Seeing the light: depth and time restrictions in the foraging capacity of emperor penguins at Pointe Geologie, Antarctica

The foraging ability of visual hunters depends critically on light conditions. Emperor penguins Aptenodytes forsteri are reported to be visual hunters, but breed and forage at high latitudes and are thus exposed to extreme changes in light conditions in the course of the year. We examined how light influenced the foraging ability of breeding emperor penguins using loggers in winter (n = 5) and spring (n = 4) 2005 at Pointe Geologie (66 degrees 40' S, 140 degrees 01' E), Antarctica, as well as by using models of light availability over the year and as a function of depth. The model showed that daylight was continuous between days of the year 350 and 363 and that the sun did not rise between days of the year 166 and 180. Logger-measured light intensity decreased with water depth, and depth-specific values were higher at greater sun elevation angles. Depth-time use (given by the maximum depth reached multiplied by the number of hours at which this depth could be achieved over 24 h) of spring-foraging penguins was 8394.5 m x h, which was more than twice that of winter-foraging birds at 3845.4 m x h, showing the severe constraints imposed by winter conditions. Average catch-per-unit effort was related to mean maximum diving depth and was highest around mid-day in winter and highest during dawn and dusk in spring. This reflects availability of prey to foraging emperor penguins.