Spatial and seasonal variations of radiocesium concentrations in an algae-grazing annual fish, ayu Plecoglossus altivelis collected from Fukushima Prefecture in 2014

To elucidate spatial and seasonal variations of radiocesium (134Cs and 137Cs) concentrations in ayu Plecoglossus altivelis, amphidromous fish samples were collected both from shallow coastal waters in March 2014 and from seven rivers in Fukushima Prefecture during May–October 2014. Of the rivers, two were located within the designated evacuation zone. After standard length (SL) and body weight measurements, fish were dissected to two parts: body without head and internal organs, BD; head and internal organs, HI. Radiocesium concentrations in these parts were measured using germanium semiconductor detectors. Those in the whole body (WB) were reconstructed using their concentrations and weights. The 137Cs concentrations in BD in oceanic samples (4.3–6.7 cm SL) were below the detection limit, whereas those in riverine samples (6.4–18.8 cm SL) were 0.719–902 Bq kg−1-wet. Statistical analyses indicated a best-fit linear mixed-effects model including “SL” and “sampling site” as explanatory variables. Significant differences in 137Cs concentrations in BD among rivers were detected. Our results indicate that 137Cs concentrations in BD of riverine ayu, which showed lower but site-specifically constant values compared with those in HI or WB, are useful as a good indicator of radiocesium contamination of the respective rivers.