Prey-field use by a Southern Ocean top predator: enhanced understanding using integrated datasets

An important challenge for understanding and managing marine ecosystems is determining the relationship between the distributions of prey species and the foraging of top predators. We examined the diet and foraging dynamics of breeding macaroni penguins Eudyptes chrysolophus from sub-Antarctic Heard Island and related these to prey distributions derived from active-acoustics and net-derived data in the foraging zone of the penguins. Consistent with previous findings, we found that penguin diets changed between the guard and crèche stages of the breeding cycle and that this change in diet corresponded with a switch from short foraging trips in the guard stage to significantly longer, offshore foraging trips in the crèche stage. We related these differences in diet and foraging to characteristics of the prey field—specifically, a relatively uniform distribution of krill over the shelf and in deeper, offshore waters, compared with an increasing abundance of fish further from shore. We developed a simple dynamic energy budget for macaroni penguins to explore whether targeting fish during the crèche stage was an energetically favourable strategy. Finally, we extrapolated our energy budget to estimate prey consumption at the colony scale as previous work has suggested that depletion near breeding colonies could contribute to prey switching. We found that prey switching during the crèche stage was energetically favourable and was most likely related to a reduction in foraging constraints, rather than prey depletion. This study shows the value of integrating data sets to address questions surrounding variation in diet and the use of alternative prey by marine predators.