Otolith δ18O of Pacific bluefin tuna Thunnus orientalis as an indicator of ambient water temperature

Abstract

To detect the relationship between ambient temperature and otolith stable oxygen isotope ( 18 O), Pacific bluefin tuna Thunnus orientalis larvae were reared at 6 different temperatures (between 23 and 28C at 35 psu; mean 18 O value, + 0.31 ) for 2 to 8 d after hatching. For the first time, otolith 18 O was measured using a continuous-flow isotope ratio mass spectrometry system. The linear relationship between otolith 18 O and temperature was determined as follows: otolith water = -2.70 (T C) + 5.193 (r 2 = 0.806, p < 0.01). The temperature-dependent fractionation of otolith 18 O was close to that reported for inorganic aragonite, indicating that the vital effects of isotopes are small. The otolith 18 O values of juveniles reared at a mean of 26.5C were not significantly different from those of larvae reared at 26C. With regard to the 2 pairs of otoliths within an individual fish, the differences in 18 O values between the left and right otoliths were subtle. The otolith 18 O values of larvae reared in 18 O-depleted seawater (24 to 26C at 32 psu; 18 O, -0.17 ) were lower, but were not significantly different from those of larvae reared in + 0.31 water, which is the 18 O value of the water in which the spawners were raised. These results suggest that the sample size for the 32 psu experiment was too small and that the rearing durations were too short to affect the otoliths completely. Our results demonstrate that otolith 18 O values of Pacific bluefin tuna larvae can provide precise and accurate estimates of their ambient temperature experience.