Spatiotemporal variability in Antarctic krill (Euphausia superba) around South Georgia derived from Antarctic fur seal (Arctocephalus gazella) diets

Despite its biological, biogeochemical, and economic importance, there is much about Antarctic krill (Euphausia superba) that remains poorly understood. This is especially true for its abundance, movement and retention in South Georgia waters, which is highly variable. Krill fluctuations exert bottom-up pressures on the entire ecosystem which are reflected in changes in breeding success and diet of dependent predators such as Antarctic fur seals (Arctocephalus gazella). This study investigates long-term datasets (2008–2021) documenting krill size distribution and abundance in Antarctic fur seal diet samples collected from two colonies that have geographically distinct and environmentally discrete foraging grounds. Krill length-frequency distributions generally followed a synchronous pattern across both sites with summer increases in length associated with growth, and winter decreases associated with senescence, mortality and possibly krill shrinkage, coupled with recruitment of smaller size classes. Bird Island seals, at the western end of the study region consistently took slightly larger krill (mean length 47.6 mm) than those at Maiviken (mean length 45.8 mm) in the east, possibly a result of continued krill growth during advection between the two areas, or differences in local oceanography. Krill cohorts visibly developed across years with large-scale recruitment of smaller krill occurring every 4–5 years. An anomalous period between 2017 and 2020 occurred when synchrony between the two sites broke down and later re-established, likely reflecting contrasting environmental conditions during this period. This study highlights the importance of having multiple spatially separated long-term monitoring sites, particularly in an ecological system recognised to be highly variable and experiencing significant changes.