Yields of fishes in marine are increased by locating large school of fish in the open sea by using satellites and echo-sounders.how can echo-sounders and satellites locate the fish in open sea?

Echo sounders and satellite imaging systems use similar techniques. they send a wave of sound inside the water which when strikes any objects they return back and are received by the same device. This information then received is used to give input to certain computer program which generate the a replicated image of the object. The calculations are made depending upon the speed of the sound in water or which ever medium the sound wave was projected into.