Photo/Illutration The Otagawa river, which runs through Minami-Soma, Fukushima Prefecture, and elsewhere, was surveyed for cesium levels in fish following the accident at the Fukushima No. 1 nuclear power plant. (Provided by the National Institute for Environmental Studies)

A team of scientists discovered that radioactive cesium released from the crippled Fukushima No. 1 nuclear power plant accumulates in freshwater fish differently in lakes and rivers.

How easily cesium is taken into bodies is determined by what the fish consume in lakes, although factors associated with water quality--such as the ratio of mud particles--are more important for those inhabiting rivers, according to the researchers.

The findings are expected to help predict the cesium concentration in aquatic creatures more accurately even when all of them are not examined individually, the researchers from the National Institute for Environmental Studies said.

“The discovery could be used for estimating how the cesium level has lessened in each fish species,” said Yumiko Ishii, a senior researcher at the institution’s Fukushima Branch.

Higher radioactive cesium levels are reported in freshwater fish than those in the ocean in regions contaminated by the nuclear accident that started to unfold at the Tokyo Electric Power Co. plant in March 2011 following the Great East Japan Earthquake and tsunami.

While cesium remains mainly in the flesh, how the substance is ingested differs greatly depending on the species, habitat and other elements.

Sweetfish, char, landlocked salmon, carp and other species are still banned from shipping in certain areas, even nine years after the nuclear accident.

The research team studied how cesium accumulated in different species in areas of Fukushima Prefecture, targeting 30 kinds of fish in lakes Hayamako, Inawashiroko and Akimotoko as well as the Udagawa, Manogawa, Niidagawa, Otagawa and Abukumagawa rivers two to four years after the accident.

The results revealed that cesium levels could change in lakes if the fish consume differing foods. Higher readings were measured for landlocked salmon, char and other creatures preying on small fish, likely because cesium becomes concentrated in their bodies by eating tiny creatures.

On the other hand, the kinds of food they consume do not affect the cesium concentration in fish species living in rivers.

The findings showed cesium accumulates more easily when the water contains a smaller amount of tiny mud particles and carcasses of living creatures. The higher the ratio of those particles, the lower the cesium level becomes.

The researchers said the reason is apparently that cesium in water is absorbed into those particles, making it difficult for cesium to remain in the fish bodies as the substance is discharged in the particles through feces.

Meanwhile, higher cesium levels were detected for larger species both in lakes and rivers.

The discovery has been published in the international magazine Journal of Environmental Radioactivity at: (https://www.sciencedirect.com/science/article/pii/S0265931X1830715X).

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