[AOS09-P08] Detecting the difference of life history in a fish group by using micro-scale oxygen isotopic analysis of otoliths
Keywords:otolith, Stable oxygen isotope, Japanese sardine
Otolith is a hard tissue composed of calcium carbonate (CaCO3: aragonite), and once it crystallizes will not be resorbed. Therefore, the environmental history experienced by individuals is preserved in otolith as chemical composition of CaCO3, especially as stable isotopic compositions. Furthermore, in the juvenile period, concentric growth ring are formed daily from the center of otolith. On the other hand, stable oxygen isotope ratio (δ18O) of CaCO3 depend on δ18O of sea water and surrounding water temperature when inorganic aragonite is formed (Kim et al., 2007). Recent study have also reported that many species of fish show temperature dependency of otoliths δ18O almost same as inorganic aragonite. Accordingly, the history of ambient water temperature through the life history of individual fish can be estimated by δ18O record in otoliths.
In this study, we analyzed the otoliths of Japanese sardine caught in three area in the Sea of Japan, off Tottori (around Oki, 25/Feb./2015), off Nagasaki (17/Mar. /2015) and Toyama bay (23/Apr. /2015).
The otoliths were continuously milled along the growth ring by high-precision micro-milling system (GEOMILL326, Izumo-web Ltd., Japan). After the milling, the stable carbon and oxygen isotopic compositions of otoliths powder were determined by microvolume isotope ratio mass spectrometry system (MICAL3c with IsoPrime100), then the migration pathway of each individuals were estimated based on high-resolution δ18O analysis of otoliths. The purpose of analysis are (1) to clarify the difference of migration routes of sardine among three sampling area, and (2) to examine individual differences in δ18O history of sardine otoliths in the same areas.
As a result, δ18O of otoliths in three area indicate different migration history clearly. As for the sardine caught in off Tottori, comparing the δ18O histories of 6 individuals, we recognized that the those are clearly divided into two groups. One group showed 18O-depleted δ18O values (-0.49 ± 0.18 ‰) and the other group showed more 18O-enriched δ18O values (-0.05 ± 0.20 ‰) after about 600 μm from the center of otolith which is calculated to be about 80 days after birth (Ohshimo et al., 1997). We concluded that these two groups experienced totally different water temperature history after around 80 days.
In this study, we identified the difference of migratory route among each sampling area. Besides, we realized the extraction of ecological information by estimating the formation time of the sardine groups by using high resolution isotopic analysis of otolith.