Otolith oxygen isotope profiles as potential indicators of large-scale migrations in Atlantic bluefin tuna (Thunnus...

Atlantic bluefin tuna (Thunnus thynnus) undertake extensive and dynamic oceanic migrations. This study investigated if secondary ion mass spectrometry (SIMS) measurements of oxygen stable isotopes (δ¹⁸O) in otoliths may be useful for inferring such movements. For methodological comparison, otoliths from the same fish were analysed using SIMS and isotope ratio mass spectrometry (IRMS). These measurements were correlated (r = 0.77, p = 1.3 x 10 ^-5) but SIMS values were lower (δ¹⁸O_IRMS = 0.37 + 0.42 δ¹⁸O_SIMS). Otoliths from adults held at a farm in the Mediterranean Sea for 32 months were analysed with SIMS. Fluctuations in δ¹⁸O values aligned with otolith annuli and reflected seasonal temperature change but absolute values and the magnitude of their fluctuations were lower than predicted by temperature dependent fractionation. Similarly, δ¹⁸O values at the edge of otoliths from young-of-the-year (25-34 cm fork length) were lower than predicted based on seawater temperature at the capture location. These discrepancies were largely resolved by using a regression-based estimate of cranial temperature as the predictor, indicating that δ¹⁸O fractionation in the otolith of Atlantic bluefin tuna is influenced by endothermy. This would likely impede using otolith δ¹⁸O values to accurately estimate experienced temperature. However, we showed using simulation, that when fractionation is dependent on cranial rather than seawater temperature, movements from the Mediterranean Sea to less saline areas of the western and northeastern Atlantic Ocean would produce detectable changes in otolith δ¹⁸O values. Therefore, SIMS analysis of otolith δ¹⁸O holds exciting promise for furthering understanding of Atlantic bluefin migratory behaviours and stock structure.