Foraging behaviour of Weddell seals (Leptonychotes weddellii) in connection to oceanographic conditions in the southern Weddell Sea

Dominik A Nachtsheim,Svenja Ryan,Michael Schröder,Laura Jensen,W Chris Oosthuizen,Marthán N Bester,Wilhelm Hagen,Horst Bornemann

doi:10.1016/j.pocean.2019.02.013

Abstract

The region of the Filchner Outflow System (FOS) in the southeastern Weddell Sea is characterized by intensive and complex interactions of different water masses. Dense Ice Shelf Water (ISW) emerging from beneath the ice shelf cavities on the continental shelf, meets Modified Warm Deep Water (MWDW) originating from the Antarctic Circumpolar Current at the sill of the Filchner Trough. These hydrographic features convert the FOS into an oceanographic key region, which may also show enhanced biological productivity and corresponding aggregations of marine top predators. In this context, six adult Weddell seals (Leptonychotes weddellii) were instrumented with CTD-combined satellite relay data loggers in austral summer 2014. By means of these long-term data loggers we aimed at investigating the influence of environmental conditions on the seals’ foraging behaviour throughout seasons, focussing on the local oceanographic features. Weddell seals performed pelagic and demersal dives, mainly on the continental shelf, where they presumably exploited the abundant bentho-pelagic fish fauna. Diurnal and seasonal variations in light availability affected foraging activities. MWDW was associated with increased foraging effort. However, we observed differences in movements and habitat use between two different groups of Weddell seals. Seals tagged in the pack ice of the FOS focussed their foraging activities to the western and, partly, eastern flank of the Filchner Trough, which coincides with inflow pathways of MWDW. In contrast, Weddell seals tagged on the coastal fast ice exhibited typical central-place foraging and utilized resources close to their colony. High foraging effort in MWDW and high utilization of areas associated with an inflow of MWDW raise questions on the underlying biological features. This emphasizes the importance of further interdisciplinary ecological investigations in the near future, as the FOS may soon be impacted by predicted climatic changes.

Highlights

The formation and export of cold, dense and oxygenated Antarctic Bottom Water (AABW) is one of the key processes that drives the global thermohaline circulation
The Filchner Outflow System (FOS) around the Filchner Trough in the southeastern Weddell Sea plays a substantial role in this context, as it is characterized by the constant outflow of Ice Shelf Water (ISW) formed below the Filchner Ronne Ice Shelf
While the two Weddell seals tagged on ice floes in the western part of the FOS travelled extensively through the pack ice, seals instrumented in a fast-ice covered inlet at the coast were more restricted to the coastline

Summary

Introduction

The formation and export of cold, dense and oxygenated Antarctic Bottom Water (AABW) is one of the key processes that drives the global thermohaline circulation. Modified Warm Deep Water (MWDW), a slightly cooler and fresher version of WDW formed via mixing with ambient water masses at the shelf break, seasonally enters the continental shelf east and west of the Filchner Trough and may even reach the Filchner Ronne Ice Shelf cavity (Nicholls et al 2008; Darelius et al 2016; Ryan et al 2017) This prominent oceanographic setting (see Fig. 1) due to intensive mixing of water masses led to the conception that the FOS could be considered a biological “hotspot” with enhanced productivity and potentially high abundances of marine top predators (Knust & Schröder 2014)

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