Abstract
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.
Highlights
Ocean warming has significant impacts on marine species and ecosystems, including high mortality, distribution shifts, and loss of spawning and nursery habitats (Perry et al 2005, Kimura et al 2010, Muhling et al 2011, 2015)
We developed a method to reconstruct ambient water temperatures experienced during the larval period of an individual fish using secondary ion mass spectrometry (SIMS) δ18Ootolith analysis. δ18Ootolith values of 5 adult Pacific bluefin tuna Thunnus orientalis (PBT) otolith samples were measured from the otolith core to edge by SIMS, and the measured SIMS δ18Ootolith values were compared to those measured by CFIRMS
22 paths were milled by a micromilling system and the δ18Ootolith of collected powders from each milled path was measured by CF-isotope ratio mass spectrometry (IRMS) (Table S2 in the Supplement)
Summary
Ocean warming has significant impacts on marine species and ecosystems, including high mortality, distribution shifts, and loss of spawning and nursery habitats (Perry et al 2005, Kimura et al 2010, Muhling et al 2011, 2015). Species that spawn seasonally in relatively limited areas are vulnerable to increasing water temperature, as their optimum range in spawning temperatures tends to be restricted. Pacific bluefin tuna Thunnus orientalis (PBT) is a highly migratory species that spawns in waters near the Nansei Islands in the western North Pacific from May to June and in the Sea of Japan from July to August (Yonemori 1989, Ohshimo et al 2017). Projected temperature in the current spawning sites is expected to increase by more than 3°C by 2100 under the most extreme Intergovernmental Panel on Climate Change (IPCC) climatewarming scenario (IPCC 2007) and become unsuitable for PBT to spawn (Kimura et al 2010). As PBT larvae are vulnerable to thermal stress, warming sea temperatures are likely to have significant impacts on their early growth and survival. The effects of ongoing climate change on the early life stages of PBT are poorly understood due to a lack of empirical evidence and methods to study such effects
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