They use a fishfinder. It's a device that sends out sound waves. The waves bounce off schools of fish and are reflected back to the boat. the reflections are like echoes, but they appear on the fishfinder's screen and show people where the fish are. A fishfinder operates in the same way as radar except that a radar set uses radio waves, while a fish finder uses ultrasonic waves. These waves emitted from a fishing boat are easily reflected when the an object under the sea because they have a short wavelength. Sound waves with a longer wavelength, such as those employed in radar detection, cannot be used because that type of sound wave is not easily reflected beneath the surface of the water.