In fast swimming fishes, propulsion is due to caudal fin. Caudal fin has an important role in fish movement. As they pass backward, a progressively greater transverse amplitude of the waves propel them forward. One wave created due to caudal fin is followed by the another and so an. This transmits a backward momentum to push the water backwards. Thus the whole body operates as a single self propelling system, and the fish is thrust forward.