Chasser dans les profondeurs : une approche biologging pour percer les mystères des interactions prédateurs-proies dans l'océan Austral

Fine-scale interactions between predators and their prey, i.e. how predators find, select, and capture their prey, and alternatively how prey detect predators, and react to imminent predation, are key factors in determining both predators’ hunting efficiency and prey survival, and are therefore a key determinant in population dynamics. This project focuses on a guild of diving predators of the Southern Ocean, i.e. Southern elephant seals (Mirounga leonina), Antarctic fur seals (Arctocephalus gazella) and King penguins (Aptenodytes patagonicus), that all depend on similar prey resources: the mesopelagic fish, a key component of marine ecosystems. Understanding the interactions between mesopelagic fish and their predators is crucial to better understand the hunting tactics of these predators and better anticipate the effects of global changes on these ecosystems. However, these interactions have been poorly studied so far, as it is almost impossible to directly observe them in the deep ocean. Using an innovative biologging approach, which involves deploying recording devices on animals, this PhD project aims to describe predator-prey interactions in the Southern Ocean for the very first time. In particular, the aim is to describe the prey selection processes and hunting tactics of the top predator guild in the Southern Ocean, and more generally the relationships between oceanographic conditions, the distribution and ecology of prey, and the foraging behaviour of their predators. The results obtained in this thesis offer a significant advance in our understanding of the fine-scale interactions between diving predators and their mesopelagic prey. Firstly, this project significantly improved our knowledge on how diving predators efficiently locate and capture their small prey in a dark, open, three-dimensional environment, and how predators belonging to different taxa use distinct hunting tactics based on their diving, locomotor and sensory abilities to rely on similar prey resources for survival. This project highlighted the importance of predators' sensory and locomotor abilities during interactions with their prey. Secondly, the combined use of sonar and oceanographic tags revealed a reduction in prey accessibility and an increase in their reactivity in warmer waters, providing preliminary cues about the potential effects of climate change on the dynamics of these ecosystems. Overall, this PhD thesis provided original and major insights on the fine-scale predator-prey interactions as well as on how species might be impacted differently by environmental changes by taking into account their sensory abilities, rarely considered to date.