Bioenergetics of Pelagic Fish: Theoretical Change in Swimming Speed and Ration with Body Size

Abstract

It is shown from hydrodynamics theory and the size composition of particles in marine food chains that there are two unique swimming speeds of importance to pelagic fish: (1) the optimal cruising speed, which maximizes the distance traveled per unit energy expenditure and (2) the optimal foraging speed, which maximizes the rate of flow of surplus energy, or production in its broadest sense. With sockeye salmon (Oncorhynchus nerka) as an example, the optimal cruising and foraging speeds were found to be proportional to the body length raised to the 0.4 power. By analogy, if pelagic fish in general tend to move at either of these speeds, their ration and growth rates relative to the body weight should be proportional to a power that varies between 0.7 and 0.8. These predictions are consistent with field growth measurements for several pelagic species. Therefore, the necessary conditions for a theory of optimal foraging exist since (1) all adaptive swimming speeds are physiologically possible and (2) there is evidence that some juvenile fish feed by moving at the appropriate speed to maximize their production rate. Key words: bioenergetics, swimming speed, optimal foraging