Under-shelf ice foraging of Weddell seals

• Main Authors: Nachtsheim, Dominik A., Held, Christoph, Owsianowski, Nils, Plötz, Joachim, Steinmetz, Richard, Naito, Yasuhiko, Bornemann, Horst
• Format: Conference Object
• Language: unknown
• Published: 2017
• Subjects: Antarc*; Antarctic; Ice Shelf; Weddell Sea; Weddell Seal; Weddell Seals
• Online Access: https://epic.awi.de/id/eprint/45560/; https://epic.awi.de/id/eprint/45560/1/Nachtsheim-etal_SCAR_Biology_Symposium_Poster_lowres.pdf; https://hdl.handle.net/10013/epic.51675; https://hdl.handle.net/10013/epic.51675.d001

The Weddell seal (Leptonychotes weddellii) inhabits the Antarctic coastal ecosystem and aggregates in areas characterized by a stable fast ice layer. Due to their extreme diving capabilities, they are able to exploit both pelagic and benthic prey resources. They mainly feed on fishes but occasionally also take cephalopods and crustaceans. Weddell seals instrumented with still-picture camera loggers in the Drescher Inlet, eastern Weddell Sea, detected an unknown cryo-benthic community underneath the floating ice shelf. Images show dense aggregations of invertebrates that likely represent an attractive food horizon for Weddell seals. In this context, we conducted a retrospective analysis of dive profiles collected in the Drescher Inlet to identify favoured hunting depths of Weddell seals and correlate those to the local physical and biological environment. A total of 34 adult Weddell seals were instrumented with dive loggers in the course of six summer field campaigns between 1990 and 2016. An automated broken stick algorithm was used to separate each dive profile 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 activities. 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 370 m corresponds to the sea floor of the Drescher Inlet, indicating demersal foraging. A second peak between 110 and 160 m 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.