Tritium and 137Cs levels in marine fishes and in their host seawater around the Fukushima Daiichi Nuclear Power Plant from 2021 to 2024

We examined distributions of tritium and cesium-137 (¹³⁷Cs) activity in marine fishes and in simultaneously collected host seawater in 2021–2024 around the Fukushima Daiichi Nuclear Power Plant (FDNPP). From April 2021 to June 2023 (before water treated by a multi-nuclide removal facility was first released into the ocean, hereafter, “before releases”), the mean tritium activity in bottom layer, host water of fishes, collected offshore (>5 km from the FDNPP) was 0.085 Bq/L. In nearshore water (2 km south of the FDNPP), tritium activity varied with fluvial input in 2021 and 2022 and correlated with dissolved ¹³⁷Cs. After the start of treated-water releases in August 2023 (hereafter, “after releases”), at both offshore and nearshore sites, several samples deviated from the line of tritium vs dissolved ¹³⁷Cs before releases, reflecting the input of treated-water-derived tritium. Means of tissue-free-water tritium (TFWT) activities in rockfish and Japanese flounder at offshore site were 0.089 and 0.075 Bq/kg-fresh, respectively, before releases, and 0.081 and 0.13 Bq/kg-fresh, respectively, after. At nearshore site, Japanese flounder TFWT activities after releases were 0.37–0.75 Bq/kg-fresh. The elevated TFWT activities after releases could have been caused by elevated tritium activities in host seawater. There, however, was no difference in the concentration ratios (CRs) of TFWT to host seawater tritium between before releases and after releases, indicating that TFWT quickly responded to changes in host water tritium. Moreover, the CRs of TFWT + total organically bound tritium for rockfish and Japanese flounder offshore after releases were 1 or less, suggesting that tritium was not accumulated in marine fish.