Arch

The arch shape comes from geometry, not from the fish's shape. As the boat moves, a fish enters the edge of the cone at longer range, passes through the centre at shortest range, and exits at longer range again. Plotted over time that traces an arch.

A full, thick arch usually means a solid fish that stayed in the cone. Half arches and straight lines mean the fish clipped the edge, or that the boat was sitting still, in which case a stationary fish draws a horizontal line rather than an arch.

Reading arches takes practice and the beginner mistake is assuming everything that is not an arch is not a fish. The other common error is trusting the fish-symbol setting, which replaces raw returns with cartoon icons and confidently labels weed and bubbles as fish.

Arch thickness relates to how long the fish was in the beam rather than to its size, so a slow-moving small fish can draw a bigger mark than a fast large one. Depth, cone width and boat speed all change what the same fish looks like.