Temperature dependence of δ¹⁸O in otolith of juvenile Japanese sardine: Laboratory rearing experiment with micro-scale analysis

We evaluated the use of the stable oxygen isotope (δ¹⁸O) in the otolith as a proxy for the temperature history of Japanese sardine Sardinops melanostictus individuals. Japanese sardine juveniles were reared in three different water temperatures over the course of a month. Otolith δ¹⁸O (δ_otolith) was analyzed by extracting the portions formed during the rearing period using a micromill. δ¹⁸O of the rearing water (δ_water) was also analyzed. A linear relationship between otolith δ¹⁸O and ambient water temperature was identified as follows: δ_otolith − δ_water = −0.18 * T + 2.69 (r² = 0.91, p < 0.01). This equation is different from that proposed for inorganic aragonite or other Sardinops spp., with resulting application to wild Japanese sardine captured in the Pacific Ocean showing that it estimates a more realistic in situ temperature. Our findings suggest that the Japanese sardine-specific isotopic fractionation equation should be used when interpreting otolith δ¹⁸O of the Japanese sardine, and the methods presented here could also be useful to understand the temperature history of other fish species.