Mechanisms of radiocesium depuration in Sebastes cheni derived by simulation analysis of measured Cs concentrations off southern Fukushima 2014-2016.

The cesium depuration mechanisms were studied in Japanese rockfish Sebastes cheni off Fukushima, in which the radiocesium level remains higher than in other teleost. Samples were collected approximately 5 km south from the nuclear power plant during 2014-2016, and the Cs concentrations in fish, stomach content and prey species were measured. The stable cesium content in fish was also analyzed and compared with fish age which was determined by annual ring analysis in otoliths. The Cs concentrations in the dominant prey species, mysids and brown shrimp, were several Bq kg-w.w.; indicating that transfer via the food chain was substantial compared to that from seawater during the study period. The Cs concentrations in S. cheni decreased from 2014 to 2016 due to the metabolic excretion and the rate of decrease in its diet. Biokinetic model analyses confirmed the slower turnover of stable cesium in S. cheni, represented as a biological half-life (Tb) of 140-215 d, and was associated with stable Cs levels in food of 5-7 ng g-w.w.. The Cs levels in S. cheni were also simulated, which showed that the Cs depuration in fish exposed to the initial contaminated plume in 2011 resulted from slower metabolic excretion, while the Cs levels in fish born after 2012 could be regarded as equilibrated with the environmental levels of Cs. Furthermore, the simulation results suggest that Cs depuration in S. cheni population was also caused by the alternation of generation, which can be substantial by the addition of new year class population hatched after 2012 that were not contaminated by the initial contaminated plume from the 2011 accident.