Catálogo de publicaciones - libros

Radiocesium (Cs) contamination histories of the Japanese flounder, , after the 2011 Fukushima Nuclear Power Plant (FNPP) accident were examined by analysis of the spatiotemporal changes in observed Cs concentrations, by comparison of the dynamics of the Cs concentrations in several year-classes of fish, and by simulation studies. Two contamination histories were revealed: (1) severe contamination by water that was directly released from the FNPP with extremely high Cs concentrations for a few months after the accident, which had a highly variable spatial distribution; and (2) long-duration contamination at relatively low concentrations resulting from consumption of contaminated food. These two histories were supported by three observations. First, high Cs concentrations with high variability were observed in the first year after the accident. Second, the highest values of the minimum Cs concentrations were observed in the autumn of 2011. Third, Cs concentrations were lower with smaller variation for fish from the 2011 year-class and younger, which were not exposed to the highly contaminated directly released water, than for fish from the 2010 year-class and older. Simulation studies also indicated that the Cs concentrations in some individuals that were exposed to the directly released water might not be in an equilibrium state even at 3 years after the accident. On the basis of these contamination histories, it can be expected that the Cs concentrations in most Japanese flounder will continue to decrease.

On 1 August 2012, a total of 25,800 Bq/kg-wet of radiocesium (Cs = 9,800 Bq/kg-wet, Cs = 16,000 Bq/kg-wet) was detected in the muscle tissue of two fat greenlings () caught approximately 20 km north of the Fukushima Dai-ichi Nuclear Power Plant (FNPP). To estimate the contamination level of this fish species off the coast of Fukushima, we measured the radiocesium concentration in the muscle tissue of individual fat greenlings in 2012 and 2013. Radiocesium concentration of fat greenlings caught in southern coastal waters from the FNPP was significantly higher than that of fat greenlings collected in other waters off the coast of Fukushima. However, fat greenlings with a radiocesium concentration greater than 10,000 Bq/kg-wet were not detected, not even from highly contaminated areas. In addition, data obtained from specimens collected off the coast of Fukushima from April to December 2012 suggested that the probability of catching fat greenlings with a concentration greater than 16,000 Bq/kg-wet of Cs was exceedingly low (less than 2.794 × 10). In contrast, highly contaminated fat greenlings were frequently caught within the FNPP port. The geometric mean of Cs was 55,400 Bq/kg-wet, as calculated from specimens obtained during December 2012 to May 2013. Our investigation suggests that fat greenlings with an extremely high concentration of radiocesium were contaminated within the FNPP port and then migrated offshore.

We analyzed the contamination process by which the fat greenling, which was caught in the area off the mouth of the Ota River of Fukushima prefecture on August 1, 2012, concentrated radiocesium (Cs + Cs) to the level of 25,800 Bq/kg-wet. The radioactivity environment of the area was insufficient to maintain or increase the radiocesium concentration in the fish at the time. Distribution of the radioactive materials in the otolith of the fat greenling estimated by beta-ray emissions suggested that the fat greenling was in a highly contaminated environment during the period immediately following the Fukushima Dai-ichi Nuclear Power Plant (FNPP) accident. We used a biokinetic simulation of the Cs concentration to demonstrate that the fat greenling had to have been exposed to radioactivity from the FNPP to achieve such a high radiocesium concentration. Thus, the extremely contaminated fat greenling originated in the heavily contaminated environment of the FNPP port or the adjoining area in the period just after the accident.

Levels of radioactive cesium (radiocesium, Cs + Cs) detected in fish caught at the Fukushima Dai-ichi Nuclear Power Plant (FNPP) Port are summarized. The mean concentration of radiocesium in three fish species (fat greenling, Japanese rockfish, and spotbelly rockfish) was significantly different from that in other fish species studied (brown hakeling, black rockfish, Japanese black porgy, olive flounder, marbled flounder). The levels of radiocesium in fat greenling decreased gradually from 100 kBq/kg-wet in 2013 to several kBq/kg-wet in 2014. A migration of fat greenling into the FNPP Port was assumed to explain the fact that fish containing low radiocesium levels were caught at the port. A low but significant correlation between the total length of the fish and the radiocesium concentration in the muscles was observed in fat greenling caught at the FNPP Port.

Levels of radiocesium (Cs + Cs) contamination in lake water, bottom sediment, plankton, and fish were investigated in three geographically separated lakes in Fukushima Prefecture (Lake Hayama, Lake Akimoto, and Lake Tagokura) between June 2012 and November 2013. Levels of contamination differed among the three lakes, with the highest levels in each measured component found in Lake Hayama, followed by Lake Akimoto, and the least contamination in Lake Tagokura. Among the lakes, the magnitude of contamination decreased with distance from the Fukushima Dai-ichi Nuclear Power Plant. Mean radiocesium concentrations were higher in piscivorous fish than in other fish, possibly reflecting differences in trophic levels. Radiocesium concentrations of the lake water, bottom sediment, plankton, and fish were significantly correlated with surface soil radiocesium content near lake sites.

We measured radiocesium (Cs + Cs) concentrations in five freshwater fish species in Lake Hayama, Fukushima Prefecture, 1 year after the Fukushima Dai-ichi Nuclear Power Plant (FNPP) accident in March 2011. The five species included bluegill (), spp. ( and ), Japanese dace (), largemouth bass (), and smallmouth bass (). We observed a “positive size effect” for radiocesium concentrations in fish muscle, but the coefficient of determination was low for bluegill, spp., and Japanese dace. In contrast, the coefficient of determination was high for the exponential relationship between body size and radiocesium concentrations in largemouth and smallmouth bass. The geometric mean radiocesium concentration in each body size class was generally higher for carnivorous fish than for omnivorous and herbivorous fish.

Ayu, , is a herbivorous fish that is an important fishery resource and a key component of the food web in many Japanese streams. After the Fukushima Daiichi Nuclear Power Plant (FNPP) accident in March 2011, ayu were exposed to highly contaminated silt while feeding on benthic microalgae attached to riverbed stones. To understand the effects of radioactive contamination on ayu, radiocesium (Cs + Cs) concentrations were analyzed in riverbed samples (microalgae and silt) and in the internal organs and muscle of ayu in five river systems in the Fukushima Prefecture between summer 2011 and autumn 2013. The concentrations of radiocesium in both the internal organs and the muscles of ayu declined over time. The radiocesium concentrations in the muscle were correlated with, but much lower than, those in the internal organs. The concentrations in the internal organs were correlated with those in the riverbed samples. The concentrations in the muscle were further correlated with ayu body size. Our results suggest that ayu ingest radiocesium while consuming silt and microalgae from the riverbed, and that a small proportion (about 15 %) is assimilated into the muscle of the fish.

Approximately 18 months (September–December 2012) after the Fukushima Dai-ichi Nuclear Power Plant accident, elevated radiocesium concentrations were detected in muscle and egg samples from masu salmon (), kokanee (), brown trout (), and lake trout () from the Lake Chuzenji system, central Honshu Island, Japan (160 km from the station). Mean muscle concentrations were 142.9–249.2 Bq/kg-wet, and mean egg concentrations were 38.7–79.0 Bq/kg-wet. No relationship between fork length and muscle radiocesium concentration was observed in any of the species, but significant relationships were found between individual muscle and egg radiocesium concentrations from masu salmon, brown trout, and lake trout.

To understand the process of radiocesium uptake in salmonids after the Fukushima Dai-ichi Nuclear Power Plant (FNPP) accident, a lake caging experiment and two captive-rearing experiments with controlled radiocesium concentrations of water and feed were conducted in and around Lake Chuzenji, central Honshu Island, Japan (160 km from the station). Substantial accumulations of radiocesium were confirmed in muscle of hatchery-reared kokanee () and masu salmon () after release into the cages, indicating that radionuclide contamination of fish is an ongoing process, 1.5 years after the nuclear accident. Two captive experiments, controlling water and feed radiocesium levels, showed that direct radiocesium transfer from water (43 mBq/l) in Lake Chuzenji to muscle tissue was undetected, at least during the approximately 90-day experimental period, whereas a rapid increase in radiocesium levels was observed when fish were cultured using radiocesium-contaminated pellets. The results revealed that radiocesium contamination in salmonids is mainly via the food chain, and that direct intake from water via the skin, gut, or gills has no major direct impact on muscle tissue concentrations.