Southern Ocean sentinels: demographic insights into the declining population of Southern elephant seals at Macquarie Island

Some of the clearest expressions of climate change have been found in the Southern Ocean, one of the biggest marine ecosystems and a major component of the Earth's climate system. Top predators living in this ecosystem are considered to be one of the best indicators of these global changes as their population fluctuations reliably integrate and reflect prevailing environmental conditions. Southern elephant seals (Mirounga leonina) are pole-ward predators that feed throughout the Southern Ocean. In the past decades, their worldwide populations have been stable or increasing with the single exception of the Macquarie Island population that has been declining for unknown reasons over the past 50 years. This study, based on an 18-year capture-recapture dataset involving more than 6,000 individually marked females, aims to explore and understand the demographic mechanisms driving this decline and the potential impact of a changing environment on a key predator. Using models that deal with the issue of uncertainty in reproductive status, I showed that breeding is extremely costly in female elephant seals and results in a substantial decrease in survival of both first-time and experienced breeders. To offset this high reproductive cost to survival and maximize their reproductive success, female elephant seals tend to skip some reproductive events during their lifetime. Environmental conditions during the beginning of the post-moult foraging trip, which also corresponds to the beginning of females' pregnancy, play a critical role in this decision to skip reproduction or not. By incorporating this information into a matrix population model, I confirmed and reevaluated the decline of this population. Taken together, these results provide a detailed understanding of the southern elephant seals demography and give potential clues to the processes driving the decline of the Macquarie Island population. More broadly, my research contributes to a body of knowledge regarding the role of evolutionary and ecological processes in shaping life histories and population trajectories in long-lived species.