Taxonomy, distribution and ecology of the four Macrourus species bycaught in the longline fishery at South Georgia (Subarea 48.3)

Macrourus species are the main bycatch across Southern Ocean longline fisheries, including around South Georgia, yet a persistent lack of species-specific data hinders effective management and ecological understanding. Aggregation of macrourids at genus level masks critical interspecific differences in life-history strategies and vulnerabilities to fishing. Here, we provide the first comprehensive biological assessment of the four macrourid species caught in South Georgia waters: M. caml, M. carinatus, M. holotrachys and M. whitsoni. Using fishery and observer data (2018–2022), we reveal pronounced female-biased sex ratios in three species (M. holotrachys, M. carinatus, M. caml), strong depth-based segregation by body length, distinct distributions and habitat preferences related to environmental features (bathymetric depth and slope, primary productivity, and temperature). Bycatch rates were highest in the southern region of South Georgia, and variable across species: M. holotrachys was the most frequently caught and had a wide range both spatially and bathymetrically (∼1000 - 1750 m); M. whitsoni was caught less frequently and in deeper water (> 1500 m) and was mostly caught in the northeast and east, and had the most restricted distribution; M. carinatus was mainly caught in the west, including Shag Rocks. These interspecific differences and fishery impacts underscore the need for species-level assessments of fisheries risk, and improved data collection to fill knowledge gaps. Overall, this study provides a critical foundation for understanding the ecology of Macrourus species in sub-Antarctic ecosystems and underpins additional requirements for data collection of bycaught species to aid species-level management in toothfish longline fisheries in the CCAMLR area.