Fishing for drifts : detecting buoyancy changes of a top marine predator using a step-wise filtering method

This research was partly funded by a Natural Environment Research Council grant [NE/E018289/1]. Further, a PhD studentship in Marine Biology partially funded by the Natural Environment Research Council [NE/L501852/1] and the University of St Andrews 600th Scholarship supported this work.In southern elephant seals (Mirounga leonina), fasting and foraging related fluctuations in body composition are reflected by buoyancy changes which can be monitored by changes in drift rate. Here, we present an improved knowledge-based method for detecting buoyancy changes from compressed and abstracted dive profiles received through telemetry. We applied this step-wise filtering method to the dive records of 11 southern elephant seals, which identified 0.8% to 2.2% of all dives as drift dives. At the beginning of the migration, all individuals were strongly negatively buoyant. Over the following 75 to 150 days, the buoyancy reached a peak close to or at neutral buoyancy, indicative of a seal’s foraging success. Ground-truthing confirmed that this new knowledge-based method is capable to reliably detect buoyancy changes in the dive records of drift diving species using abstracted dive profiles. This affirms that the abstraction algorithm conveys sufficient detail of the geometric shape of drift dives for them to be identified. It also suggest that using this step-wise filtering method, buoyancy changes could be detected even in old datasets with compressed dive information, for which conventional drift dive classification previously failed.Publisher PDFPeer reviewe

Pinniped diving behaviour in geographic and oceanographic space

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.Natural Environment Research Council (NERC) grant number: [NE/L501852/1]Natural Environment Research Council (NERC) grant number: [NE/L501852/1