Otolith-temperature estimates in Atlantic bluefin tuna (Thunnus thynnus) from the Mediterranean Sea: Insights from clumped isotope measurements

Abstract

The subpopulation and/or contingent structure of Atlantic bluefin tuna (Thunnus thynnus) within the Mediterranean Sea is undefined, leading to uncertainty regarding the best strategy for an effective assessment and management of this highly exploited stock. This study aimed to reconstruct temperatures experienced by Atlantic bluefin tuna during the early life period (<3.5 months) using clumped isotope temperature proxy, an innovative geothermometer for carbonates, that does not require previous knowledge of other environmental parameters such as water oxygen composition. We examined otolith chemistry in fish captured from 3 different areas of the Mediterranean Sea and adjacent waters. We found that mean seasonal temperature estimates from clumped isotopes did not differ significantly from satellite derived and otolith oxygen stable isotopic ratios derived temperatures, except for the central Mediterranean Sea, were clumped isotopes derived temperatures were significantly higher than satellite derived temperatures. However, the sensitivity of the clumped isotope thermometer was found to be lower than that based on oxygen fractionation equation, with high variance observed in the clumped isotopes derived temperature estimates. We also observed that clumped isotope derived temperatures were undistinguishable among bluefin tuna captured in the Gibraltar Strait, the central, and eastern Mediterranean Sea. In this paper, we discuss the major sources of uncertainty in temperature reconstructions using bluefin tuna otoliths.