Competition differentiates 137Cs concentrations in sympatric salmonids

Sympatric headwater salmonids compete over food items that are the primary ¹³⁷ Cs sources for fishes. This ecological process driven by competition is expected to be an important determinant of ¹³⁷ Cs concentrations in headwater salmonids, but little is known regarding the process. Here, we investigated how a stronger competitor salmon ( Oncorhynchus masou ) affects ¹³⁷ Cs concentrations in them and a sympatric weaker competitor charr ( Salvelinus leucomaenis ) in reaches of a highly contaminated headwater system in Fukushima, Japan. The results indicated that higher proportion of sympatric O. masou individuals within salmon communities lowers ¹³⁷ Cs concentrations in both species, and the effect is more pronounced for S. leucomaenis . Because stronger salmonid competitors exploit falling and drifting terrestrial prey, which is generally more contaminated than aquatic one, O. masou could monopolize terrestrial prey, thereby be more contaminated than did S. leucomaenis as a consequence of interspecific competition. Although previous studies have reported the effects of abiotic environmental factors such as water quality and ¹³⁷ Cs deposition on ¹³⁷ Cs activity concentrations in freshwater salmonids, the present study is the first to discover that competition for food resources is an important determinant of ¹³⁷ Cs concentrations in salmonids. The findings of this study open a new research area in which the ecological process of interspecific competition can affect radiocesium dynamics in headwater salmonids.