Fish arches, and why they are arches

The sound does not go straight down in a thin line. It spreads out in a cone, wider the deeper it goes. As the boat moves, a fish first touches the edge of the cone, where it is a bit farther from the transducer. Then the boat comes directly over it, and the fish is as close as it will get. Then it slides out the far side again.

The unit draws that change in distance, so a fish passing through the cone makes an arch: it starts far, gets closer, then gets farther. If the fish is on the edge of the cone, only a short piece shows. If the boat passes beside it, you may see only a small half arch or a dot, or nothing at all.

Try it below. Drag the boat across a fish and watch the arch build on the screen. Then change the beam, the boat speed and how far off to the side the boat passes.

An interactive drawing goes here in the app.

On the water

• Look for arches in a row at one depth: that is usually fish, or something fish-sized holding at a depth. A single odd shape is harder to trust.
• Slow down. A slower boat gives the fish more time in the cone and draws a fuller, easier-to-read arch.
• A fish that is not an arch can still be a fish. Fish that are moving with the boat, sitting still in the cone, or very close to the bottom often draw dots, short dashes or smudges.

Easy mistakes

• Judging the size of the fish by the size of the arch. How wide the arch is depends on the cone, the speed and how close to the middle you passed, not how big the fish is.
• Counting only arches. Plenty of fish do not make one, and plenty of arches are not what you think.
• Expecting an arch on every fish when the boat is moving fast. At speed the arch gets squeezed into a short line.

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