The Great East Japan Earthquake and consequent giant tsunami on 11 March 2011 resulted in serious damage to the Fukushima Dai-ichi Nuclear Power Plant (FNPP). Radioactive cesium ( 134 Cs and 137 Cs) was then released into the ocean through atmospheric deposition and as direct releases. As is a concern for the fisheries community, the contamination level of fisheries products is the most important issue. The monitoring of the concentration of radioactive materials (mostly radioactive cesium and

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... odine) in fisheries products started immediately after the accident. The standard limit, 100 Bq kg -1 for the sum of radioactive cesium ( 134 Cs + 137 Cs), was set as the safe limit for food (https://www.mhlw.go.jp/english/topics/2011eq/dl/new_standard.pdf). This standard limit ensures that the lifetime effect of radioactive materials in fish is sufficiently low as to be safe even when a person continues to eat the fish-one milli-sievert or less per year. The monitoring results of radioactive cesium for fisheries products are published on the Fisheries Agency Japan website (https://www.jfa.maff.go.jp/e/inspection/index.html). Soon after the accident, ~40% of tested samples exceeded the standard limit. After that, the concentrations of radioactive cesium in fisheries products decreased rapidly, and very limited samples (< 10%) have exceeded the standard limit since FY2012. More recently, only 0.01% of inspected samples recorded more than 100 Bq kg -1 during FY2019 and FY2020. Such a rapid decrease of radioactive cesium in marine organisms is reflected by the presence of radioactive cesium in the environment they inhabit, such as seawater and seabed sediment. Generally, cesium is a conservative element and mostly occurs in the dissolved phase in the marine environment. The distribution of radioactive cesium as a dissolved phase is fundamentally important to assess how the FNPP accident affects the oceanic environment.