Locomotion is energetically expensive. This may create selection pressures that favor economical locomotor strategies, such as adoption of low cost speeds and efficient propulsive movements. For swimming fish the energy expended to travel a unit distance, or cost of transport (COT), has a U-shaped relationship to speed. The relationship between propulsive kinematics and speed summarized by the Strouhal number (St = fA/U, where f is tail beat frequency, A is tail tip amplitude in m and U is swimming speed in m s^-1) allows for maximal propulsive efficiency where 0.2