The determining factor of radiocesium contamination levels in smallmouth bass Micropterus dolomieu in Lake Hayama after the Fukushima fallout

We examined the radiocesium concentration, age, standard length, and stomach contents of smallmouth bass Micropterus dolomieu collected from Lake Hayama, Fukushima Prefecture, Japan, in 2014. Across three seasons sampled, fish and aquatic insects were common foods for M. dolomieu; the most frequently occurring forage items were Osteichthyes. The mean radiocesium concentration in M. dolomieu muscle was 582 Bq kg–1 wet weight, and the concentrations in different categories of stomach contents ranged from 297 to 811 Bq kg–1 wet weight. Considering the radiocesium concentrations in the lake water and in plankton (which included some particulate organic matter), it was possible that M. dolomieu radiocesium contamination is mainly transferred via the food chain. A positive correlation existed between M. dolomieu standard length and radiocesium concentration. However, in M. dolomieu of the same size range, the concentrations did not differ significantly between fish that existed at the time of the Fukushima Daiichi Nuclear Power Plant accident and those born afterward. Although some fish sampled in 2014 were highly contaminated, the initial Fukushima fallout does not necessarily explain the high radiocesium concentrations in some M. dolomieu in the lake several years after the accident. The contamination levels observed in M. dolomieu might result from radiocesium concentrations in their available food at any time, whether immediately or several years after the Fukushima accident.