Environmental and biological drivers of 137Cs accumulation in freshwater fish across forested and downstream sites in Fukushima

To elucidate ecological factors governing ¹³⁷Cs accumulation in freshwater fish, we examined 10 species collected from forested headwater streams and downstream reaches of rivers flowing through the Fukushima evacuation zone between 2020 and 2022. By integrating land cover data with isotopic and body size metrics, our study clarifies mechanisms shaping radiocesium distribution across fish communities in post-accident environments. Individuals exceeding the Japanese regulatory limit of 100 Bq/kg-wet were found at all sites, and fish/water concentration ratios of ¹³⁷Cs varied widely, from 9.7 $\times$ 10 L/kg-wet in a sea-run masu salmon to 1.2 $\times$ 10⁶ L/kg-wet in a Japanese dace. White-spotted charr and masu salmon, both typical stream-dwelling salmonids, exhibited significantly higher concentration ratios than the other species. Fish from areas with greater forest cover exhibited consistently elevated ¹³⁷Cs concentrations, indicating that forests serve as persistent sources of radiocesium to aquatic food webs more than a decade after the accident. Stable isotope analysis showed a significant positive association between terrestrial carbon contribution and ¹³⁷Cs concentration, demonstrating that individuals assimilating more terrestrial-derived resources tended to accumulate higher contamination. A positive relationship was also observed between relative body size and ¹³⁷Cs concentration, suggesting that growth-related traits influence accumulation. These results suggest that forest connectivity, dietary reliance on allochthonous resources, and individual growth characteristics collectively influence the accumulation of ¹³⁷Cs in freshwater fish. The approach and findings provide quantitative evidence for species- and site-specific processes underlying long-term radiocesium dynamics in forested river networks, informing risk assessment and fisheries management in radiologically impacted landscapes.