Autumn food availability in Bransfield Strait for Antarctic krill Euphausia superba and the relationship between body size and fatty acid content

Bransfield Strait is an important spawning, feeding and overwintering ground for Antarctic krill. Increases in both the biomass of krill and the ice-free extent of the strait make this area of great interest for krill fisheries. While krill are known to have access to food supplies beyond summer, lipid accumulation plays a pivotal role in their ability to survive the food-limited winter. Understanding the condition and dietary habits of krill during the pre-winter season is important for assessing how they respond to this period that is often characterized by short-lived phytoplankton blooms. For 5 consecutive autumns, from 2015 to 2019, krill obtained from the fishery in Bransfield Strait were used to assess their diet and to evaluate the roles of individual body condition and habitat features in shaping krill fatty acid profiles. Analysis of dietary fatty acids in krill indicated that they were generally in an active feeding condition in autumn, showing significant levels of essential fatty acids such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Fatty acid biomarkers displayed substantial inter-annual variation, with satellite data suggesting that sea surface temperature could be a potential factor contributing to this variation. Additionally, we observed strong correlations between krill body size and most of the fatty acid contents. There were positive correlations between size, EPA and DHA as well as negative correlations between size and a carnivorous diet. These findings suggest that krill exhibit differentiated feeding abilities and lipid retention based on their size.