Remote monitoring of resource allocation to growth vs energy storage in space and time by sexually dimorphic elephant seals foraging at sea

Despite considerable recent progress in studies of movements and behaviour of large marine vertebrates at sea, continuous measures of foraging activity and associated changes in body condition such as total mass and fat content have remained elusive. Such information is essential for understanding the links between foraging ecology and population regulation of important marine foragers, for identifying areas of special management significance more accurately in space and time, and for identifying food chain effects of global climate change. Newly developed technologies have been used in archival data loggers to make high-resolution measurements of animal swimming behaviour, such as identifying when an animal is gliding. Animal morphology and body condition (density and total mass) determine the drag and buoyancy forces affecting animal movement during a glide. Therefore, by analysing closely how seals slow down during glides, we can estimate both animal density (i.e. body composition) and mass of the seal. We will deploy sensors on 5-6 year old male and female elephant seals to track changes in body composition and mass during the 8-month feeding migration they make in the Southern Ocean. We will carefully measure the mass and density of the seals when the tag is attached prior to the migration, and again when the tags are recovered when the seals return to the breeding grounds. Elephant seals of this age are expected to use different tissue allocation strategies depending on sex, with males benefiting most from lean-tissue growth for increased body size and females benefiting from increased energy stores for transfer to their offspring through lactation. However, during the final few months before, a pregnant female is under strong pressure to provide lean tissue to the rapidly growing foetus. With this new approach, we will be able to identify the timing when, and location in the Southern ocean where, seals make these critical contributions to different tissues within their body. As the health of such top predators depends on the food web, this technique will allow us to use them to study variations in the health of the ecosystem on which they depend. Once developed, the instrumentation and methodologies developed will be applicable to other seal and marine vertebrate species, across the world's oceans. To achieve this goal, we will build on existing technologies and skills of the researchers and collaborators to develop an instrument with dual functions: 1.) measuring and archiving large quantities of data for high-resolution analyses of changes in body composition and behavior throughout long at-sea migrations, and 2.) meaningfully compressing and processing data on-board the tag for relay via the ARGOS satellite system in near real-time. Apart from the specific scientific objectives of this proposal, the development of these instruments and analysis techniques has the potential to significantly add to our understanding of the foraging ecology and free-ranging energetics of many marine vertebrates, such as pinnipeds, turtles, cetaceans and sharks.