Radioecological behaviour of ¹³⁷Cs in rockfish of the southern coastal waters off Fukushima during 2017-2021

Following the accident at the Fukushima Dai-ichi Nuclear Power Station, radiocaesium concentrations were specifically elevated in rockfish species compared to other fish species. To clarify the likely reasons, a caesium metabolic rate in the Japanese rockfish Sebastes cheni was derived by an aquarium experiment of live fish collected from the area off Fukushima. Stable caesium and ¹³⁷Cs concentration in prey organisms, stomach contents and muscle of rockfish were measured and the bioavailable fraction in prey organisms was evaluated. Using derived transfer parameters, ¹³⁷Cs radioactivity levels in S. cheni and prey organisms were simulated by a model, and verified by the measured radioactivity concentrations of biota in coastal waters south of the Fukushima Dai-ichi Nuclear Power Station. As a result, slow caesium metabolism in S. cheni was confirmed with the biological half-life (Tb_1/2) of 190 d. The determining factor for the initial ¹³⁷Cs radioactivity levels in S. cheni, was the maximum radioactivity levels in surrounding seawater which was constrained by the sedentary nature of rockfish. Controlling factors of depuration rate of ¹³⁷Cs levels in S. cheni were slow caesium metabolism, enhanced ¹³⁷Cs radioactivity level of prey organisms, and survival of older contaminated individuals due to a long life-span. During the study period 2017-2021, ¹³⁷Cs radioactivity concentrations in seawater decreased close to the level measured before 2010, whereas those in prey organisms and rockfish in southern Fukushima waters were still above the levels that existed before 2010. An additional source for enhancing ¹³⁷Cs radioactivity in rockfish and biota of the food chain was indicated by the greater ¹³⁷Cs/¹³³Cs atom ratios in rockfish compared to those in the surrounding seawater, however it was considered to be radiologically insignificant in relation to seafood safety limits.