Antarctic fur seals as bioindicators of seasonal and ocean basin scale variation in the Southern Ocean food web

The current CCAMLR Ecosystem Monitoring Program (CEMP) review aims to identify environmental variables that can be used to assess the impact of climate changes on the Southern Ocean ecosystems. Using novel biomolecular analysis of blood from wide-ranging female Antarctic fur seals (Arctocephalus gazella) from three key breeding sites (Bird Island, Cape Shirreff and Marion Island), this paper provides the first cross-basin and seasonal comparison of food web baselines (i.e. primary producers), and trophic ecology for this species. Results show that Bird Island females feed at the lower trophic levels in summer, primarily krill, while Cape Shirreff individuals exhibit a more mixed diet including some higher trophic level preys, probably fish. In contrast, Marion Island seals constantly target higher trophic level preys year-round. During winter, Bird Island and Cape Shirreff females shift to higher-trophic preys (squid, myctophids), reducing krill consumption. Part of the observed increase in trophic position may also reflect seasonal shifts in the stable isotope values of krill themselves. By accounting for spatial and temporal variation in baseline δ¹⁵N values, this study provides more accurate trophic position estimates and highlights the importance of considering isotopic baseline dynamics when interpreting predator diet. This study presents support for integration of advanced biomarker techniques across a wider array of CEMP sites for detecting krill consumption by wide-ranging species such as the Antarctic fur seal across all seasons, and ongoing data on ecosystem shifts to inform adaptive management strategies.