Once you know what you’re looking at, a fish finder screen stops being noise and starts being useful. Here’s how to read the three things that matter most: arches, structure and bottom composition.
Fish arches: why fish don’t look like fish
A fish shows up as an arch, not a fish-shaped blob, because of how the sonar cone sweeps past it as your boat moves. As the cone’s edge reaches the fish the return signal is weak, strengthens as the fish passes directly under the transducer, then weakens again as it exits the cone — plotted over time, that traces an arch. A tight, complete arch usually means a single fish moving through the full width of the beam; a flat mark often means the fish stayed under you without crossing the whole cone.
Reading structure
Structure — weed beds, rocks, drop-offs, wrecks — shows as solid or textured returns rather than clean arches. Weed typically appears as a fuzzy, inconsistent band a little off the bottom; hard structure like rock or wreckage returns a strong, well-defined shape. Watching how fish marks cluster around structure, rather than scattering in open water, is often more useful than the fish marks alone.
Bottom composition
The thickness and colour intensity of the bottom line tells you about the sea or lake bed. A thin, sharp bottom line usually means soft bottom (mud or sand), which absorbs more of the sonar signal. A thick, strong return usually means hard bottom (rock or gravel), which reflects sound more efficiently. Many fish, particularly bottom feeders, relate closely to these transitions between hard and soft ground.
Practical tips for cleaner reads
- Slow down — arches form more clearly at lower boat speeds
- Adjust sensitivity so the bottom line stays crisp without excess clutter above it
- Use CHIRP sonar if you regularly fish mixed depths or want cleaner target separation
- Cross-reference fish marks with structure rather than chasing marks in open water alone
Still not sure what you’re seeing on screen? Send us a screenshot and we’ll help you work out what’s fish, what’s structure, and what’s just noise.