Diet and foraging effort of Adélie penguins in relation to pack-ice conditions in the southern Ross Sea

Abstract

We investigated the diet and aspects of for- aging eAort among Adelie penguins (Pygoscelis adeliae) breeding at three colonies on Ross Island, in the southwestern Ross Sea - Capes Royds, Bird and Crozier - during the chick-provisioning period of three austral summers, 1994-1995, 1995-1996 and 1996-1997. During the study period, pack-ice cover diAered in waters oA- shore of these colonies, by colony, seasons and year. Diet diAered among colonies only slightly. The fish Pleuragramma antarcticum was the most important prey, especially during years or periods within years when little pack ice was present. With respect to krill, which composed the remainder of diet, juvenile Euphausia crystallorophias were consumed predominantly in a year of heavy pack-ice cover; more adult krill were consumed in 2 years when pack ice was sparse. Foraging trip du- ration diAered by colony, season and year and was related directly to distance from the colony to the nearest pack ice. The amount of food brought to chicks increased as trip duration increased, to a point (2 days), but then decreased as duration increased further (up to 4 days). On the basis of data on mass of parents and of meal sizes to chicks, it appeared that on the longest trips more of the food gathered by parents was used for self maintenance; on the longest trips, parents lost body mass. Successful foraging during chick rearing, the pe- riod when adult foraging is most intense, appears to depend on the proximity of pack ice to nesting colonies for this penguin species.