Southern bluefin tuna (Thunnus maccoyii; SBT) hold significant ecological, nutritional, and economic importance. Each year, SBT undertake seasonal migrations through the Indian Ocean to their feeding grounds in the Great Australian Bight, South Australia, where juveniles typically reside during the Austral summer. However, over the past decade, shifts in their distribution have been reported by the fishing community. Since 2012, SBT have not returned to their historic feeding grounds and are instead found further east, but with an inconsistent distribution likely driven by pulse environmental events in the Encounter Bay region (e.g., River Murray plume, upwelling, marine heatwaves). This study aimed to determine the changes in diet and nutritional value of wild SBT in Encounter Bay using biochemical tracers (i.e., stable isotopes and fatty acids). Results revealed considerable interannual variation in diet, with changes in nitrogen isotope values and fatty acid profiles observed between years. These patterns suggest shifts in trophic position and foraging habitats over time, with more recent years also showing higher contributions of polyunsaturated fatty acids, particularly EPA and DHA. Overall, the findings highlight the feeding plasticity of SBT and their capacity to adapt to environmental variability. However, these dietary shifts also appear to influence their nutritional quality, with potential implications not only for tuna health and condition, but also for human consumers who rely on SBT as a dietary source of essential fatty acids.