Fatty acid profiles of mackerel icefish Champsocephalus gunnari reflect diet variability of its main prey, Antarctic krill Euphausia superba

Understanding predator-prey interactions in Southern Ocean ecosystems is critical for assessing trophic dynamics. Antarctic krill Euphausia superba , a keystone species in the Southern Ocean ecosystem, supports numerous populations of higher trophic level predators. Despite a critical need to understand its feeding ecology, access limits direct studies, particularly during winter when extensive sea ice constrains sampling. This study therefore explores whether the fatty acids of mackerel icefish Champsocephalus gunnari , a key predator of krill, can provide insights into krill diet composition. By analyzing the fatty acids of C. gunnari and E. superba from South Georgia during winter and from the South Orkney Islands during the summer, we assessed differences in krill diets and the transfer of dietary signals from prey to predator. Stomach content analysis confirmed E. superba as the predominant prey of C. gunnari , whereas fatty acid analysis revealed significant differences between the South Georgia and South Orkney Islands samples. The fatty acid patterns of C. gunnari closely reflected those of E. superba , suggesting that prey diet composition drove predator fatty acid variations rather than dietary diversity of the predator. Calibration coefficients further highlighted consistent patterns of fatty acid transfer between regions. These findings demonstrate that C. gunnari can act as a natural sampler for E. superba , offering a novel approach to infer E. superba feeding ecology during periods of limited direct sampling. These insights enhance understanding of predator-prey interactions in the Southern Ocean ecosystem, where shifts in prey availability and composition could have cascading effects on higher trophic level predators.