Marine predator foraging strategies in response to broad- and fine-scale resource variability

Foraging ecology is key to understanding the drivers of habitat selection, elucidating the abiotic and biotic factors associated with animal decision-making and the ecological context within which these choices occur. In particular, the resilience and response of upper-trophic-level populations to trans-seasonal resource variability is an ongoing question in marine ecology. New analytical tools are now opening new avenues for understanding such questions in the marine environment. However, while a considerable body of literature exists on the effects of in situ physical conditions on marine species; few studies have investigated the effects of in situ biological conditions. The overall aim of this thesis was to assess the feasibility of using light, collected by a marine predator, to estimate biological conditions in the water column, and demonstrate its application, in an ecological context. First, light level and depth data collected by elephant seals were used as a relative index to estimate plankton density in the water column, and subsequently compared with trans-seasonal chlorophyll-a patterns as revealed by satellite-derived ocean colour. Second, different aspects of broad-scale elephant seal foraging behaviour (search intensity, dive effort and mass gain rate) estimated from time-depth recorders was tested in response to the daily plankton density index. Seal foraging strategies were compared between summer and winter foraging trips. Finally, feeding activity of elephant seals (prey encounter events - PEE) estimated from high-resolution accelerometry was examined in relation to concurrent light data used to estimate both plankton density and possible bioluminescent prey encounter events (BiolumPEE), within the recurrent Kerguelen‚ÄövÑvp phytoplankton plume in summer, at both the meso- and dive-scale. Thesis results demonstrated that light collected by elephant seals, used as an index for plankton density in open waters of the Southern Ocean, correlated well with seasonal chlorophyll-a estimates derived from satellite ocean colour. Despite contrast phytoplankton production between summer (bloom period) and winter (oligotrophic waters), the foraging behaviour of elephant seals responded positively to plankton density at the broad-scale, regardless of time of year. This provided insight into how marine predators may utilise different foraging strategies to exploit prey resources throughout the year. High-resolution data collected within the Kerguelen plume alluded to the possible function of plankton in relation to seal foraging behaviour within the frontal zones of the Antarctic Circumpolar Current; plankton density is likely to facilitate seal vertical access to prey as the predator moves towards the Kerguelen plume 'front' where the system is less mature (i.e. less time has passed for energy to transfer from lower-trophic to higher-trophic species). Fine-scale data collected by bio-logging devices in remote polar regions can provide useful biological information that is concurrent with predator foraging behaviour at relevant scales. Differences in behavioural responses reflect resource variability during the period of prey acquisition at sea. My results suggest likely foraging strategies adopted by elephant seals (that targets meso-pelagic fish) in response to trans-seasonal resource variability, while scale-dependent analysis alludes to their foraging plasticity in a system largely structured by a phytoplankton plume.