This study aimed to reconstruct temperatures experienced during the larval period by adult Pacific bluefin tuna Thunnus orientalis using high-resolution otolith stable oxygen isotope (δ18O) analysis. A novel otolith sample preparation protocol for secondary ion mass spectrometry (SIMS) analysis developed in this study reduced the background noise of SIMS measurements, enabling analyses of >10 times higher resolution around the otolith core compared to previous studies using conventional isotope ratio mass spectrometry (IRMS). The values obtained from SIMS were compared to those obtained by microvolume δ18Ootolith analysis using micromilling and conventional continuous-flow IRMS (CF-IRMS). There was a systematic offset (average 0.41‰ with SIMS resulting in lower values) most likely caused by matrix effects on SIMS δ18Ootolith values that can be calibrated using a strong linear relationship between SIMS and CF-IRMS measurements (r2 = 0.78, p < 0.001). The core-to-edge δ18Ootolith of 5 Pacific bluefin tuna revealed fine-scale seasonal variations in water temperature agreeing with known migration patterns. In addition, the ambient water temperature experienced during larval stages (about 10-20 d post hatch) estimated from otolith core δ18O ranged from 26.7 to 30.7°C, overlapping with temperatures associated with the occurrence of larval Pacific bluefin tuna. Combining SIMS and microvolume CF-IRMS δ18O otolith analyses offers a microscale examination of fish ecology that is not possible with conventional IRMS techniques. This novel method is particularly useful for understanding the early life history of fish that may be affected by climate change and reconstructing a well-resolved migration history for fish species that have small otoliths and/or narrow growth increments.
Otolith δ18O of Pacific bluefin tuna Thunnus orientalis as an indicator of ambient water temperature
