Dr. Tsubokura's Radiation Lecture Vol.47
Author: Masaharu Tsubokura
Editors: Akihiko Ozaki M.D., Yuki Senoo
93. The level of the marine zone determines the level of radioactive contamination in fish.
Since the 2011 Fukushima Daiichi nuclear accident, regular measurements of radioactive contamination in sea products have been conducted once a week. Each different type of saltwater fish lives under a different layer of the ocean, with regard to the depth: the surface layer, the middle layer, or the bottom layer.
Immediately after the nuclear accident, radioactive substances leaked into the ocean and spread to broad areas, and, later, the substances sank to the ocean floor. For this reason, the degree of contamination among sea products is different between species. For saltwater species, the degree of radioactive contamination can be easily understood by dividing them into their habitats (surface layer, middle layer, and bottom layer) and types (migratory fish, coastal fish, and others [seaweeds and invertebrates]).
Indeed, the results of the routine inspections are published according to these categories as well. In the last article, we explained contamination of fish at the surface layer has currently disappeared. This is the same for migratory fish as well.
Species such as skipjack tuna, mackerel, and saury migrate to broad areas, including the offshore Fukushima Prefecture. They are monitored every week at many landing ports across the country, including those in Fukushima. Until today, the levels of radioactive contamination detected among these species have never exceeded the reference value.
94. The contamination of fish at the bottom is also decreasing.
Since the 2011 Fukushima Daiichi nuclear accident, regular measurements of radioactive contamination in sea products have been continuously conducted once a week. Each different type of saltwater fish is lives under a different marine zone: the surface layer, the middle layer, or the bottom layer. Immediately after the nuclear accident, radioactive substances leaked into the ocean and spread to broad areas, and, currently, they are on the ocean floor.
The extent of the radioactive substance contamination differs between three types. It has been revealed that radioactive contamination tends to accumulate more in fish in the bottom layer than other layers. However, the level of contamination in the bottom-layer fish has been also decreasing.
According to a report by the Fisheries Agency in Japan, various bottom-layer fish in the Fukushima Prefecture have been investigated more than 3,000 times from April 2016 to the end of September 2016. The results prove that the level of contamination exceeding the reference value of 100 Becquerel of food per kilogram was not detected, and more than 90% of them had contamination below the detection limit.
Even if you ingest 1 kilogram of seafood with a contamination level of 100 Becquerel/kilogram (reference value) every day, the total annual radiation dose will not exceed 1 millisievert. It takes time for the radioactive substance to disappear, but the contamination level of deep-layer fish is gradually decreasing.
The Japanese version of the manuscript was originally published in Fukushima Minyu, a local newspaper in Fukushima prefecture, Japan, on October 16th and 23th 2016 was reproduced for MRIC Global under the author's permission.