Do fish prey size affect the foraging patterns and breeding output of the Antarctic shag Phalacrocorax bransfieldensis?

Concurrent information on diet composition, foraging patterns and breeding output of the Antarctic Shag Phalacrocorax bransfieldensis was obtained at three colonies in the Danco Coast (Py Point, Midas Island and Primavera Island), Antarctic Peninsula, during the 1997/98 breeding season. Overall, demersal-benthic fish were the most frequent and important prey at all the colonies sampled, followed by octopods and gastropods. Between colonies there were marked differences in the size of the fish consumed, being the smaller specimens eaten by shags from Py Point. This was mainly influenced by the number of specimens of the smallest fish species, Harpagifer antarcticus, consumed at that colony. Differences in the composition of the diet might be related to different foraging areas used by the shags. Compared to Midas Island and Primavera Island, the shags from Py Point displayed longer foraging trips and spent significantly more time in foraging activities. Although at the beginning of the study the number of chicks per nest was similar in the three colonies, the breeding output at Py Point was markedly lower. The most likely explanation for the higher foraging effort and the lower breeding output of shags at Py Point might be the difference in fish prey consumption between these and shags from Midas Island and Primavera Island. Present results suggest that the decline in the inshore populations of Gobionotothen gibberifrons and Notothenia rossii observed in the last eighteen years around the South Shetland Islands, may be one of the reasons to explain the steady decrease in the number of breeding Antarctic shags observed in the last twelve years at colonies monitored in that archipelago.