11:55 AM - 12:10 PM
[AOS03-11] Reproducing migration history of Japanese sardine using otolith δ18O and a data assmilation model
Keywords:sardine, otolith oxygen stable isotope, data assimilation model
Using the combination of otolith oxygen stable isotope ratio (δ18O) and data assimilation model, a new method to reproduce migration histories of Japanese sardine (Sardinops melanostictus) was developed. Firstly, dependence of otolith δ18O on temperature was examined for the first time for Japanese sardine. Juveniles were reared in three different water temperature (14.6, 18.7, 22.0 °C) for a month. Sagittal otoliths were collected and areas formed in latest 28 days were extracted by micromill for δ18O analysis. δ18O of rearing water was also measured and a linear relationship between otolith δ18O and temperature was determined as follows: δotolith = δwater – 0.186 (T) + 2.770, r2 = 0.91 (1).
Secondly, the distribution of seawater δ18O in the western North Pacific and relationship between salinity was investigated. During 2012-2015, surface water samples were taken from 90 different locations for δ18O analyses. Surface δ18O showed a clear poleward gradient and linear regression analysis revealed that δ18O and salinity were strongly correlated: δwater = 0.601(S) – 20.564, r2 = 0.93 (2), which enabled us to estimate seawater δ18O from salinity. These results were essential to convert the otolith δ18O profile into migration history. Micro-volume δ18O analysis and our original microsampling technique enabled us to extract otolith δ18O profile in a temporal resolution of 10-15days through whole life of juveniles approximately 200 days post hatch. For dates corresponding to each value of the profile, surface temperature and salinity in the range of 30-55 °N, 130-180 °E were extracted from FRA-ROMS, a data assimilation ocean model which reproduce ocean environment realistically. Temperature and salinity in each grid were converted into otolith δ18O value using Eq. (1) and (2). Grids in which the calculated otolith δ18O value was equivalent to actually analyzed value were considered to be the location of the individual on the date. Movements of the juveniles reproduced by this method clearly showed the northward migration from the Kuroshio-Oyashio transition zone to the Oyashio region and the estimated location on the date approached to the actual sampling point, which indicated the high accuracy of the method.
Secondly, the distribution of seawater δ18O in the western North Pacific and relationship between salinity was investigated. During 2012-2015, surface water samples were taken from 90 different locations for δ18O analyses. Surface δ18O showed a clear poleward gradient and linear regression analysis revealed that δ18O and salinity were strongly correlated: δwater = 0.601(S) – 20.564, r2 = 0.93 (2), which enabled us to estimate seawater δ18O from salinity. These results were essential to convert the otolith δ18O profile into migration history. Micro-volume δ18O analysis and our original microsampling technique enabled us to extract otolith δ18O profile in a temporal resolution of 10-15days through whole life of juveniles approximately 200 days post hatch. For dates corresponding to each value of the profile, surface temperature and salinity in the range of 30-55 °N, 130-180 °E were extracted from FRA-ROMS, a data assimilation ocean model which reproduce ocean environment realistically. Temperature and salinity in each grid were converted into otolith δ18O value using Eq. (1) and (2). Grids in which the calculated otolith δ18O value was equivalent to actually analyzed value were considered to be the location of the individual on the date. Movements of the juveniles reproduced by this method clearly showed the northward migration from the Kuroshio-Oyashio transition zone to the Oyashio region and the estimated location on the date approached to the actual sampling point, which indicated the high accuracy of the method.