What is a Fish Arch?


If you are new to reading a fish finder it might be a little difficult to interpret what you see on the screen and whether or not there are fish where your depth finder is looking. If the boat is moving, usually at a slow speed, the classic „Fish Arch“ will be on the screen. A Fish Arch looks just like an arch, hence its name. The next question is generally, „Why do fish look like an arch on the display?“ The reason for the shape is the interaction between the cone angle of the transducer and the fish while the boat moves over the fish.

When the fish enters the outer boundary of the cone, pixels on the fish finder’s display are activated. As the fish moves closer to the center line, the distance between the transducer and the fish decreases. The decreasing distance causes pixels representing shallower water to be activated thereby causing a slightly curved line. When the fish is directly on the center line of the cone, the first half of the fish arch is completed. As the fish moves away from the center line, the pixels at greater depths become activated and complete the second half of the arch.

Fish arches are caused by the difference between the fish’s location as the boat moves. If the boat is not moving, the fish will display as a horizontal line as they swim in and out of the cone.

If the fish does not swim through the center line of the cone, the arch will not be as defined. Since they are not in the cone as long, the signals are weaker and there are not as many signals to display. In shallow water, it is more difficult to show a well defined Fish Arch because the cone angle is often too narrow to allow for a full arch to display.