[A-OS12] Marine ecosystems and biogeochemical cycles: theory, observation and modeling

[AOS12-15] Marine fish migration history reconstructed from novel radiocarbon in otolith retrieved from walleye pollock around Hokkaido, Japan

*Kozue Ando¹, Yusuke Yokoyama¹, Yosuke Miyairi¹, Osamu Sakai², Tomonori Hamatsu², Ichiro Terashima³, Toshi Nagata¹ (1.Atmosphere and Ocean Research Institute, the University of Tokyo, 2.Fisheries Research Institute, Japan Fisheries Research and Education Agency, 3.Department of Biological Sciences, Graduate School of Science, The University of Tokyo)

Keywords:radiocarbon, fish migration, walleye pollock, otolith

Otoliths have been widely studied as a natural recorder of the entire life cycle of aquatic teleosts. Although numerous trace elements and stable isotope ratios have been studied as potential proxies, little success has been achieved with regards to the reconstruction of the migration history of oceanodromous teleosts due to the relatively uniform chemical property of seawater compared to that of freshwater. This study reports the use of ¹⁴C in otolith carbonate to reconstruct the migration history of three different subpopulations of walleye pollock Gadus chalcogrammus around Hokkaido, Japan.

Seawater radiocarbon concentrations around Japan of northward warm current (i.e. Kuroshio and Tsushima warm current) and southward cold subarctic current (e.g. Oyashio) differ considerably due to the global thermohaline circulation. Since Hokkaido is surrounded by waters that originate from these two water masses, ¹⁴C concentrations vary greatly among different coasts around Hokkaido. The physiological effect on elemental abundances in biological materials that have long been problematic in otolith microchemistry studies can be also canceled using ¹⁴C. In this study, we introduce novel stepwise dissolution of otoliths to obtain chronological data of otoliths that requires much less time and cost compared to the conventional micromilling technique, which allows for a greatly increased sample size. Three subpopulations of walleye pollock around Hokkaido (n=14) were analyzed. Distinct radiocarbon values were observed for three different groups; the Japan Sea subpopulation showed Δ¹⁴C values of Tsushima warm current (ca. 25‰), whereas the Pacific subpopulation matched that of Oyashio (ca. -45‰). The Okhotsk Sea subpopulation showed the intermediate value of the two water masses (ca. -25‰). Thus, migration histories of fish have been uniquely recorded as radiocarbon values. The results show that the pollocks of all three subpopulations generally inhabited the sampling area throughout their life cycle, though some pollocks of the Okhotsk and Japan Sea subpopulations respectively showed a possibility of periodic migration to other areas. This study confirmed a novel highly sensitive method using radiocarbon to reconstruct the migration history of marine animals that move between different water masses.