Use of at-sea distribution data to derive potential foraging ranges of macaroni penguins during the breeding season

Macaroni penguins are the main avian consumer of Antarctic krill in the vicinity of South Georgia. The largest breeding colonies occur on small offshore islands to the northwest of the main island. To delimit the foraging range of penguins from these colonies during the chick-rearing period, we used a series of ship-based survey transects to estimate their distribution and abundance. The survey transects were positioned so as to radiate from a point close to those islands with the largest colonies. Spatial and temporal variations within the data are examined. Penguins were mostly over the continental shelf in water less than 350 m depth and with more than 70% of birds at distances less than 40 km from the main breeding site. To model the observed distribution of penguins, a theoretical at-sea distribution is developed together with a 'potential foraging footprint'. The results are compared with those from previous models based solely on the use of foraging trip duration and at-sea activity budgets. The foraging 'footprint' is probably the best basis currently available for assessing the nature of interactions between macaroni penguins and the commercial fisheries for krill.