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in commercial fishing, high-frequency sonar device for locating schools of fish. It transmits sound waves downward and receives echoes from the bottom of the sea, or from intervening schools of fish, also indicating distance from ship to fish. Two different types are used, one of which is a simple "echo sounder" that points directly downward from the ship and indicates the depth of the water as well as the presence of fish. More complex sonars or scanners send signals in all directions, covering a much wider area and providing more extensive information about the location of schools of fish