Variation in the diet of the blue-eyed shag Phalacrocorax atriceps throughout the breeding season at Half-Moon Island, South Shetland Islands

One hundred and seventy three regurgitated casts of the blue-eyed shag or imperial cormorant Phalacrocorax atriceps were collected in a colony at Half-moon Island, South Shetland Islands, by weekly sampling throughout the breeding season from December 1993 to February 1994. Demersal-benthic fish were the main prey (91% in number) followed by mollusks and polychaetes. Six fish species were represented: Notothenia coriiceps, Nototheniops nudifrons, Trematomus newnesi, Harpagifer antarcticus, Gobionotothen gibberifrons and Pagothenia bernacchii. Notothenia coriiceps was the most frequent (F%=73) and important in mass (79%), followed by N. nudifrons and T. newnesi. The overall composition of the diet was qualitatively and quantitatively similar to that reported for the 1992/93 austral summer at the same colony. However, the pellet analysis throughout the 1993/94 breeding season showed that large N. coriiceps were the main prey during the chick rearing period, whereas when chicks were fledging/post- fledging the diet was more diverse and comprised chiefly prey of lower energetic content like other smaller fish species and non- fish organisms. This may reflect different degrees of energy demand by chicks at different stages of the season.