Beneath the Antarctic sea-ice: Fine-scale analysis of Weddell seal (Leptonychotes weddellii) behaviour and predator-prey interactions, using micro-sonar data in Terre Adélie

Description

Notes

Methods

Sonar tags were deployed for a week on three breeding female Weddell seals in Terre Adélie (East Antarctica) in November 2019, to study animals' movements and dives at high resolution (3D acceleration, magnetometry, time and depth and GPS location), as well as information on prey and predator-prey interactions using acoustic data (Goulet et al. 2019). The individuals are identified respectively as #314a, #314b and #315a. The sonars tags also included 3D acceleration and magnetometry sensors providing unprecedented data to document the approach and pursuit of prey by predators. Deployment started on November 11th (2 individuals) and November 12^th (1 individual) and ended on November 17^th and 18^th (7 days deployment) respectively. Females were temporarily captured with a canvas head-bag and anesthetized with a 1:1 combination of Tiletamine and Zolazepam (Zoletil), at a dosage of 0.5 mg.kg^-1 (Wheatley et al. 2006). Seals were monitored until fully recovered from anaesthesia and were allowed to go back to sea when no longer sedated. The tags were glued to the fur using quick-setting two-part epoxy (Araldite AW 2101 and Hardener HW 2951). Two types of equipment were deployed: i) an Argos-transmitting GPS tag was glued on the back of the animal, and recorded GPS position at the surface (Wildlife Computers Inc., Splash10-BF, 86 x 55 x 27 mm, 132 g in air); ii) a sonar tag designed by M. Johnson and P. Goulet, containing several types of sensors (SMRU tags, 85x44x32 mm, 120 g in air) was attached on the head of the animal. The sonar tag recorded pressure and magnetometry at 50Hz. It also recorded acceleration in the three axes at 250Hz (individuals #314a and #314b) or 200Hz (individual #315a). The tag was configured to emit 10µs pings at a rate of 25Hz (i.e., 25 sound signals per second), with a -3dB beamwidth of approximately 3.4 degrees and detection range of 7m. The tag was programmed to record continuously throughout the deployment. For one individual, #314a, the depth sensor stopped working properly after approximately 142.3 hours (the 370^th dive, at the end of the fifth day) until the end of the deployment. Data analysis was conducted in Matlab version R2021b (The MathWorks, Natick, MA, USA), using functions from www.animaltags.org, and in R Studio software, version 1.3.1093 (R Core Team, 2020). Movement data (depth, magnetometry, tri-axial acceleration) were calibrated using custom-made script and function do.cal (www.animaltags.org). Magnetometry was calibrated based on location of true magnetic south and date. Magnetometry and depth were downsampled to a sampling frequency of 5Hz for the rest of the analysis. Acceleration data were kept at their full bandwidth.

All experiments were conducted under the ethical regulation approval of the French sub-Antarctic and Antarctic territories (TAAF) ethic committee. All animals in this study were treated in accordance with the "Comité de l'Environnement Polaire" Ethic Committee guidelines and the work was conducted under permit #2019-107 of Terre Australes et Antarctiques Française. We warmly thank all the fieldworkers of the 69^th (2019) and 70^th (2020) expeditions to Dumont D'Urville, and the IPEV program 109 who assisted in the field.

Animal Tag Tools Wiki. 2017. URL https:// animaltags.Org/

Goulet P, Guinet C, Swift R, Madsen PT, Johnson M. 2019. A miniature biomimetic sonar and movement tag to study the biotic environment and predator-prey interactions in aquatic animals. Deep Sea Res Part Oceanogr Res Pap. 148:1–11. Doi:10.1016/j.Dsr.2019.04.007.

Wheatley KE, Bradshaw CJ, Harcourt RG, Davis LS, Hindell MA. 2006. Chemical immobilization of adult female Weddell seals with tiletamine and zolazepam: effects of age, condition and stage of lactation. BMC Vet Res. 2(1):8. Doi:10.1186/1746-6148-2-8.