Sonar Technology Fish Arch

A fish arch is the curved mark that appears on a traditional 2D sonar display when a fish passes through the sonar cone. The arch shape results from the changing distance between the transducer and the fish as it moves through the beam. When the fish first enters the edge of the cone, it is at maximum distance, creating the rising left side of the arch. At the center of the cone, the fish is closest, forming the peak. As the fish exits the far edge, the distance increases again, creating the descending right side.

A full arch only appears when a fish passes completely through the cone at a consistent depth. In practice, many fish returns appear as partial arches, half arches, or simple dots. A fish that only clips the edge of the cone shows as a short mark. Fish that are directly below the transducer at the center of the cone produce the tallest, most defined arches.

Arch size does not directly indicate fish size. Boat speed, cone angle, sensitivity settings, and the fish's depth all affect arch appearance. Larger fish do tend to produce thicker arches because they return a stronger signal, but a small fish at the center of the cone can create a bigger arch than a large fish at the edge. Signal strength, shown as color intensity on color displays, is a better indicator of target size than the arch dimensions.

Some fish finders offer a Fish ID mode that replaces arches with fish icons. While easier for beginners, experienced anglers generally prefer the raw arch view because it conveys more information about target size, depth, and position within the water column.