Dive efficiency versus depth in foraging emperor penguins

Dive duration generally decreases with dive depth in air-breathing vertebrates. In most penguin species, this occurs due to increasing transit (descent and ascent) durations. However, in U-shaped dives, this is also because the duration of the bottom phase of the dive increases with increasing depth. We considered the time-based efficiency with which birds can use different diving depths by using depth recorders on 9 foraging emperor penguins Aptenodytes forsteri, studied during the early and late chick-rearing period in Adélie Land, Antarctica. Dive and post-dive duration positively correlated with diving depth, but particularly long recovery durations were apparent for dives longer than 456 s. This inflection point (i.e. 456 s) corresponded to a theorized maximum rate of gain of prey per dive cycle. By using the number of undulations in the bottom phase of the dive as a proxy for prey capture success, we conclude that the most lucrative dive depths for the birds studied were between 50 and 225 m. Since these depths were also those most often visited, we think that foraging emperor penguins focus on depths where profitability is highest.