Consideration on the Long Ecological Half-Life Component of (137)Cs in Demersal Fish Based on Field Observation Results Obtained after the Fukushima Accident.

Radiocesium concentrations in most marine fish collected off the coast of Fukushima and surrounding prefectures have decreased with time, and four years after the Fukushima Daiichi Nuclear Power Plant accident occurred, radiocesium concentrations have generally fallen below the detectable level (ca. < 10 Bq kg(-1)-raw). Only in some demersal fish species have detectable concentration levels still been found, and even these species have showed slow radiocesium decreases. The food web was considered as the major factor causing this phenomenon; however, slow elimination rates of radiocesium from these fish species also could be the cause. The latter effect was examined by considering that the (137)Cs concentration decreasing trend in fish could be fit with a set of three exponentially decreasing components; that is, having short, intermediate, and long biological half-lives. The long ecological half-life component was calculated using a 400-1500 d period of monitoring results for Japanese rockfish (Sebastes cheni) and compared with previous reported laboratory results for biological half-life. The obtained ecological half-lives ranged from 274-365 d, and these values agreed with the biological half-life of this fish species. This result implied that the long biological half-lives of radiocesium in some demersal fish species made their radiocesium contamination periods longer.