ワカサギ中の放射性セシウム濃度の体重依存性と経年変化  [in Japanese] Body-size Effect and Dynamics of Radiocesium for Wakasagi <i>Hypomesus nipponensis</i>  [in Japanese]

The accident of the Fukushima Dai-ichi Nuclear Power Plant in March, 2011, released large amounts of radiocesium into the atmosphere, and contaminated the environment of Gunma Prefecture in eastern Japan. In particular, 640 Bq kg^–1-wet of radiocesium concentration was found in wakasagi (<i>Hypomesus nipponensis</i>) in Lake Onuma on Mt. Akagi in August, 2011. Thus, to elucidate the body-size effect in weight-dependent and dynamics of radiocesium in the ecosystem of Lake Onuma, we determined the age of wakasagi, the body-weight class of the radiocesium concentration in wakasagi and the effective ecological half-life (<i>T</i>_eff) of radiocesium in wakasagi 0+ from 2012 to 2016. The body-size effect was found for the ¹³⁷Cs concentration of wakasagi fished from 2012 to 2015, <i>i.e.</i>, the concentration of ¹³⁷Cs increased with the increase in its body weight. On the other hand, no body size effect was found in 2016. This result may be caused by the following two factors: the ¹³⁷Cs concentration of the lake water reached a steady state after May 2014; wakasagi is a small plankton-feeding fish, while it is known that larger piscivorous fishes show the strong body size effect. <i>T</i>_eff of the ¹³⁷Cs concentration in wakasagi 0+ consists of two components, fast- and slow-term ones, and the decay rate of the ¹³⁷Cs concentration in wakasagi 0+ was greatly reduced. As stated above, the radiocesium contamination in Lake Onuma has still been lasting; we are thus continuing our monitoring studies.