Fishfinders mainly send and receive ultrasonic waves from the ship toward the seafloor. Scanning sonar, on the other hand, scans ultrasonic waves around the ship to detect fish schools far from the ship. Here are some echograms of both of them. In the movie below, the left side is the echogram of the fish finder and the right side is the echogram of the scanning sonar. In the echogram of the scanning sonar, the position of your ship is almost in the center of the screen. You can see a red mass moving from the top to the bottom. This red mass is a school of fish. Since the ship is currently moving, you can see that the ship is approaching the school of fish. When the ship passes almost directly over the school of fish, you can see the school of fish appear in the echogram of the fishfinder on the left side. This video shows that the scanning sonar has a very wide detection range.
