Assessment of the cesium-137 concentration in algae-grazing ayu Plecoglossus altivelis collected from Tomioka River in Fukushima, Japan.

Large amounts of the cesium-137 (Cs) released into the atmosphere during the Fukushima Dai-ichi Nuclear Power Plant accident in March 2011 had deposited on land, especially in Fukushima Prefecture. The long-term persistent Cs contamination of freshwater fish has resulted in fishing activities being restricted or suspended in some rivers and lakes, mainly in the central to eastern areas of Fukushima Prefecture. The ayu Plecoglossus altivelis, an important inland resource in Japan, is one of the species for which fishing activities are suspended. This fish feeds on periphytic algae attached to riverbed stones, ingesting fine sediments along with the algae. Although whole-body monitoring of ayu has been conducted, whether they contain the Cs concentration that allows the resumption of fishing activities is difficult to ascertain because the Cs in fine sediments affects the concentration in the whole fish body. Therefore, in this study, substances (refractory portion) affecting the whole-body Cs concentration were removed to gain an accurate assessment of the actual concentration. We measured the Cs concentrations in the muscle, head, and internal organs of ayu individuals collected from Tomioka River in Fukushima in May and October 2022 and reconstructed the whole-body Cs concentration from those measurements. Through HO-HNO digestion of the fish and microalgae, we also measured the Cs concentrations in the refractory portions that affect the whole-body measurement. The percentages of non-digested fish with whole-body Cs concentrations exceeding the Japanese regulatory limit were 18.0 % in May and 50.0 % in October. By contrast, when the refractory portions were removed, none of the specimens showed whole-body Cs concentrations that exceeded the limit. These findings suggest that the procedure for monitoring Cs in ayu should be reviewed.