1National Research Institute of Fisheries Science, Fisheries Research Agency, Yokohama, Kanagawa 236-8648, 2Fisheries Research Agency, Yokohama, Kanagawa 220-6115, Japan
After the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in March 2011, the National Research Institute of Fisheries Science (NRIFS) urgently measured the radiocesium concentration of 63 samples of small epipelagic fishes (such as sardine and Japanese anchovy) sampled by commercial fishery boats off the Kashima-Boso area between 24th March 2011 and 21st March 2013. Fluctuations of radiocesium concentration of fish muscle were synchronized with a delay from that of sea water near the fishing ground; radiocesium concentration in fish muscle reached a maximum of 31 Bq/kg-wet in July 2011, and then declined gradually. Radiocesium concentration in fish muscle subsequently indicated a low level in 2012 and 2013 (0.58-0.63 Bq/kg-wet). Compared to the 137Cs concentration before the FDNPP accident, 137Cs of fish muscle in 2013 was still approximately 10 times higher, whereas 137Cs of sea water near the fishing ground in 2012 was approximately 4.5 times higher.