the cooling surface water becomes greater than the density of the deep water, and the lake 'turns over.' It's Mother Nature's fashion of redistributing the nutrients in Delavan Lake.

This is important to anglers because there is less oxygen in the water below the thermocline. That colder water doesn't reach the surface, and organisms living below the thermocline use up what oxygen is there. Less oxygen means fewer fish below the thermocline. Sure, you'll find some near underwater structures, but they're less active and they won't stay long below the thermocline: Their dinner is above it. (Which explains why predator fish have a larger lower jaw since they are usually striking their meal from below.)

Experienced anglers use a sonar to figure out where the thermocline is and set their maximum scan depth to it so they don't waste time looking where the fish aren't. For the weekend angler visiting Delavan, fish the thermocline – typically around 11-13 feet – and above. Dingy water in other lakes puts a thermocline shallower because the sunlight's warmth can't reach any deeper than six feet. For fall fishing, there is no thermocline (the water is the same temp) and the fishing can be good at any depth. You just have to find them.

Photo by Todd Arena, used with permission