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As a result of the Fukushima Dai-Ichi Nuclear Power Plant accident in March 2011, a large volume of radionuclides was released into the environment, thus contaminating marine and freshwater systems. The Fisheries Research Agency has conducted research beginning soon after the accident. Our research addressed the contamination processes of radionuclides (mainly radiocesium) through water, sediments, and food chains, in both marine and freshwater systems, based on a large volume of original in situ data. Our research has also provided important information on when and how marine fish have been contaminated. This chapter gives an overview of our research.

Enormous quantities of radionuclides were released into the ocean via both atmospheric deposition and direct release as a result of the Fukushima Dai-ichi Nuclear Power Plant (FNPP) accident. The evaluation of FNPP-derived radioactive cesium (Cs) in the marine environment is important in addressing risks to both marine ecosystems and public health through consumption of fisheries products. Understanding the distribution patterns of radioactive Cs in the ocean throughout the water column is key in assessing its effects on marine ecosystems. This chapter summarizes the dispersion pattern of FNPP-derived radioactive Cs in the North Pacific and around Japan, based on our observational studies as follows: (1) eastward dispersion in surface seawater; (2) southwestward intrusion with mode water; and (3) background level Cs without any detectable Cs in the Japan Sea, East China Sea, Seto Inland Sea, and Bering Sea, along with highly radioactive Cs off the coast of East Japan.

The Fukushima Dai-ichi Nuclear Power Plant (FNPP) accident following the Great East Japan Earthquake in 2011 resulted in the release of enormous quantities of anthropogenic radionuclides into the ocean off the east Japanese coast, especially radioactive cesium (Cs and Cs). FNPP-derived radioactive Cs might have consequently accumulated within marine food webs via seawater intake and predator–prey interactions. This study provides evidence of temporal variability in Cs concentrations in seawater and zooplankton samples collected off the Joban–Sanriku coast and in Sendai Bay between June 2011 and December 2013. In Sendai Bay, seawater Cs concentration was more than 1 Bq/kg in June 2011 and rapidly decreased over the study period. Cs concentration in zooplankton was also measured to be as high as high 23 Bq/kg-wet in June 2011, and this concentration decreased at a slower rate than seawater concentrations. The difference in the rate of decrease of Cs concentration between seawater and zooplankton resulted in an elevated apparent concentration ratio (aCR) for zooplankton. The observed relationship between Cs in seawater and the aCR of zooplankton reflected the progression of Cs contamination in zooplankton from the beginning of the FNPP accident to the restoration phase.

This section introduces results of an investigation for radiocesium (Cs and Cs) in sea sediment. The three-dimensional spatial distributions of radiocesium in sea sediment to a 14-cm core depth were surveyed from off the northern part of Ibaraki Prefecture to off Fukushima Prefecture with 5-min horizontal resolution in July 2012, approximately 16 months after the Fukushima Dai-ichi Nuclear Power Plant (FNPP) accident. A high concentration band was observed along the 100-m isobaths where the concentration of the Cs reached 1,240 Bq/kg-dry at the maximum and where vertical profiles of the concentration generally had an exponential-type decline with depth. The concentrations were very low at the area shallower than 100 m of depth north from the FNPP, where vertical concentration peaks often occurred in deeper layers. These horizontal and vertical distribution patterns are suggested to be mainly determined by the supplied amount of radiocesium from the radiocesium-contaminated bottom seawater and the ability of radiocesium adsorption as dependent on the grain size of the sediment.

Sequential chemical extraction of radiocesium was performed on 22 surface sediment samples to assess radiocesium concentration in the organic fraction of sea sediments (Cs). Our results showed that Cs of sea sediments was significantly larger than that of bulk sediments (Cs). The concentration factor of radiocesium in organic fraction against the bulk concentration (CF) varied from 3 to 50 off the Fukushima continental margin and showed a proportional relationship with median grain size and an inversely proportional relationship with organic content (OC) of the sediment. By using these relationships, the regression equation of Cs based on median grain size, organic content, and Cs was determined to construct a two-dimensional (2-D) distribution of Cs along the continental margin off the Fukushima region. The resultant map showed that the continental margin north of Fukushima Dai-ichi Nuclear Power Plant (FNPP) had moderate Cs values despite very low Cs. On the other hand, sediments sampled at the mouth of Abukuma River showed extremely low CF, which might have been caused by the existence of river-derived sediment particles.

Long-term monitoring and intensive field experiments for the bottom layer off the Fukushima coast were performed from October 2012 to November 2014 to understand the bottom processes, which are closely related to the spatial distribution and temporal variations of radiocesium in sea sediment. In this section, focusing on autumn 2012, we examine the bottom processes for a 32-m depth site (Sta. B) off Iwaki, Fukushima. Observational results showed that the bottom shear stresses from waves generally dominated over those from currents in this depth region, and the bottom turbidity increased in high wave conditions. Stepwise and significant southward cumulative transports of bottom turbidity were observed when high waves with long periods (LPW) coming from an E–ENE direction were superimposed on the southward current flow that has a periodicity of 5 days; both phenomena are influenced by successive passages of low pressure systems and the associated spatial distribution of atmospheric pressure. The combination of waves and currents caused by meteorological disturbance is a key process in the transport of suspended particulate material off the Fukushima coast.

We estimated the radiocesium translation from contaminated sediments to benthic organisms off the coast of Fukushima. We conducted field investigations and an experiment with a benthic polychaete () reared on highly contaminated sediments collected from a station 1 km off the Fukushima Dai-ichi Nuclear Power Plant. Results of the field investigations revealed that radiocesium contamination in benthic organisms depended on their feeding habitat. The radiocesium concentration in carnivore or herbivore feeder polychaetes was higher than that in deposit feeders. Radiocesium concentrations of all benthic organism specimens were lower than that in sediments collected from the same sampling point. Results of the rearing experiment showed that the concentration ratio (CR) of Cs for and contaminated sediments (wet/wet) was less than 0.10. Moreover, 4 days after separation from the contaminated sediments, the Cs concentrations in rapidly decreased. Based on the results of our field investigations and rearing experiment, we conclude that the intake of radiocesium through the benthic food web is limited for benthic organisms, despite the high contamination of the surrounding sediments.

The artificial radionuclides I, Cs, and Cs released from FNPP were detected in Japanese sardine () and Japanese anchovy () off the Kanto area of Japan. In the research period from 24 March 2011 to 13 July 2011, the maximum concentrations of I, Cs, and Cs were detected in the internal organs of Japanese anchovy collected on 24 March 2011. The concentration of I in the internal organs tended to be higher than that in muscle and the whole body, although no clear tendency was observed for Cs and Cs; it was thought that that was caused by I of the planktonic contents in the internal organs. These radionuclides detected in sardine and anchovy would be derived through the atmospheric pathway from FNPP to off the Kanto area, because these radionuclides were detected before the direct release of contaminated water into the ocean from FNPP.

After the Fukushima Dai-ichi Nuclear Power Plant (FNPP) accident, which occurred in March of 2011, the National Research Institute of Fisheries Science (NRIFS) undertook emergent radioactivity monitoring of 63 samples of small epipelagic fishes (such as sardine and Japanese anchovy) collected by commercial fishery boats off the Kashima-Boso area (located to the south of the Fukushima coast) from 24 March 2011 to 21 March 2013. Fluctuations in the radiocesium concentration in fish muscles synchronized with the decreasing concentration from seawater near the fishing ground; the radiocesium concentration in fish muscles reached a maximum of 31 Bq/kg-wet in July 2011, after which it declined gradually. From 2012 to 2013, the radiocesium concentrations in fish muscles were low (0.58–0.63 Bq/kg-wet). Compared to the Cs concentration before the FNPP accident, Cs concentration in fish muscles in 2013 was still about 10 times higher, whereas it was about 4.5 times higher in seawater near the fishing ground in 2012.

We examined year-class-related differences in radiocesium concentrations in Pacific cod () and evaluated the potential factors affecting the differences after the release of large amounts of radionuclides from Fukushima Dai-ichi Nuclear Power Plant (FNPP) in March 2011. The concentration of radiocesium was highest in the 2009 and earlier year-classes (yc) (≤2009 yc), followed by the 2010 yc, and was rarely detected in the 2011 yc. Trawl surveys throughout the year revealed that a proportion of Pacific cod born in or before 2009 and 2010 were distributed in the coastal area from winter to early summer, whereas all individuals were on the upper continental slope from early summer to winter. The concentration of radiocesium decreased more rapidly in the 2010 yc than in the ≤2009 yc. The diet of cod changed ontogenetically and spatiotemporally. The organisms preyed upon on the upper continental slope by cod of all year-classes and in the coastal area by the 2010 yc contained very low concentrations of radiocesium. However, some food items ingested in the coastal area by the ≤2009 yc had relatively high radiocesium levels. These results suggest that Pacific cod primarily accumulated radiocesium during the first few months after the FNPP accident. Age- and body size-dependent differences in growth, metabolic rate, and diet, as well as seasonal migration patterns, also affected the rate of decrease in radiocesium levels, which likely led to the differences we observed between year-classes.