Assessing the impact of large-scale farmland abandonment on the habitat distributions of frog species after the Fukushima nuclear accident.

Rice paddies function as wetlands; therefore, abandoned paddy fields cause a loss of habitats for aquatic species, such as amphibians. Following the accident at the Fukushima Daiichi Nuclear Power Plant in 2011, paddy fields around the plant were abandoned and rapidly dried. To identify the impact of large-scale abandonment of paddy fields on the habitats of frogs, we investigated changes in the distributions of four frogs that breed in paddy fields using niche modeling and field surveys. The spatial distributions of suitable habitats before and after the accident for each frog were created using MaxEnt. In the area where rice cropping was restricted due to radioactive contamination, the areas of suitable habitats decreased for Pelophylax porosus porosus but increased or remained unchanged for other frogs after the accident. Additionally, field surveys conducted in 2014 indicated that the ratios of breeding sites in the area where rice cropping was restricted were significantly lower for P. p. porosus and Hyla japonica than outside this area. Thus, 3 years after the accident, one species strongly dependent upon paddy fields rapidly disappeared over a large area. Although amphibian populations or monitoring data were not available to examine changes directly after the accident in the study area, this research showed that the combination of niche modeling and field survey was effective for predicting species response after an accident and revealed that large-scale disasters sufficient to disrupt agricultural activity could markedly change the distribution of species reliant on habitats created by human activity.

Effects of chronic γ-irradiation on growth and sexual maturation of the Tohoku hynobiid salamander, Hynobius lichenatus.

There are still considerable gaps in knowledge regarding the biological effects of chronic ionising radiation exposure in amphibians. To fill these gaps, Tohoku hynobiid salamanders, Hynobius lichenatus (Amphibia, Caudata), were chronically irradiated with 137Cs γ-rays from embryonic to adult stages over 1954 days, and the effects on their growth and sexual maturation were examined under laboratory conditions. Irradiation at a dose rate of 33 µGy h-1 had some stimulatory effects on growth (body weight increase) of H. lichenatus, while growth was temporarily or permanently suppressed at 150 or 510 µGy h-1, respectively. On day 1802, secondary sexual characteristics (a tubercle at the anterior angle of the cloacal vent for males and ovisac development for females) were observed in 91% of the salamanders irradiated at 33 µGy h-1, and in a similar percentage of non-irradiated controls. At 150 and 510 µGy h-1, secondary sexual characteristics were not observed in any individuals. These results suggest that the derived consideration reference level (DCRL) of the International Commission on Radiological Protection (ICRP) for Reference Frog, i.e. 40-400 µGy h-1, is applicable for the protection of H. lichenatus, and that growth and sexual maturation of this salamander may not have been adversely affected even in the most severely contaminated area in Fukushima, where the highest dose rate to salamanders was estimated to be 50 µGy h-1. However, observations in the contaminated area are required to confirm this conclusion, considering the possible confounding factors which may make this salamander more sensitive to radiation in the natural environment than under laboratory conditions.

Radiocaesium contamination and dose rate estimation of terrestrial and freshwater wildlife in the exclusion zone of the Fukushima Dai-ichi Nuclear Power Plant accident.

To characterise the radioactive contamination of terrestrial and freshwater wildlife caused by the Fukushima Dai-ichi Nuclear Power Plant accident, biological samples, namely, fungi, mosses, plants, amphibians, reptiles, insects, molluscs, and earthworms, were collected mainly from the forests of the exclusion zone in the Fukushima Prefecture from 2011 to 2012. Caesium-134 and 137Cs were detected by gamma spectrometry in almost all the samples. Fungi, ferns, and mosses accumulated high amounts of radiocaesium, as they did in Chernobyl, with 134Cs + 137Cs activity concentrations of 104-106 Bq kg-1 fresh mass (FM). Earthworms, amphibians, and the soft tissue of the garden snail Acusta despecta sieboldiana, also had levels as high as 104-105 Bq kg-1 FM of 134Cs + 137Cs. Most of the estimated total (internal + external) dose rates to herbaceous plants, amphibians, insects, and earthworms were below the corresponding derived consideration reference levels (DCRLs) recommended by the ICRP. This suggests that, in most cases, there was little chance of deleterious effects of ionising radiation on these organisms in the exclusion zone for the first year after the accident, though the dose rates were underestimated mainly due to the lack of consideration of short-lived radionuclides.

Radiocaesium contamination of wild boars in Fukushima and surrounding regions after the Fukushima nuclear accident.

Analysis of radioactivity data obtained under the food monitoring campaign in Japan indicates that elevated 134Cs+137Cs activity concentrations in wild boar meat remained constant or slowly decreased in Fukushima and surrounding prefectures from 2011 to 2015. The activity concentrations in some samples are still over the regulatory limit of 100 Bq kg-1 fresh weight, even in 2015. Activity concentrations of 137Cs in muscle of wild boars we captured in 2011 were higher than those in kidney, liver, spleen, heart and lung. A food processing retention factor, Fr, was 0.5 or 0.6 for 137Cs when the wild boar meat was boiled, suggesting that a parboiling process is effective for reduction of radiocaesium intake from wild boar meat.

Assessment of radiocesium contamination in frogs 18 months after the Fukushima Daiichi nuclear disaster.

We investigated the accumulation of radionuclides in frogs inhabiting radioactively contaminated areas around Fukushima Daiichi Nuclear Power Plant (FDNPP) to search for possible adverse effects due to radionuclides. We collected 5 frog species and soil samples in areas within and outside a 20-km radius from FDNPP in August and September 2012 and determined their radiocesium concentrations ((134)Cs and (137)Cs). There was a positive correlation between radiocesium concentrations in the soil samples and frogs, and the highest concentration in frogs was 47,278.53 Bq/kg-wet. Although we conducted a histological examination of frog ovaries and testes by light microscopy to detect possible effects of radionuclides on the morphology of germ cells, there were no clear abnormalities in the gonadal tissues of frogs collected from sites with different contamination levels.

Dose rate estimation of the Tohoku hynobiid salamander, Hynobius lichenatus, in Fukushima.

The radiological risks to the Tohoku hynobiid salamanders (class Amphibia), Hynobius lichenatus due to the Fukushima Dai-ichi Nuclear Power Plant accident were assessed in Fukushima Prefecture, including evacuation areas. Aquatic egg clutches (n = 1 for each sampling date and site; n = 4 in total), overwintering larvae (n = 1-5 for each sampling date and site; n = 17 in total), and terrestrial juveniles or adults (n = 1 or 3 for each sampling date and site; n = 12 in total) of H. lichenatus were collected from the end of April 2011 to April 2013. Environmental media such as litter (n = 1-5 for each sampling date and site; n = 30 in total), soil (n = 1-8 for each sampling date and site; n = 31 in total), water (n = 1 for each sampling date and site; n = 17 in total), and sediment (n = 1 for each sampling date and site; n = 17 in total) were also collected. Activity concentrations of (134)Cs + (137)Cs were 1.9-2800, 0.13-320, and 0.51-220 kBq (dry kg) (-1) in the litter, soil, and sediment samples, respectively, and were 0.31-220 and <0.29-40 kBq (wet kg)(-1) in the adult and larval salamanders, respectively. External and internal absorbed dose rates to H. lichenatus were calculated from these activity concentration data, using the ERICA Assessment Tool methodology. External dose rates were also measured in situ with glass dosimeters. There was agreement within a factor of 2 between the calculated and measured external dose rates. In the most severely contaminated habitat of this salamander, a northern part of Abukuma Mountains, the highest total dose rates were estimated to be 50 and 15 µGy h(-1) for the adults and overwintering larvae, respectively. Growth and survival of H. lichenatus was not affected at a dose rate of up to 490 µGy h(-1) in the previous laboratory chronic gamma-irradiation experiment, and thus growth and survival of this salamander would not be affected, even in the most severely contaminated habitat in Fukushima Prefecture. However, further studies of the adult salamanders may be required in order to examine whether the most severe radioactive contamination has any effects on sensitive endpoints, since the estimated highest dose rate to the adults exceeded some of the guidance dose rates proposed by various organisations and programmes for the protection of amphibians, which range from 4 to 400 µGy h(-1). Conversely, at one site in Nakadori, a moderately contaminated region in Fukushima Prefecture, the dose rate to the adult salamanders in spring of 2012 was estimated to be 0.2 µGy h(-1). Estimated dose rates to the overwintering larvae in spring of 2012 were 1 and 0.2 µGy h(-1) at one site in Nakadori, and in Aizu, a less contaminated region in Fukushima Prefecture, respectively. These results suggest that there is a low risk that H. lichenatus will be affected by radioactive contamination in these districts, though further studies on dose rate estimation are required for definitive risk characterisation.

Effects of chronic γ-irradiation on growth and survival of the Tohoku hynobiid salamander, Hynobius lichenatus.

The Tohoku hynobiid salamanders, Hynobius lichenatus, were chronically irradiated with γ-rays from embryonic to juvenile stages for 450 days. At 490 µGy h(-1) or lower dose rates, growth and survival were not significantly affected by irradiation, and any morphological aberrations and histological damages were not observed. At 4600 µGy h(-1), growth was severely inhibited, and all the individuals died mostly at the juvenile stage. Chronic LD50 was 42 Gy as a total dose. In the liver, the number of hematopoietic cells was significantly reduced in the living juveniles, and these cells disappeared in the dead juveniles. In the spleen, mature lymphocytes were depleted in the living larvae, and almost all the heamtopoietic cells disappeared in the dead juveniles. These results suggest that this salamander died due to acute radiation syndrome, i.e., hematopoietic damage and subsequent sepsis caused by immune depression. The death would be also attributed to skin damage inducing infection. At 18,000 µGy h(-1), morphological aberrations and severe growth inhibition were observed. All the individuals died at the larval stage due to a multiple organ failure. Chronic LD50 was 28 Gy as a total dose. Assuming that chronic LD50 was 42 Gy at lower dose rates than 4600 µGy h(-1), a chronic median lethal dose rate could be estimated to be <340 µGy h(-1) for the whole life (>14 years). These results suggest that, among guidance dose rates, i.e., 4-400 µGy h(-1), proposed by various organisations and research programmes for protection of amphibians and taxonomic groups or ecosystems including amphibians, most of them would protect this salamander but the highest value may not on the whole life scale.

(135)Cs/(137)Cs isotopic ratio as a new tracer of radiocesium released from the Fukushima nuclear accident.

Since the Fukushima Daiichi nuclear power plant (FDNPP) accident in 2011, intensive studies of the distribution of released fission products, in particular (134)Cs and (137)Cs, in the environment have been conducted. However, the release sources, that is, the damaged reactors or the spent fuel pools, have not been identified, which resulted in great variation in the estimated amounts of (137)Cs released. Here, we investigated heavily contaminated environmental samples (litter, lichen, and soil) collected from Fukushima forests for the long-lived (135)Cs (half-life of 2 × 10(6) years), which is usually difficult to measure using decay-counting techniques. Using a newly developed triple-quadrupole inductively coupled plasma tandem mass spectrometry method, we analyzed the (135)Cs/(137)Cs isotopic ratio of the FDNPP-released radiocesium in environmental samples. We demonstrated that radiocesium was mainly released from the Unit 2 reactor. Considering the fact that the widely used tracer for the released Fukushima accident-sourced radiocesium in the environment, the (134)Cs/(137)Cs activity ratio, will become unavailable in the near future because of the short half-life of (134)Cs (2.06 years), the (135)Cs/(137)Cs isotopic ratio can be considered as a new tracer for source identification and long-term estimation of the mobility of released radiocesium in the environment.

Isotopic evidence of plutonium release into the environment from the Fukushima DNPP accident.

The Fukushima Daiichi nuclear power plant (DNPP) accident caused massive releases of radioactivity into the environment. The released highly volatile fission products, such as (129m)Te, (131)I, (134)Cs, (136)Cs and (137)Cs were found to be widely distributed in Fukushima and its adjacent prefectures in eastern Japan. However, the release of non-volatile actinides, in particular, Pu isotopes remains uncertain almost one year after the accident. Here we report the isotopic evidence for the release of Pu into the atmosphere and deposition on the ground in northwest and south of the Fukushima DNPP in the 20-30 km zones. The high activity ratio of (241)Pu/(239+240)Pu (> 100) from the Fukushima DNPP accident highlights the need for long-term (241)Pu dose assessment, and the ingrowth of (241)Am. The results are important for the estimation of reactor damage and have significant implication in the strategy of decontamination.

Identification of Euryhelmis costaricensis metacercariae in the skin of Tohoku hynobiid salamanders (Hynobius lichenatus), northeastern Honshu, Japan.

Since 2000, cutaneous nodular lesions were observed in Tohoku hynobiid salamanders (Hynobius lichenatus) in the northern Abukuma Mountains, northeastern Honshu, Japan. The distribution of diseased salamanders has expanded, along with the increasing severity of the disease. Samples (foreleg, hindleg, and tail) from five diseased salamanders caught in the spring of 2009 contained 11 to 383 metacercarial cysts (0.43-0.48 mm in diameter) per individual. Experimental inoculations of the metacercariae into rats (Rattus norvegicus) resulted in the development of pentagonal adults with intrauterine eggs that were collected at 3 days, 2 wk, and 5 wk postinoculation. Although adults obtained from rats were smaller, the flukes were morphologically similar to Euryhelmis costaricensis, recorded from Japanese martens (Martes melampus) in northeastern Honshu. The ribosomal RNA gene sequences of the metacercariae collected from salamanders and the E. costaricensis collected from a Japanese marten were identical, confirming the morphology-based species identification. Environmental changes responsible for the increased frequency of diseased salamanders and the increased parasite load are unknown, although increases in feral populations of introduced raccoons (Procyon lotor) and American minks (Neovison vison) may be associated.