Radiocesium biokinetics in olive flounder inhabiting the Fukushima accident-affected Pacific coastal waters of eastern Japan.

Radiocesium ((134)Cs and (137)Cs) originating from the Fukushima Dai-ichi Nuclear Power Plant (1FNPP) has contaminated coastal waters and been subsequently transferred to the marine biota along the Pacific coastal region of eastern Japan. To clarify the mechanism of radiocesium biokinetics in olive flounder, a commercially valuable and piscivorous predator, the biokinetics of (137)Cs was simulated using a dynamic biological compartment model and then validated with the measured concentrations in available monitoring data. The (137)Cs concentrations in seawater of the Pacific coastal sites of eastern Japan, from Kesen-numa (170 km north from the 1FNPP) to Choshi (190 km south from the 1FNPP), were reconstructed by fitting the simulated levels to the observed concentrations. Simulated values were verified by measured radiocesium levels in sedentary organism such as macro-algae and mussels inhabiting each study site which had accumulated radiocesium in their ambient environment from the beginning of the accident. Using reconstructed (137)Cs concentrations in seawater, the (137)Cs levels in olive flounder and its main planktivorous prey fish, e.g. anchovy, sand lance, whitebait, etc., were simulated and compared with observed concentrations to clarify the biokinetics of radiocesium in these organisms. This assessment showed that the determining factor for the maximum radiocesium concentrations in fish in the plankton food chain is likely to be the initial radiocesium concentration which they were exposed to during the contamination stage. Furthermore, the simulated (137)Cs concentrations in gut contents of olive flounder were verified by measured (137)Cs concentrations in the stomach contents of this fish collected within 30 km from the 1FNPP. These results indicated that the decrease of (137)Cs levels in their prey organisms was the primary determining factor of radiocesium depuration, and the resultant ecological half-lives were 140-160 d in the olive flounder, by the simulation.