Elucidating the Cause of Changes in Radioactive Cesium Levels in Mountain Stream Fish
—The Study Revealed Link with Migration of Cesium in Forests—

The levels of radioactive cesium in natural freshwater fish species collected in Fukushima Prefecture have steadily decreased with time and the restrictions of their distribution are being gradually lifted. Regardless of that, the cause of changes in cesium levels still need clarifying, to obtain a solid future outlook.

This study focused on freshwater fish species living in mountain streams and analyzed the transfer paths of cesium from forests to those mountain stream fish species, by utilizing the environmental monitoring data accumulated so far. The results revealed that cesium taken in by mountain stream fish was supplied from three different paths.

The levels of cesium are following decreasing trends with time, which is considered to be highly dependent upon the contribution of organic soil layers. It is assumed that cesium present in organic soil layers will be gradually transferred along the depth direction and ultimately absorbed into inorganic soil layers. In order to steadily predict future levels of cesium in mountain stream fish species, we will take another step into elucidating such transfer of cesium from organic soil layers to inorganic soil layers, as well as understanding where and how cesium exists under the ground.