Relationship between ¹³⁷Cs concentration and body size in Japanese dace Tribolodon hakonensis in the rearing environment

Although seven years have passed since the Fukushima Dai-ichi Nuclear Power Plant accident, some wild fish species, including the Japanese dace Tribolodon hakonensis, inhabiting radioactively contaminated rivers and lakes in Fukushima Prefecture are still listed as restricted foodstuffs for shipment by the Japan Ministry of Health, Labour and Welfare. Japanese dace is widely distributed in Fukushima Prefecture. In this study, to provide a basic knowledge of the uptake and elimination of radiocaesium, rearing experiments using this species with two different body sizes (about 35 and 71 g weight) at the same temperature of 20℃ were conducted. All the fish was reared using radiocaesium-contaminated pellets for 0-41 days (¹³⁴Cs: 12.6 Bq/kg+¹³⁷Cs: 85. 7 Bq/kg) and non-contaminated pellets for 42-161 days, respectively. Daily feeding rate was set as 1 % body weight of them. Three samples were periodically taken from each size-group, and ¹³⁷Cs concentration in muscle tissues were measured. Physical decay constant of ¹³⁷Cs was set as 0. As a result, the growth rate of fish (expressed by body weight) was not significantly different between both groups. The rate of increase of ¹³⁷Cs concentration tended to be higher in smaller fish, while that of decrease had no significantly difference between size-groups. (author)