Pacific Bluefin Tuna maintain trophic stability despite shifting diet composition over 12 years in the California Current Ecosystem

Abstract Understanding predator responses to shifting prey availability and environmental variability is crucial for ecosystem monitoring, particularly for highly migratory species that link multiple ocean regions through their movements. We examined Pacific Bluefin Tuna (PBF; Thunnus orientalis ) trophic ecology in the California Current Ecosystem using stomach content analysis as well as bulk and amino acid compound-specific stable isotope analysis (CSIA-AA) over 12 years (2012–2023). Stomach contents identified taxa from 31 families representing diverse horizontal and vertical habitats. PBF diets exhibited pronounced temporal shifts: cephalopods and pelagic crustaceans dominated early years, while Northern Anchovy ( Engraulis mordax ) and mesopelagic fishes became prevalent later. Despite dietary variability, bulk δ 15 N values (maximum annual mean variation 0.8‰) and trophic position estimates (4.3–4.7) remained stable. In contrast, annual mean source and trophic amino acid δ 15 N values varied temporally but were highly correlated. Environmental predictors, particularly sea surface temperature anomalies, were associated with source and trophic amino acid δ 15 N variation, suggesting isotopic changes reflected shifting isotopic baselines rather than trophic changes. Our results are consistent with PBF maintaining a stable trophic position while switching among prey taxa with broadly similar functional traits—small-bodied, zooplanktivorous schooling species—despite year-to-year shifts in dominant taxa consumed. Simultaneously, our results suggest CSIA-AA signatures in PBF may offer a useful integrated record of ecosystem-level isotopic baseline variability in the California Current Ecosystem.