1 Introduction

Large amounts of artificial radionuclides such as 131I, 134Cs, and 137Cs, were released into the environment by the Fukushima Dai-ichi Nuclear Power Plant (FNPP) accident, which was caused by the Great East Japan Earthquake and tsunami on 11 March 2011. Tokyo Electric Power Company (TEPCO) estimated that 4.7 × 1015 Bq of radioactive materials including 131I, 134Cs, and 137Cs were released directly into the ocean from the FNPP Unit 2 reactor during April 1–6 in 2011 (Nuclear Emergency Response Headquarters 2011), although it was reported that the direct release to the ocean had already occurred on 26 March 2011, and showed the estimation that the total amount of 137Cs directly released was 3.5 ± 0.7 × 1015 Bq from March 26 to the end of May 2011 (Tsumune et al. 2012). The total quantity of 131I and 137Cs released into the atmosphere between 12 March 2011 and 1 May 2011 was estimated to be approximately 2.0 × 1017 Bq and 1.3 × 1016 Bq, respectively. Furthermore, the quantities of 131I and 137Cs deposited on the ocean surface from the atmosphere were estimated as 9.9 × 1016 Bq and 7.6 × 1015 Bq, respectively (Kobayashi et al. 2013).

Monitoring research detected 131I, 134Cs, and 137Cs in marine organisms (Fisheries Agency 2014). The source of the 131I and 134Cs detected in marine organisms clearly originated from FNPP because of the short physical half-lives, 8.02 days for 131I and about 2.06 years for 134Cs. However, it has been unclear whether the radionuclides were released into the atmosphere or directly into the ocean from FNPP. In this report, we focus on the detection of 131I, 134Cs, and 137Cs in small epipelagic fishes, sardine and anchovy, off the Kanto area of Japan. Our results indicate that the 131I, 134Cs, and 137Cs detected in small epipelagic fishes was released into the atmosphere from FNPP.

2 Experimental Procedure

Fish samples were commercially collected from 24 March 2011 to 13 July 2011 at regions shown in Fig. 8.1. An individual fish sample contain only small amounts of 131I, 134Cs, and 137Cs, so to determine the concentrations we used multiple fish samples for each measurement specimen. Therefore, we prepared the specimen for measurement consisting of muscle, internal organs, and whole body from multiple samples, a total of 5–10 kg individuals. The previous report showed 134Cs and 137Cs concentrations in most of the fish samples obtained from raw measurement specimens (Takagi et al. 2014). In this report, those samples were re-measured after ashing. On the other hand, 131I concentrations were obtained from the measurement using a raw measurement specimen. For preparation of the ashed measurement specimen, raw samples were dried in an oven at 105 °C for 72–120 h, carbonized in a gas furnace at 350–400 °C for approximately 6 h, and ashed in an electric furnace at 450 °C for 48–72 h. The ashed samples were ground to a fine powder, transferred to a plastic cup, and pressed using a hand press. The concentrations of 131I, 134Cs, and 137Cs were measured using a high-purity germanium (HpGe) semiconductor detector with a multichannel analyzer (Seiko EG & G; ORTEC, Oak Ridge, TN, USA). This HpGe semiconductor detector has a resolution of 1.44 keV at a peak of 662.15 keV (137Cs). The counting efficiency of the Ge semiconductor detector was calibrated using volume standard sources (MX033U8PP; Japan Radioisotope Association, Tokyo, Japan). Coincidence summing effects of 134Cs were corrected with 134Cs standard solutions (CZ005; Japan Radioisotope Association, Tokyo, Japan). The counting times were about 7,200 s for the raw specimen and from about 3,000 s to about 7,200 s for ashed specimens. All radionuclide concentrations were corrected for decay from the respective sampling date. The concentration of three standard deviations (σ) from counting error was defined as the detection limit.

Fig. 8.1
figure 1

Location of the Fukushima Dai-ichi Nuclear Power Plant (FNPP) and sampling regions (A and B). Respective sampling regions are surrounded by dashed lines indicating shorelines

3 Concentrations of 131I, 134Cs, and 137Cs in Sardine and Anchovy

There was no difference in the 134Cs/137Cs concentration ratio between sardine and anchovy and among the respective measurement specimens. Considering that the half-life for 134Cs is 2.1 years, the 134Cs/137Cs concentration ratio in these small epipelagic fishes was close to 1.0 (Fig. 8.2, Table 8.1). This ratio is consistent with the 134Cs/137Cs concentration ratio already reported in seawater and marine organisms (Aoyama et al. 2012; Wada et al. 2013). The ratio indicated that the 134Cs and most of the 137Cs detected in these small epipelagic fishes originated from the FNPP accident. The concentration of 137Cs in muscle and whole bodies without internal organs of sardines collected off the Kanto area in 2010, before the FNPP accident, was 0.052 ± 0.0038 Bq/kg-wet and 0.030 ± 0.0044 Bq/kg-wet, respectively (Fisheries Research Agency 2012).

Fig. 8.2
figure 2

Relationship between 134Cs/137Cs concentration ratio and concentration of 137Cs detected in this study. The 134Cs/137Cs concentration ratio was calculated from the data decay corrected on 11 March 2011

Table 8.1 Concentrations of 131I, 134Cs and 137Cs in sardine and Japanese anchovy

The previous report showed the summed concentration of 134Cs and 137Cs in raw measurement specimens for muscle of sardine and anchovy (Takagi et al. 2014). These concentrations were 61.0 % to 155.9 % of the sum concentration of 134Cs and 137Cs in the ashed measurement specimen consisting of the same samples as the raw measurement specimen. Figure 8.3 shows the temporal variation of 131I and 137Cs concentrations in the internal organs of sardine and anchovy. The maximum concentrations of 131I, 134Cs, and 137Cs were 309.08 ± 2.06 Bq/kg-wet, 61.01 ± 0.52 Bq/kg-wet, and 59.63 ± 0.39 Bq/kg-wet, respectively, in the internal organs of anchovy collected 24 March 2011 (Fig. 8.3, Table 8.1). There was no detection of 131I on 26 April 2011 because of the short physical half-life, 8.02 days. The concentrations of 131I in the internal organs of sardine and anchovy until 26 April 2011 decreased to half by 4.4 and 4.6 days, respectively.

Fig. 8.3
figure 3

Temporal variation in the concentration of 131I (a) and 137Cs (b) in internal organs of sardine and anchovy. Circles and square symbols indicate data for sardine and anchovy, respectively. Open and closed symbols indicate data for samples collected in regions A and B, respectively. Error bar shows 1 σ value derived from counting statistics. Errors for many of the data are too small to show an error bar

The respective concentrations in fishes collected in region B were obviously lower than those in region A. It was clear that the reason was the distance from FNPP to each sampling region. The concentration of 131I in the internal organs tended to be higher than those in other measurement specimens, although no clear tendency was observed for 134Cs and 137Cs (Table 8.1). Although the concentration factor of iodine in fish is from 9 to 10, the factor of iodine in phytoplankton and zooplankton is from 800 to 1,000 (IAEA 2004). The measurement specimen from the internal organs of sardine and anchovy, which are plankton feeders, could include some plankton. Therefore, the higher concentrations of 131I would be detected in the internal organ specimens from sardine and anchovy.

4 Detection of 131I, 134Cs, and 137Cs Released into the Atmosphere from FNPP

The 137Cs concentration in sardine gradually decreased until the end of May in 2011, but the concentration suddenly increased in the first week of June in 2011 (Fig. 8.3b, Table 8.1). It was considered that this sudden increase was caused by the disappearance on 30 May 2011 of a warm water eddy, the center of which was located off Iwaki between Onahama and Hasaki from the middle of May (Takagi et al. 2014; Fig. 9.4 in Chap. 9). The warm water eddy prevented the seawater, including 131I, 134Cs, and 137Cs, from moving southward to sampling region A (Aoyama et al. 2012). In this time, 131I was again detected in the internal organs of sardine, although there had come to be no detection of 131I on 26 April 2011. This detection of 131I also could indicate the southward movement of contaminated seawater.

134Cs and 137Cs were detected in sardine and anchovy collected in sampling region A before the southward movement of contaminated seawater. According to the previous report, the reason for these detections was considered to be that the contaminated sardine and anchovy migrated southward to the region earlier than the southward movement of contaminated seawater (Takagi et al. 2014). It is well known that the radioactively contaminated fishes are able to migrate from a contaminated area to a noncontaminated area. Radionuclides were transported from Russia to Japan by walleye pollock and from Japan to the United States of America by Pacific bluefin tuna (Morita et al. 2007; Madigan et al. 2012). However, in the large amount of 131I, 134Cs, and 137Cs deposited on the ocean surface off the Kanto area (Kobayashi et al. 2013), it would be difficult to distinguish between directly released and atmospheric pathway radionuclides.

The 131I/137Cs concentration ratio in the internal organs of sardine and anchovy that were collected during from 16 March 2011 to 29 March 2011 in regions A and B was from 4.2 to 7.8. The 131I/137Cs concentration ratio of the radionuclides that were released directly from FNPP to 30 km offshore from 26 March to 6 April 2011 agreed with the radioactive decay curve of 131I (Tsumune et al. 2012). However, it was unclear whether this agreement applied to the region A. In addition, the 131I/137Cs concentration ratio shows variations during atmospheric transport (Kinoshita et al. 2011) because of differences in the wet deposition rate depending on the size of particles (Hirose et al. 1993), whereas the simulation estimated that the 131I/137Cs ratio deposited in the ocean during 22 March 2011 to 24 March 2011 around region A was 6.7–40.4 (T. Kobayashi, personal communication). Therefore, it was also difficult to determine the route (as direct release or via the atmospheric pathway) of these radionuclides by the 131I/137Cs concentration ratio because of the range variation in the estimated ratio and the difference in incorporation rate into the internal organs between 131I and 137Cs. On the other hand, it was estimated that the direct release of the contaminated water from FNPP into the ocean occurred from 26 March 2011 (Tsumune et al. 2012). We detected 134Cs and 137Cs in Japanese anchovy collected on 24 March 2011 in region A and on 16 March 2011 in region B. Consequently, it was clear that these radionuclides were released into the atmosphere from FNPP; these would deposit on the surface water in this region through the atmospheric pathway. In addition, we also detected 134Cs and 137Cs in sardines collected on 28 March 2011 in region A and in both sardine and anchovy collected on 29 March 2011 in region B. Considering the distance between the FNPP and these sampling regions, these radionuclides were clearly released into the atmosphere from FNPP.