How to Read a Fish Finder: Sonar Basics for Anglers

A fish finder is one of the most valuable tools an angler can own. But when you first turn one on, the screen can look like a confusing mess of lines, blobs, and numbers. Here's how to read a fish finder and actually understand what you're seeing.

How Sonar Works

A fish finder sends sound waves (sonar) down into the water. These waves bounce off objects — fish, the bottom, structure — and return to the transducer. The device measures how long each echo takes to return and displays it on the screen.

The basic principle: the longer the echo takes, the deeper the object.

Understanding the Display

The Bottom

The bottom appears as a thick, continuous line across the bottom of the screen. The thickness of the line indicates the hardness of the bottom:

• Thick, bright bottom line — Hard bottom (rock, sand, gravel)
• Thin, faint bottom line — Soft bottom (mud, silt, clay)
• Fuzzy line above the bottom — Weeds or vegetation growing on the bottom

Fish Arches

Fish appear as arches on the screen. When a fish swims through the sonar cone, it appears first as a small mark that grows larger as the fish passes through the center of the cone, then shrinks again — creating an arch shape.

• Full, clear arches — Larger fish passing through the center of the cone
• Partial arches or straight lines — Smaller fish or fish at the edge of the cone
• Very large, thick arches — Trophy-sized fish
• Tight arches close to the bottom — Bottom-hugging fish like walleye
• Arches suspended in the water column — Open-water fish like salmon or lake trout

Structure

Trees, rocks, docks, and other structure appear as distinct shapes:

• Vertical lines rising from the bottom — Standing timber, stumps, or pilings
• Irregular bumps on the bottom — Rocks, boulders, or humps
• Cloudy patches near the bottom — Schools of baitfish
• Sharp drop-offs — A sudden deepening of the bottom line indicates a drop-off or breakline

Understanding Sonar Settings

Frequency

• High frequency (200 kHz) — Narrower cone, more detail, better for shallow water (< 60 ft)
• Low frequency (50 kHz / 83 kHz) — Wider cone, less detail, better for deep water (> 60 ft)
• CHIRP (Compressed High-Intensity Radiated Pulse) — Sends a range of frequencies for better target separation and clearer images

Sensitivity

Sensitivity controls how much detail is shown on the screen. Too low and you'll miss fish and structure. Too high and the screen will be cluttered with noise. Adjust until you can clearly see the bottom and fish arches without excessive noise.

Depth Range

Set the depth range to show the bottom at about 2/3 of the way down the screen. This gives you a good view of both the bottom and the water column above it.

Reading Advanced Displays

Down Imaging / Side Imaging

Down Imaging provides a photo-like image of what's directly under your boat. Side Imaging shows what's to either side. These technologies make it much easier to identify specific structure and fish.

Water Temperature

Most fish finders display surface water temperature. This is critical for understanding fish behavior — especially for species like walleye and trout that are sensitive to temperature.

GPS and Waypoints

Modern fish finders include GPS. Mark waypoints on productive structure, and you can return to the exact spot on future trips.

Tips for Using a Fish Finder

• Watch for baitfish — Where there are baitfish, there are game fish. Baitfish appear as clouds or patches near the bottom or in the water column.
• Look for edges — Weed lines, drop-offs, and temperature breaks are prime locations.
• Slow down — You miss a lot of detail at high speed. Idle speed gives you the best picture.
• Log your findings — The CatchTales app lets you log waypoints, depths, and structure you find on your fish finder for future reference.

Published July 2026 by the CatchTales team.