A test of hypotheses based on optimal foraging considerations for a diving mammal using a novel experimental approach

Abstract

The response of marine predators to changes in fine-scale prey distribution is poorly understood. Precipitous declines in marine apex predators necessitate a better understanding of the magnitude of fluctuations in prey availability that are within the compensatory behavioural plasticity of predators. We experimentally manipulated the fine-scale prey field for a marine carnivore in a controlled, captive setting and examined changes in behaviour and efficiency with changes in prey encounter rate. We hypothesized (i) a minimum prey encounter rate below which the cost of foraging always exceeds the benefit, (ii) foraging effort should increase with increasing prey encounter rates, and (iii) a maximum threshold prey encounter rate at which foraging efficiency is optimized. Dive duration, foraging time, and dive and foraging efficiency increased significantly with increasing prey encounter rate up to an asymptote of ~13 fish per dive cycle, supporting two of the three hypotheses. The results also support predicted responses to changing prey encounter rates derived from an optimal foraging model for diving animals and are the first experimental validation of optimal foraging model predictions in a marine mammal. We believe that these results provide new insight and suggest new experimental techniques for examining the foraging ecology of large marine predators.