| Summary: | Amblyraja hyperborea is the cartilaginous fish with the widest distribution across the Arctic seas and shelves and yet, large parts of its biology remain unknown. In a changing climate where the ice cover in the polar regions is retreating and fisheries are expanding northward, studying poorly known species is more important than ever. To provide a more exhaustive understanding of this species, horizontal and vertical distributions and temporal trends in the North Atlantic Ocean were evaluated, length at first maturity (L 50 ) was estimated and potential nursery grounds were searched for. Additionally, the most common patterns of ventral colouration were described, and the variation of the colouration coverage was investigated. A transboundary approach was applied with data provided by four countries. Amblyraja hyperborea was found in every area surveyed, though not being evenly distributed and clustering along the shelf breaks, and the abundance observations of was found to decrease below 65ºN. The vertical distribution did not depend on sex nor size, and the majority of the observations were made from 200 to 1000 m depth. No conclusive temporal trends could be defined. The estimated L50 for females was of 70.5 cm total length (TL) and for males of 66.8 cm TL, and some indices of potential nursery grounds were found in Iceland and the Barents Sea. Lastly, distribution was found to play a role in the ventral colouration coverage, with lighter individuals being dominant in the eastern side of the study area, and darker individuals being most present in the western side. Overall, the transboundary approach was successful in the in-depth study of A. hyperborea , even with the limitations of the data. This study can serve as a baseline for future studies regarding other poorly known transboundary species.
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