Distribution and ecology of the four <scp> <i>Macrourus</i> </scp> species by‐caught in the longline fishery at South Georgia, Southern Ocean

Although Macrourus species are the main by-catch across Southern Ocean longline fisheries, including around South Georgia, the lack of species-specific data has hindered 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: Macrourus caml, Macrourus carinatus, Macrourus holotrachys and Macrourus whitsoni. Using fishery and observer data collected in 2018-2022, we reveal pronounced female-biased sex ratios in three species (M. holotrachys, M. carinatus and 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). By-catch rates were highest in the southern region of South Georgia and varied across species: M. holotrachys was the most frequently caught and had a wide spatial and bathymetric distribution (∼1000-1750 m); M. caml was the second most caught and appeared the most flexible in terms of habitat use; M. carinatus was mainly caught in the west, including at Shag Rocks; and M. whitsoni was caught less frequently and in deeper water (> 1500 m), mostly in the northeast and east, and had the most restricted distribution. These interspecific differences 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 highlights that additional data are required for by-caught species to aid species-level management in toothfish longline fisheries in the CCAMLR area.