<P> Many fish swim through water by creating undulations with their bodies or oscillating their fins . The undulations create components of forward thrust complemented by a rearward force , side forces which are wasted portions of energy , and a normal force that is between the forward thrust and side force . Different fish swim by undulating different parts of their bodies . Eel - shaped fish undulate their entire body in rhythmic sequences . Streamlined fish , such as salmon , undulate the caudal portions of their bodies . Some fish , such as sharks , use stiff , strong fins to create dynamic lift and propel themselves . It is common for fish to use more than one form of propulsion , although they will display one dominant mode of swimming Gait changes have even been observed in juvenile reef fish of various sizes . Depending on their needs , fish can rapidly alternate between synchronized fin beats and alternating fin beats </P>
what part of the fish is used to propel the fish through the water
their fins their bodies