Structure, thermoclines and bait

Fish rarely spread out evenly. They hold near things: a drop-off, a brush pile, a standing tree, a rock ledge, a layer where the temperature changes, or a cloud of baitfish. Finding those is most of what a fish finder is for.

On the screen, structure shows up as a rise or break in the bottom band, or as clutter that stands up from it. Brush and trees look like rough, scraggly shapes above the bottom. A drop-off looks like the bottom band sliding down. Bait often looks like a cloud or ball of small dots that moves and changes shape from pass to pass.

A thermocline is a layer where the temperature drops quickly with depth. In many deeper lakes in summer the water separates into a warm top layer, a cooler bottom layer and the band between. Fish and bait often stay near that layer, and the sonar may show it as a faint line or as a level at which bait and marks pile up. A fish finder does not measure the layer directly, so you read it from where the marks hold.

Use the buttons below to add and remove each thing and see what it would look like on the screen. This is a drawing of an idea, not a real lake.

An interactive drawing goes here in the app.

On the water

• Mark what you find. When the screen shows structure, bait or a layer, drop a waypoint at the edge of it so you can come back and fish it from a better angle.
• Look for two things together: structure with bait on it, or bait at a depth that keeps showing up. One alone is a hint. Two together is a stronger one.
• Remember that shallow lakes and rivers often do not layer up the way deeper lakes do, so do not expect a thermocline everywhere.

Easy mistakes

• Calling any flat line a thermocline. A faint level line is a possible sign, not proof.
• Mixing up bait and weeds. Both can look like a fuzzy cloud; weeds are rooted to the bottom and bait is usually off the bottom and shifts.
• Fishing the top of a brush pile because that is what the screen shows. The fish might be on the side you did not drive over.

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