Pinniped diving behaviour in geographic and oceanographic space

Abstract

Southern elephant seals (SES) are far-ranging top predators and sentinels of the Southern Ocean ecosystem status. Their well-being depends on the resources available in different oceanographic habitats. This study presents methods for long-term monitoring of SES movements and body condition within the Atlantic sector. Collecting information on SES, who spend months at sea, is often only possible using telemetry. I thus present a method designed for detecting buoyancy changes from low-resolution dive data. Detected buoyancy changes, which reflect body condition changes (i.e. lipid gain or loss), are examined in relation to when and where they geographically occur. For replenishing resources SES rely on macroscale latitudinal fronts, which provide favourable foraging conditions through aggregating prey and enhancing productivity. Generally, SES associate more frequently with higher latitude fronts/zones. Body condition improvements associated with a given frontal system vary strongly according to year, season and month. The variability in body condition improvements is higher in some frontal systems than in others, likely due to shifts in the Subantarctic and Polar Front. During a migration, some individuals stay within ≤3 frontal systems, whilst others change between several frontal systems and primarily improve their body condition in upper ocean waters. Body condition improvements, related to particular water masses, differ between the sexes and seasons. SES do not trace particular water masses across different frontal systems. Large inter-individual variability exists in how fast, when and where successful resource acquisition occurs. Conclusions drawn from track-based behavioural metrics regarding foraging activity are biased by the influence of currents on the horizontal movements of SES. The presented current-correction methods reveal that movements in geographic and hydrographic space differ. Currents primarily alter the horizontal travel direction, and SES show an ability to compensate for such deflections. However, even after current-correcting trajectories, track-based behavioural metrics fail to reflect body condition improvements, but rather display different foraging strategies. Individuals are flexible in using different foraging and movement strategies, depending on which oceanographic habitat they exploit. They exhibit large plasticity towards spatio-temporal variability. This could indicate resilience against environmental changes. However, long-term monitoring is necessary to fully capture the vulnerability of SES towards climate change.