Trophic ecology and niche segregation among coexisting Macrourus species at South Georgia, Southern Ocean

Despite their importance in food webs and predominance among species bycaught in Southern Ocean toothfish fisheries, the trophic ecology of grenadiers of the genus Macrourus, remains poorly understood. Here, we provide the first comprehensive assessment of trophic position, niche segregation and dietary composition of four Macrourus species: M. caml, M. carinatus, M. holotrachys, and M. whitsoni. M. holotrachys had the highest trophic position (highest δ15N values) and the highest mean δ13C values, indicative of a benthic diet; M. caml and M. carinatus had intermediate and to some extent overlapping δ15N and δ13C values, and the widest isotopic niches, and; M. whitsoni, had the lowest δ15N and δ13C values, indicative of a lower trophic position and more pelagic diet. These differences in stable isotope values were reflected in their feeding strategies and diets; although all four species are generalist feeders, with crustaceans and fish as their main prey, M. holotrachys exhibited a more specialized benthic feeding strategy (hence a higher vulnerability to bycatch), whereas M. caml and M. carinatus had broader benthopelagic diets. The niche segregation is likely shaped by depth distribution and habitat preferences, which reduces competition for prey and enables their coexistence. This study is the most comprehensive assessment to date of the trophic ecology of Macrourus species in the Southern Ocean, increasing our understanding of their ecological role in the deep-sea food web and providing valuable insights for their management.