How long should a dive last? A simple model of foraging decisions by breath-hold divers in a patchy environment

Although diving birds and mammals can withstand extended periods under water, field studies show that most perform mainly short, aerobic dives. Theoretical studies of diving have implicitly assumed that prey acquisition increases linearly with time spent searching and have examined strategies that maximize time spent foraging. We present a simple model of diving in seals, where dive durations are influenced by the seal's assessment of patch quality, but are ultimately constrained by oxygen balance. Prey encounters within a dive are assumed to be Poisson distributed and the scale of the patches is such that a predator will encounter a constant prey density during a dive. We investigated the effects of a simple giving-up rule, using recent prey encounter rate to assess patch quality. The model predicts that, for shallow dives, there should always be a net benefit from terminating dives early if no prey are encountered early in the dive. The magnitude of the benefit was highest at low patch densities. The relative gain depended on the magnitude of the travel time and the time taken to assess patch quality and the effect was reduced in deeper dives. As the time taken to decide decreased, the relative benefit increased, up to a maximum, but fell as decision time was reduced further. Frequency distributions of different aspects of dive durations from three deep-diving and two shallow-diving seal species are presented and compared with the model's predictions. (C) 2001 The Association for the Study of Animal Behaviour.