How to Read a Fish Finder | Sonar Screen Decoded

Reading a fish finder means interpreting sonar echoes where the right edge shows your current position and fish appear as signature arches.

A fish finder screen is a historical graph of sonar returns—not a live photograph. The rightmost edge shows what’s directly below your transducer now; everything to the left is what the boat has passed over. Depth runs vertically, time horizontally. Once you know each return type, you can identify fish, bottom type, and structure in seconds.

How a Fish Finder Screen Is Organized

The vertical axis measures depth (surface at top, bottom at the unit’s range setting). The horizontal axis tracks time and boat movement. The extreme right edge is the live feed; everything left is history. Moving faster creates steeper depth-change lines; idling produces shallower ones. Key data appears along the top or side: current depth, water temperature at the transducer, and boat speed over ground.

A common mistake is treating the screen like a side-view picture. It’s a scroll of sonar echoes over time. Faster movement covers more ground per screen width but loses detail; slowing down gives a tighter, more detailed view.

What Fish and Bottom Structure Look Like

Fish show as arches. A clean, well-defined arch means a single fish passed through the sonar cone. Clusters of small dots indicate baitfish schools. The arch’s vertical thickness—not its horizontal length—tells you the fish’s size: thick, bright arches mean dense-bodied fish; thin, faint arches mean smaller or less dense targets. Half-arches still count as fish.

Color density adds information. On standard 2D sonar, yellow and red mean strong returns (dense fish or hard structure); blue and green mean weaker returns (smaller fish, weeds, soft bottom). Large fish like striped bass appear red or orange; baitfish appear blue or green. Closer fish produce larger, stronger returns.

The bottom is the first solid horizontal line. A thick, solid line means hard bottom (rock or sand); a thin, faint line means soft bottom (mud or weed). A second faint return below the main bottom line is a secondary return—it usually confirms hard bottom, as the signal bounced off the bottom, reflected off the surface, and hit bottom again.

Sonar Settings That Make a Difference

Start with auto settings to calibrate, then fine-tune. Raising sensitivity helps see more fish, but too much creates clutter—a setting around 9 on many units works at cruising speed. Set depth range to match your water: a smaller range for shallow, larger for deep.

Enable the vertical flasher (A-Scope) if available—it shows real-time return thickness for judging target size more precisely. On the Fish Deeper app, find it under Settings > Sonar > Vertical Flasher. Some units overlay fish icons on identified arches—useful but not essential once you can read raw returns. If shopping for your first unit, our roundup of the best budget fish finders covers solid options. Ensure your transducer has a clear view with no air bubbles—bubbles scatter sonar and cause false readings. Wired2Fish’s detailed guide covers advanced scenarios like side imaging and forward-facing sonar.

FAQs

Why do fish show up as arches?

The sonar cone is conical. As a fish enters, distance to the transducer decreases, then increases as it leaves—creating the arch. A full arch means the fish passed through the cone’s center, where the signal is strongest.

Does a wider arch mean a bigger fish?

Yes, but vertical thickness matters, not horizontal length. Horizontal length reflects how long the fish stayed in the cone (varies with boat speed and position); vertical thickness correlates with the fish’s body depth and density.

What does the second bottom line mean?

A second faint return below the main line is a secondary return: the signal bounces off the bottom, reflects off the surface, and echoes off the bottom again. A strong secondary return usually confirms a hard bottom like rock or sand.