RESUMO
To elucidate 137Cs contamination levels and mechanisms of fish inhabiting river and pond environments near the Fukushima Dai-ichi Nuclear Power Plant, 137Cs activity concentrations in fish (15 species, n = 164) and water collected from Maeda River (3.3-8.9 km from the plant) and Shimofukazawa Pond (2.9 km) in 2017 were analyzed. Also, an 8-week rearing experiment using Japanese dace Pseudaspius hakonensis fed on non-contaminated pellets and the pond water (mean 137Cs concentration of 2.0 Bq/L) was conducted to evaluate 137Cs accumulation from water to fish. The 137Cs concentrations in Japanese dace, the only species collected throughout five sampling sites from estuarine to upstream areas in Maeda River, were found to be correlated with ambient air dose rates and fish size, exhibiting large variations (16.5-2.6×103 Bq/kg-wet). By contrast, dissolved 137Cs in river waters increased from the upper to lower course (0.025-0.28 Bq/L), which caused large variations of the water-to-body concentration ratio (CR) in Japanese dace (60.0-35700 L/kg-wet). These CRs (geometric mean of 3670 L/kg-wet) were much higher than the steady-state CR of reared fish (9.7 L/kg-wet), indicating that river fish uptake 137Cs mainly from prey items from aquatic and riparian zones, rather than from water. Statistically significant negative correlations between K+ concentrations in water and river fish CRs were detected, resulting in the decreasing trend of CRs from upstream to estuarine areas. These results suggest that the large heterogeneity of air dose rates, K+ concentration, and estuarine processes in brackish water habitats, in association with the feeding habit and size effect in fish, can engender wide variation of 137Cs concentrations and CRs of river fish along a river course. In contrast, 137Cs concentrations in pond fish (4.3-14.6 kBq/kg-wet) were higher than in river fish. The CRs of pond fish were constantly high but the range was smaller (1010-3440 L/kg-wet) with larger values in fish of higher trophic levels. These findings suggest that biomagnification within a pond was inferred as the main cause of 137Cs contamination of pond fish.