How to Use a Fish Finder: A Simple Guide to Reading Sonar

The transducer — the part mounted in the water — sends out pulses of sound in a cone shape. When the sound hits something (the bottom, a fish, a tree), it bounces back. The unit measures how long the echo took and how strong it was, then draws it on the screen.

The screen scrolls from right to left: the newest information is on the right, and older readings move off to the left.

CHIRP sends a sweep of frequencies instead of one, which gives clearer separation between fish and the bottom. It's worth having even on budget units — see best budget fish finders.

• The bottom: the thick band near the bottom of the screen. A thick, bright bottom usually means hard ground like rock or gravel. A thin, dim bottom usually means soft mud or silt.
• Fish arches: on 2D sonar, a fish shows as an arch because it enters the edge of the cone, passes through the middle, and leaves. Arches show best when the boat is moving slowly. When you're sitting still, fish often show as lines instead.
• Baitfish: clouds or balls of small marks. Find bait and you often find bigger fish nearby.
• Structure: trees and brush show as vertical, branchy shapes rising from the bottom; rocks and drop-offs show as bumps and steep changes in depth.
• Thermocline: a faint horizontal band of clutter in summer, where warm and cold water meet. Fish often hold near that depth.

1. Turn off fish symbols (Fish ID). They guess — often wrongly — and hide real information. Learn the raw arches instead.
2. Sensitivity (gain): start on auto, then raise it until you see a little speckled clutter, and back off slightly.
3. Depth range: auto works, but setting the range just below the bottom makes everything bigger and clearer.
4. Frequency: higher frequencies (such as 200 kHz and above) show more detail in shallow water; lower frequencies (such as 50–83 kHz) reach deeper with a wider cone.
5. Speed: move at slow trolling speed while searching. Fast boat speeds make readings messy.

• Transom mount: on the back of the boat — the most common and easiest.
• Trolling motor mount: points where you're fishing from the front of the boat.
• Kayak mount: through a scupper hole or an arm on the side. See best kayak fish finders.
• Castable: a floating sonar ball you cast from shore that sends readings to your phone. See best castable fish finders.
• Ice fishing: a transducer that hangs down your hole. See best ice fish finders.

Mount it where water flows smoothly with no bubbles, or the picture will break up at speed.

1. Idle slowly over likely areas — points, drop-offs, weed edges, brush piles.
2. Look for baitfish and changes in the bottom, not just fish arches.
3. When you find fish or structure, drop a GPS waypoint or a marker buoy.
4. Circle back and fish the spot from a distance so you don't spook the fish.
5. Note the depth where you catch fish, then look for that depth in other areas.

Units with GPS and mapping make it easy to save spots and follow contour lines — see fish finders with GPS.