Interannual variability in fatty acids revealing autumn food availability for Antarctic krill (Euphausia superba) in Bransfield Strait

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 makes this area of 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 prewinter season is important for assessing how they respond to this period that is often characterized by short-lived phytoplankton blooms. For five 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 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. This study also indicates that the length-weight relationships and lipids of krill vary interannually, which should be considered cautiously while performing the stock assessment tasks involving in these parameters whatever the age-/length-based or acoustic stock assessments.