A sum greater than its parts: merging multi-predator tracking studies to increase ecological understanding

Understanding how animals find prey in heterogeneous environments is a central goal of ecology. Placing this process in an environmental context requires a lot of information regarding the characteristics of both the habitat selected by the animal and its surroundings. In high-latitude marine systems, information about subsurface habitats of marine predators is often very limited. Animal-borne oceanographic instruments have added a new modality to improve our understanding of marine predators and their habitats. While these instruments do not collect environmental information beyond that experienced by the animals carrying them, our study makes use of an oceanographic dataset collected by southern elephant seals (Mirounga leonina; N = 15), to provide environmental context for two sympatrically foraging penguin species in the waters close to the subantarctic island of Bouvetøya. The seals collected 154 CTD profiles during the study period, averaging 4.9 (±3.67) profiles per day, documenting the stratification of the upper water layer in terms of both seawater density and temperature. Using these data, we quantitatively describe the relationship between the diving behavior of the penguins (N = 3,745 dives) and the hydrographic properties of the three-dimensional area in which they were foraging. Both penguin species appeared to favor water characterized by a shallow mixed layer. The chinstrap penguins (Pygoscelis antarctica) dove within a shallow, unstable body of water close to the colony, whereas macaroni penguins (Eudyptes chrysolophus) exploited the bottom of the surface mixed layer further offshore. The hydrographic properties preferred by the penguins match closely those that describe the highest densities of their preferred prey, krill (Euphausia superba), identified during a temporally and spatially concurrent study. We demonstrate how merging multiple telemetric data streams from animals can shed new light on aspects of foraging behavior beyond simply relating movements to two-dimensional, remotely sensed measurements of the environment.