Under-shelf ice foraging of Weddell seals

Horst.Bornemann [ at ] awi.de


The Drescher Inlet is a 25km long crack in the Riiser-Larsen Ice Shelf, eastern Weddell Sea, and is characterized by a stable fast ice layer. The fast ice represents an attractive habitat for Weddell seals, aggregating along the tidal cracks of the inlet in numbers of ca. 200 – 300 individuals. Multiple field campaigns aimed to investigate the seals’ diving behaviour and foraging ecology in relation to their environment. A recent joint seal and ROV project was focussed on the investigation of the cryo-benthic community underneath the floating ice shelf of the Drescher Inlet, which were previously detected by seal-borne cameras, and 3D-multi-channel dive loggers (Watanabe et al. 2006; Liebsch et al. 2007). Images show dense aggregations of isopods that likely represent an attractive food horizon, where seals could benefit from a local hotspot of high biological productivity. This context triggered a retrospective analysis of available high-resolution dive profile data to identify within-dive hunting phases and correlate those to the local physical environment. A total of 34 adult Weddell seals were instrumented with different types of dive loggers (time-depth recorders, digital still image loggers) in the course of six summer field campaigns between 1990 and 2016. Dive profiles were zero-offset corrected to reduce noise in the dataset and subsequently only dives deeper than 20m were regarded as ‘true’ dives, accounting for the thickness of the local fast ice and platelet ice layer. An automated broken stick algorithm (Heerah et al. 2014) was used to separate the dive into different segments. Segments with a high sinuosity were considered to indicate hunting. Segments characterized by a straight dive trajectories (low sinuosity), were assumed to be transit phases with no hunting behaviour. A tri-modal distribution of mean hunting depths suggests that Weddell seals concentrated their foraging activities in three depth strata. A peak in hunting depths below 370m corresponds to the sea floor of the Drescher Inlet, indicating demersal foraging. A second peak between 110 and 160m matches with the depth of the underside of the floating ice shelf, which suggests shelf ice associated foraging. The third peak probably represents hunting in the pelagic realm. Our investigation highlights the importance of the shelf ice underside as an attractive food horizon for Weddell seals suggesting a re-evaluation of trophic interactions and bentho-pelagic processes in the coastal Antarctic ecosystem.

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Conference (Poster)
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Event Details
XIIth Biology Symposium of the Scientific Committee of Antarctic Research (SCAR) "Scale Matters", 10 Jul 2017 - 14 Jul 2017, Leuven, Belgium.
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Nachtsheim, D. , Held, C. , Owsianowski, N. , Plötz, J. , Steinmetz, R. , Naito, Y. and Bornemann, H. (2017): Under-shelf ice foraging of Weddell seals , XIIth Biology Symposium of the Scientific Committee of Antarctic Research (SCAR) "Scale Matters", Leuven, Belgium, 10 July 2017 - 14 July 2017 .

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ANT > XXXI > 2

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