Factors affecting variations in 137Cs concentrations of masu salmon during summer–autumn in a forest river and connected dam reservoir near the Fukushima evacuation zone

This study aims to elucidate factors affecting variations in 137 Cs concentrations of masu salmon Oncorhynchus masou inhabiting a forest river and its connected dam reservoir (an artificial lake) in the upper reaches of the Ota River system during summer–autumn (August and October 2017). A clear size effect was observed in river-resident individuals, with significantly higher 137 Cs concentrations in autumn, particularly in 1-year-old (age-1) prespawning adults. River residents (mean 1990 Bq/kg) showed significantly higher 137 Cs concentrations than the larger lake migrants (mean 703 Bq/kg). The main prey items of river residents were terrestrial and aquatic insects, whereas lake migrants predominantly consumed small fish (pond smelt), as indicated by distinct δ 13 C and δ 15 N values. Larger age-1 river residents consumed a greater proportion of terrestrial prey than smaller age-0 individuals. A significant linear relationship between 137 Cs concentrations in diets and those in age-1 river residents was observed in autumn, when terrestrial insects had higher 137 Cs concentrations than aquatic insects. These results suggest that larger river residents efficiently fed on terrestrial insects with elevated 137 Cs concentrations, resulting in rapid 137 Cs accumulation during summer–autumn. By contrast, the lower variation in lake migrants likely reflects their consistent feeding on small fish with lower 137 Cs concentrations. Potential factors contributing to interspecific differences in 137 Cs concentrations and their variability among masu salmon and other river and lake fish are also discussed.