This study investigated the degree to which the behavior of a predator in the marine environment can be used to indicate the availability of prey. It examined this in Antarctic fur seals (Arctocephalus gazella) feeding on krill, with a view to understanding how behavioral choices could lead to a nonlinear functional response. The study showed how fur seals adjusted their time budgets to maximize the mean rate of energy intake in response to varying prey abundance. The behavioral indicator of prey abundance was the amount of time spent by fur seals at the bottom of dives after adjustment for dive depth. This indicator was well correlated with independent measures of prey abundance. The indicator was calibrated in terms of net rate of energy intake from information about energy expenditures in Antarctic fur seals and was used within the framework of optimal foraging theory to examine how female fur seals organized their foraging activity within and between bouts. Foraging in Antarctic fur seals conformed to many of the predictions from energy rate-maximizing behavior. For example, the apparent net rate of energy intake by fur seals declined with time spent in a patch, and the rate of energy intake on leaving a patch declined as apparent food availability in the environment declined. In addition, the time spent in patches increased as apparent food availability declined. Antarctic fur seals were able to adjust their behavior to track highly variable prey distributions and densities. The simple decision about when to leave a patch in relation to current environmental prey availability appears to be responsible for allowing fur seals to maintain saturation of the foraging rate as food density declines
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