The role of juvenile foraging ecology and growth in the evolution of life history strategies for southern elephant seals
thesisposted on 2023-05-26, 08:58 authored by Field, IC
In highly dynamic and unpredictable environments such as the Southern Ocean, species that have evolved behaviours that reduce the effects of intra-specific competition may have a selective advantage. This is particularly true when juveniles face disadvantages when foraging due to morphological or physiological limitation, such as in the case of many marine mammals. Southern elephant seals (Mirounga leonina) are a major consumer of biomass in the Southern Ocean with a global distribution. Recent modelling of the Macquarie Island population concluded that juvenile survival is a key parameter in influencing the rates of population change and as an important demographic component of the population. Resource limitation has been suggested as the primary reason for the change in numbers of these populations and this coupled with the importance of juvenile rates of survival influencing population change may provide some insight into explaining any reduction in juvenile survival. Until now, little has been known about these juveniles, ontogenetic and intra-specific differences in life history and foraging ecology have been suggested but not investigated. During this juvenile stage individuals undergo many morphometric and physiological changes as they develop toward maturity. Therefore, it would seem likely that studying the foraging ecology and growth and development patterns of this demographic group may show the proximate processes in affecting population dynamics. This study has followed juvenile seals as they grow and develop rapidly toward adulthood observing changes in foraging areas or strategies and associated changes in prey availability, differences in the seasonal availability of prey, changes in morphology and physiology for growth, maintenance or provisioning toward adulthood. In this thesis I present data for: 1) Anaesthesia for safe handling - I assessed the effects of variation in body condition and age at on the characteristics of anaesthesia, including induction time and dose-specific recovery rate which has increased the control over immobilisation level and duration, and reduces handling times for wild pinnipeds. 2) Foraging range and 3) Habitat use of the Southern Ocean -I tracked the at-sea movements of juvenile southern elephant seals using locations derived from recorded light levels. 4) Diet - I describe intra-specific dietary differences in prey composition and size. 5) Metabolic estimates and energy use and 6) Growth and body condition changes - I examined changes in mass and body composition of juvenile southern elephant seals during and between their annual moult and mid-year haul-outs. General discussion - These key ecological areas of an important predator has increased our understanding of the evolutionary and ecological interactions that influence the population dynamics of southern elephant seals at Macquarie Island and the structure of the Southern Ocean ecosystem.