Re-defining Trophic Dynamics of Antarctic toothfish in Subarea 88.3 by Compound-specific Stable Isotope Analyses: Individual Size and Spatial Variability

The Antarctic toothfish (Dissostichus mawsoni) is a long-lived apex predator that functions as an important mediator of energy flow between pelagic and benthic ecosystems of the Southern Ocean. Yet, its trophic ecology is not well characterized in the Bellingshausen Sea (Subarea 88.3), due to a region of high environmental variability but limited ecological sampling. Here, we applied compound-specific stable isotope analysis of amino acids (CSIA-AA) to quantify trophic position and basal resource contributions of Antarctic toothfish and their prey. Bulk isotope measurements (δ13Cbulk, δ15Nbulk) show a significant positive correlation between body size and δ15N (R = 0.76, p < 0.01), confirming ontogenetic enrichment. However, bulk isotopes did not resolve prey taxa (PERMANOVA, p > 0.05). In contrast, CSIA-AA resolved trophic structure more clearly (PERMANOVA, p < 0.05). According to contribution of direct prey using a Bayesian mixing model, Channichthyidae emerged as the most readily available prey for Antarctic toothfish. Furthermore, the contribution of cephalopods (Psychroteuthis glacialis) was also significant for some Antarctic toothfish individuals, demonstrating that stable isotope analysis overcomes the limitations of under- or overestimation due to digestive efficiency. δ13CEAAs fingerprints indicated contrasting basal resource use, with haptophytes. However, depending on the Research Block (RB), some prey taxa with higher carbon contribution from diatoms existed. The results of this study suggest the following. First, juvenile toothfish share trophic position overlapping with prey taxa, suggesting potential competition before shifting to higher predator role with growth. Second, Antarctic toothfish appear to integrate the pelagic and benthic ecosystems of the Bellingshausen Sea by feeding on both pelagic prey (e.g., Channichthyidae) and benthic prey (e.g., Macrouridae). Third, prey taxa showed spatial variability in nitrogen baselines and basal sources among Research Blocks. This variability is likely related to the Antarctic Slope Current (ASC), which flows along the complex seafloor topography of the Bellingshausen Sea, and to sea-ice dynamics. Therefore, this study is the first report of CSIA-based quantitative assessments of Antarctic toothfish in the Bellingshausen Sea and shows that isotopic approaches can resolve both size-related trophic shifts and environmentally driven variability in basal resources. We suggest that effective CCAMLR management should account not only for stock size but also for prey availability, baseline heterogeneity, and climate-driven changes in primary production that may influence the resilience of this predator.