Otoliths of Atlantic bluefin tuna (Thunnus thynnus) collected from the Mediterranean Sea and North Atlantic Ocean were analyzed to evaluate changes in the seawater isotopic composition over time. We report an annual otolith delta C-13 record that documents the magnitude of the delta C-13 depletion in the Mediterranean Sea between 1989 and 2010. Atlantic bluefin tuna in our sample (n = 632) ranged from 1 to 22 years, and otolith material corresponding to the first year of life (back-calculated birth year) was used to reconstruct seawater isotopic composition. Otolith 8180 remained relatively stable between 1989 and 2010, whereas a statistically significant decrease in delta C-13 was detected across the time interval investigated, with a rate of decline of 0.05 parts per thousand yr(-1) (-0.94 parts per thousand depletion throughout the recorded period). The depletion in otolith delta C-13 over time was associated with the oceanic uptake of anthropogenically derived CO2. (C) 2016 Elsevier B.V. All rights reserved.