Factors affecting 137Cs radioactivity and water-to-body concentration ratios of fish in river and pond environments near the Fukushima Dai-ichi Nuclear Power Plant.

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.

Strong contrast of cesium radioactivity between marine and freshwater fish in Fukushima.

A proper understanding of radioactive contamination levels of food resources near the Fukushima Dai-ichi Nuclear Power Plant is necessary to estimate the potential effects of radionuclide contamination on human health. This study was conducted to present a direct comparison of radiocesium (134Cs and 137Cs) concentrations in marine and freshwater fish inhabiting different water bodies in Fukushima Prefecture (coastal waters, 6.3-54.5 km from the plant; forest rivers and irrigation ponds, 1.4-71.6 km), and to reveal plausible contamination mechanisms for each habitat. In contrast to marine demersal fish (7 species, n = 50), which showed lower and less variable radiocesium concentrations (0.234-3.41 Bq kg-1-wet), freshwater fish (6 species, n = 463) showed higher and more site-specific variations for each species and habitat (4.09 Bq kg-1-wet - 25.6 kBq kg-1-wet) in 2015-2016. The apparent concentration ratio (aCR, L/kg) of 137Cs in fish to water is higher for fish of freshwater habitats (mean 1240-12900 for each site) than in those of coastal waters (mean 200). Radiocesium contamination is more severe and persistent in freshwater fish, especially those distributed within the designated evacuation zone (salmon in rivers and bass in ponds). Continuous radiocesium uptake through the food web in relation to fish feeding habits and size (size effect), and biotic/abiotic characteristics in water and surrounding environments are main factors affecting site/habitat-specific bioaccumulation of radiocesium in freshwater fish. By contrast, uniformly lower radiocesium concentrations in marine demersal fish are mainly attributable to decreased radiocesium transfer intensity from the benthic food web because of lowered radiocesium contamination in sediments, and low physiological ability to retain radiocesium. Our results revealed a strong contrast of radiocesium contamination levels and mechanisms between marine and freshwater fish in natural habitats. Particularly, a close relation between 137Cs accumulation in river salmon and contamination of prey items in forest ecosystems (mainly terrestrial and aquatic insects) is peculiar to the upstream areas affected by the Fukushima accident.