Possibility of radionuclide originated from the Fukushima accident as oceanic tracer by fish muscle as bio-indicator

The Fukushima Dai-ichi Nuclear Power Station (F1NPS) accident released radionuclide was believed to circulate in the North Pacific Ocean and was suggested to arrive at East China Sea ECS (Aoyama et al., 2022). Temporal fraction of ¹³⁷Cs from F1NPS was estimated to be 0.5 mBq l^-1 at ECS in 2023 (Inomata et al., 2023). Appeared ¹³⁷Cs radioactivity 1.4 mBq l^-1 off Okinawa seawater in 2025 seems to suggest still having contribution of 0.6 mBq l^-1 as F1NPS originated fraction even after 14 years of the accident, compared to assumed global fallout originated level 0.8 mBql^-1 in ECS at 2025 (ENVRDB, 2025). This level was suggested to be caused by recirculation of ¹³⁷Cs within the north western Pacific waters by Subtropical Mode Water (STMW) and Central Mode Water (CMW)(Kumamoto et al., 2025). Similarly, F1NPS-¹³⁷Cs may be brought by North Equatorial Current (NEC) within 10-18 years (Chen et al., 2023). However, in contrast, there is other possibility as depuration delay of global fallout-¹³⁷Cs in surface water by depression of vertical mixing to deeper layer due to high surface water temperature after 2010 as observed global warming. Since F1NPS-orginated ¹³⁴Cs originated F1NPS almost decayed and being difficult to detect, it is still unknown the precise contribution rate of F1NPS-¹³⁷Cs compared to global fallout ¹³⁷Cs fractions. Possible method to derive F1NPS fraction may be using fish muscle which has approximately 50-100 times greater radioactivity in equivalent sample size. Successful detection of F1NPS-originated radio-caesium will is expected not to understand up-to date ocean circulation environment, but also to find the usefulness of bioindicator as oceanic tracer.