How efficient is a fish?

Compared with other modes of locomotion, swimming is energetically inexpensive. The costs of moving a unit mass of swimming animal are lower than for animal flight or terrestrial locomotion, but higher than those of human-engineered vehicles (SchmidtNielsen, 1972). To understand why, the efficiencies of the underlying processes that convert metabolic energy into mechanical power need to be determined. In his 1971 study Paul Webb determined the efficiency of swimming trout by comparing metabolic rate data (the power input) with estimates of mechanical power output derived from added drag loads (Webb, 1971a) and mathematical modelling of momentum transfer to the wake based on tail kinematics (Lighthill, 1969; Lighthill, 1970). The paper isn’t a classic because it gives us all the answers concerning swimming efficiency; we’re still wrestling with the problems of quantifying fish thrust and the cost of its production (Schultz and Webb, 2002). For me its strength lies in the way it adopts a new approach to the problem: clarifying where the uncertainties lie; systematically addressing them; and combining distinct but complementary techniques to yield new information.