Prey-field use by top predators of the Southern Ocean: understanding foraging dynamics of macaroni penguin (Eudyptes chrysolophus) near Heard Island

This study used data collected during the 2003/04 Heard Island Predator Prey Investigation and Ecosystem Study (HIPPIES project). It is the first study to directly compare diet, foraging, and prey-field dynamics with energetics of macaroni penguins (Eudyptes chrysolophus) around Heard Island.Diet and foraging dynamics were evaluated for the guard and crèche stages of breeding macaroni penguins from Heard Island. Diet changed significantly between the two stages (p < 0.005), from a diet dominated by krill (83%) in the guard stage, to a more variable diet in the crèche stage (43% krill, 33% fish, 23% amphipods). The observed change in diet corresponded with a change in foraging behaviour. During the guard stage foraging trips were short (mean = 109.1 km ± se 7.2 km), but became significantly (p < 0.005) longer (mean = 660 km ± 139.2 km) in the crèche stage.These changes lead to interesting questions regarding the relationship between diet, foraging and prey-field dynamics. The prey-field of macaroni penguins was sampled with nets and acoustic techniques, and showed that krill is available both close to the island and offshore. In contrast, fish are more abundant over deeper waters offshore. The observation that penguins choose to forage further offshore, where fish are more abundant (but the availability of krill is unchanged) suggests that macaroni penguins may prefer fish to krill.This theory was analysed by examining energy contents of prey species, and energetic requirements of breeding penguins. Energetically, fish found in the stomach samples of the macaroni penguins have a higher value than krill (8.42 kJ/g and 5.05 kJ/g respectively). This suggests that when macaroni penguins at Heard Island are limited in the distances they can forage in the guard stage, they settle for lower-energy krill as their main food source. However, in the crèche stage when they are able to forage further, they preferentially feed on food with higher energy content.By extrapolating from the energy and food requirement budgets for one breeding pair, the prey consumed for the entire study colony at Heard Island was estimated. This suggested another possibility for the observed changes between the two breeding stages; that of prey depletion. This appears to be a real possibility around the island, particularly when looking at the large number of land-based predator populations on Heard Island. Further analysis of predator diets, better estimates of available prey biomass, and species-specific energy budgets are required to confirm this suggestion.