Interannual variation in the diets of two albatross species breeding at South Georgia: implications for breeding performance
The diet and breeding performance of Grey-headed Albatrosses Thalassarche chrysostoma and Black-browed Albatrosses Diomedea melanophris breeding at Bird Island, South Georgia, were studied simultaneously during the chick-rearing period between 1996 and 2000. When samples for all years were combined, cephalopods and crustaceans were the main components in the diet of Grey-headed and Black-browed Albatrosses, respectively. However, their diets exhibited interannual variations. Cephalopods were the most important component in the diet of Grey-headed Albatrosses between 1996 and 1999 (60-75% by mass) but decreased significantly in 2000 (17%), when crustaceans dominated (61%). The Black-browed Albatross diet varied greatly, with cephalopods being the most important component in 1996 (49% by mass) and 1997 (48%), fish in 1998 (32%) and 1999 (40%), and crustaceans in 2000 (63%). In 1998 and 2000 there was a significant change in the cephalopod species present in the diet of both albatross species, when their breeding success was low. The consumption of the ommastrephid Martialia hyadesi was significantly and positively correlated with Grey-headed Albatross breeding success. For Black-browed Albatrosses significant correlations were found between its consumption of the Icefish Champsocephalus gunnari and breeding success, and between its consumption of M. hyadesi and M. hyadesi CPUE (Catch per Unit Effort). These findings suggest that Grey-headed Albatrosses are more reliant on Antarctic Polar Frontal Zone prey (M. hyadesi and Lamprey Geotria australis) whereas Black-browed Albatrosses are more dependent on Antarctic prey (Icefish and Antarctic Krill Euphausia superba). The differences between diets of Grey-headed and Black-browed Albatrosses breeding on different islands of the Southern Ocean showed that Grey-headed Albatrosses feed more on oceanic cephalopods (e.g. M. hyadesi) whereas Black-browed Albatrosses feed primarily on shelf fish (e.g. Blue Whiting Micromesistius australis), suggesting that albatross diets are likely to be influenced by the geographical position of those islands, albatross foraging preference and prey availability.