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Between 3 February and 30 March 2021 the research icebreaker RV Polarstern conducted an extensive multidisciplinary research expedition across the southern Weddell Sea, Antarctica. During the Continental Shelf Multidisciplinary Flux Study (COSMUS) expedition (designated research expedition PS124), the Ocean Floor Observation and Bathymetry System (OFOBS) was deployed 20 times to collect high-resolution seafloor image and acoustic data with a 26-megapixel resolution camera, HD (high-definition) video camera, forward-facing acoustic camera and multibeam side-scan system from heights of between 1.5 and 4.5 m above the seafloor. To localise the collected data, the OFOBS was equipped with a Posidonia transponder for ultra-short baseline position triangulation, an inertial navigation system (INS) and a seafloor-tracking dynamic velocity logger (DVL). The 20 deployments were made across contrasting areas of the Weddell Sea, Filchner Trough, Filchner Sill and Filchner Shelf. Here, we present the full image dataset collected from across the various deployments as well as raw side-scan data and derived high-resolution bathymetric maps produced from these acoustic data after the cruise. All images are available from 10.1594/PANGAEA.936205 (Purser et al., 2021d) and 10.1594/PANGAEA.932827 (Purser et al., 2021a). Acoustic data for all deployments are also available from PANGAEA, with the raw side-scan data available at 10.1594/PANGAEA.939322 (Hehemann et al., 2021b) and forward-facing acoustic camera data at 10.1594/PANGAEA.939341 (Purser et al., 2021c). Seafloor topographic maps derived from the side-scan data are available at 10.1594/PANGAEA.939087 (Hehemann et al., 2021a). [ABSTRACT FROM AUTHOR]

A breeding colony of notothenioid icefish (Neopagetopsis ionah , Nybelin 1947) of globally unprecedented extent has been discovered in the southern Weddell Sea, Antarctica. The colony was estimated to cover at least ∼240 km2 of the eastern flank of the Filchner Trough, comprised of fish nests at a density of 0.26 nests per square meter, representing an estimated total of ∼60 million active nests and associated fish biomass of >60,000 tonnes. The majority of nests were each occupied by 1 adult fish guarding 1,735 eggs (±433 SD). Bottom water temperatures measured across the nesting colony were up to 2°C warmer than the surrounding bottom waters, indicating a spatial correlation between the modified Warm Deep Water (mWDW) upflow onto the Weddell Shelf and the active nesting area. Historical and concurrently collected seal movement data indicate that this concentrated fish biomass may be utilized by predators such as Weddell seals (Leptonychotes weddellii, Lesson 1826). Numerous degraded fish carcasses within and near the nesting colony suggest that, in death as well as life, these fish provide input for local food webs and influence local biogeochemical processing. To our knowledge, the area surveyed harbors the most spatially expansive continuous fish breeding colony discovered to date globally at any depth, as well as an exceptionally high Antarctic seafloor biomass. This discovery provides support for the establishment of a regional marine protected area in the Southern Ocean under the Convention on the Conservation of Antarctic Marine Living Resources (CCAMLR) umbrella. [Display omitted] [Display omitted] • A vast icefish breeding colony has been discovered in the southern Weddell Sea • Extremely high benthic biomass provides food for predators and scavengers • Oceanographic conditions influence brooding habitat suitability for icefish • Unique ecosystem supports establishment of a marine protected area (MPA) In this report, Purser et al. describe the largest yet discovered fish breeding colony ecosystem. Found in the Weddell Sea, Antarctica and covering at least 240 km2 of seafloor, Neopagetopsis ionah icefish brood eggs in ∼60-cm-diameter nests at a density of 0.26 nests m−2. [ABSTRACT FROM AUTHOR]