Reading Your Depth Finder

How fishing sonar & depth finders work

A transmitter sends out an electrical impulse; a transducer converts it into a sound wave and sends it into the water. The sound wave rebounds off an object, striking the transducer. It is converted back to an electrical signal, amplified by a receiver and sent to a display. The process repeats several times per second. Environmental factors such as wind, wave action, silt, minerals, salt, and microorganisms reflect, absorb, and scatter signals.

Displays calculate the echoes and provide different resolutions, shades, and colors. Pixels determine the clarity of readings. The formula for a 240-vertical-pixel unit in 60 feet of water is: 240 divided by 60 = 4 (one fourth). Targets are then separated by a quarter-inch.

Data determination

Check your batteries. Most depth finders use D-size batteries. The transducer must be level above the hole so you know what lies beneath. This allows you to see the lure and the fish as it approaches the bait. Getting a good reading on your lure allows you to place the bait right above the fish. Depth determines the lure and bait you will use.

Registering a green line (a weak signal) indicates a fish is on the edge of the cone, a yellow line means it's closer, and a red line shows the fish directly below the hole. A line a distance up from the bottom is a fish making his way through open water. Several stationary lines between the bottom and the ice are weeds. Short weeds in deep water could contain fish that show up as additional lines on the flasher. Locating a tight ball of baitfish indicates a predator in pursuit.

A thick bottom line indicates a hard bottom and a thin bottom line registers a soft bottom. Muck at the bottom of the river will absorb the signal. Receiving the sub-bottom echoes helps locate shell beds and other areas where fish hang out.