[AOS09-P09] Stable isotopic responses of otoliths of juvenile sardine Sardinops melanostictus experimentally cultured at different temperatures
Keywords:Otolith, Stable oxygen isotope, Stable carbon isotope, Sardine, Water temperature, Fish
29 specimens of S. melanostictus caught in Suzaki Bay, Kochi Prefecture were cultured for 60 days with a flow-through experimental tank system at the Hakatajima station (Ehime Prefecture) of the National Research Institute of Fisheries and Environment of Inland Sea, FRA, Japan. Before the temperature experiment, the experimental specimens had been treated in ALC to mark a distinct red ring in each otolith under UV light. During the experimental period, water temperature in each tank was recorded every 30 minutes by digital data loggers, and was 13.8 ± 0.4, 19.2 ± 0.6, 24.2 ±0.5°C (mean ± 1SD). The experimental fishes were fed to satiation daily with 2-6 % of their body weight (g) of commercial dry pellets in relation to temperature regimes. Carbonate powder was collected from newly-formed areas during the experiment by using a high-precision micromilling system (GEOMILL326, Izumo-web, Japan), and δ18O and δ13C of these samples were determined using a continuous-flow isotope ratio mass spectrometry system (MICAL3c with IsoPrime 100) at the National Institute of Technology, Ibaraki College.
The average growth increments of individual specimens for 60 days were 71 ± 16 μm, 168 ± 34 μm, and 229 ± 47 μm for treatments at temperatures of 14, 19, 24°C, respectively; thus the otolith growth rate tended to be higher at higher water temperature. From the measured δ18O values of otoliths and seawater, we obtained the following relationships between water temperature and aragonite–water fractionation in S. melanostictus:
δ18Ootolith – δ18Oseawater = –0.16 × T + 2.56 (R2 = 0.96, P < 0.01) (N = 17)
where T is water temperature. Our equation is close to the equation reported by Sakamoto et al. (2017); thus our study evaluated their reported equations by the rigorous experimental methods. Therefore, these equations can be a useful temperature proxy for understanding the migration history. The otolith δ13C values showed a significant negative correlation with water temperatures, and also had a significant negative correlation with otolith growth rates. Thus, the physiological variation affected by water temperature might be recorded in δ13C of S. melanostictus otoliths.