Radiological impact of the nuclear power plant accident on freshwater fish in Fukushima: An overview of monitoring results

Radionuclide (131I, 134Cs, and 137Cs) concentrations of monitored freshwater fish species collected from different habitats (rivers, lakes, and culture ponds) in Fukushima Prefecture during March 2011-December 2014 (total 16 species, n = 2692) were analyzed to present a detailed description of radionuclide contamination after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, and to elucidate species-specific spatiotemporal declining trends of 137Cs concentration for their respective habitats. Low concentrations of 131I (≤24 Bq kg−1-wet) were detected from only 11 samples collected during March-June 2011, demonstrating that 131I transferred to freshwater fish were not intense. In river and lake fishes, a more gradual decrease and higher radiocesium (134Cs, 137Cs) concentrations were observed than in culture pond fishes, which strongly implied that radiocesium in freshwater fish species was mainly bioaccumulated through the food web in the wild. During 2011-2014, percentages above the Japanese regulatory limit of 100 Bq kg−1-wet for radiocesium in river and lake fish (14.0% and 39.6%, respectively) were higher than in monitored marine fish (9.9%), indicating longer-term contamination of freshwater fish species, especially in lakes. Higher radiocesium concentrations (maximum 18.7 kBq kg−1-wet in Oncorhynchus masou) were found in the northwestern areas from the FDNPP with higher deposition. However, radiocesium contamination levels were regarded as 1-2 orders of magnitude less than those after the Chernobyl accident. Lagged increase of 137Cs concentration and longer ecological half-lives (Teco: 1.2-2.6 y in the central part of Fukushima Prefecture) were observed in carnivorous salmonids (O. masou, Salvelinus leucomaenis), whereas a rapid increase and decrease of 137Cs concentration and shorter Teco (0.99 and 0.69 y) were found in herbivorous and planktivorous osmerids (Plecoglossus altivelis, Hypomesus nipponensis) with younger age at maturity. Comparison of Teco among salmonids, osmerids, and cyprinids suggests that, in addition to the fish feeding habits and life-cycles, hydraulic conditions in rivers and lakes (e.g., turnover time), which are expected to affect radiocesium concentration in prey items, are an important factor affecting the 137Cs decreasing rate of freshwater fish.