This study reports the first long-term monitoring survey of cesium-137 (137Cs) activity concentrations in wild boar (Sus scrofa) muscle samples collected inside the Fukushima difficult-to-return zone (DRZ) over a 5-year period following the 2011 Fukushima Daiichi nuclear power plant accident. All 221 boar captured inside the DRZ exceeded the 100 Bq/kg fresh mass (FM) consumption standard, with annual means in the thousands to tens of thousands of Bq/kg FM.
Key numbers
137Cs activity in wild boar muscle inside the DRZ (Table 1, p. 3; Table 2, p. 5):
By year (Bq/kg FM):
- 2016: mean 15,028; max 132,210; min 608; n = 56.
- 2017: mean 3,030; max 26,110; min 542; n = 29.
- 2018: mean 8,303; max 38,385; min 427; n = 30.
- 2019: mean 5,180; max 42,018; min 42; n = 52.
- 2020: mean 3,568; max 20,010; min 213; n = 54.
By month inside DRZ (Table 2, p. 5): monthly means ranged from 1,463 Bq/kg FM (March, n = 5) to 17,289 Bq/kg FM (February, n = 44).
Seasonal pattern: 137Cs activity was higher in the high-activity-concentration season (December to March; winter) and lower in the low-activity-concentration season (July to September; summer). Significant differences between February and August (Kruskal-Wallis test, df = 11, P < 0.001; Steel-Dwass test P < 0.01).
Comparison with outside-DRZ regions: 137Cs concentrations inside the DRZ were significantly higher than in all three external regions (Hamadori excluding DRZ, Nakadori, Aizu) in all time periods examined (Figure 2, p. 3).
The 100 Bq/kg FM standard was exceeded by all 221 DRZ samples; levels ranged from 42 to 132,210 Bq/kg FM (Results, p. 2-3).
Effective ecological half-life: T_eff range 3.0-5.5 years (Results/Discussion, p. 4). The 5-year trend in 137Cs activity concentration in DRZ boar muscles showed little to no decline and in some cases a slight increase.
Correlations: Significant positive correlations between muscle 137Cs concentration and both body length and body weight were observed in the low-activity-concentration season (July to September), but not across all seasons (Figures 5-6, p. 6-7). No significant correlation was found between muscle 137Cs and soil 137Cs deposition within the DRZ in any season (Figures 4-6, p. 6-7).
Methods (brief)
Muscle tissue samples were collected from 221 wild boar culled under “The habitat survey and capture of the wild animals project” and similar surveys authorized by the Ministry of the Environment of Japan. Samples were collected from January 2016 to November 2020 in five municipalities (Okuma, Katsurao, Tomioka, Namie, and Futaba). Hind-leg muscle tissue was minced, freeze-dried, and crushed; 137Cs activity was measured with a germanium semiconductor detector (Canberra GC3018). Results were expressed as Bq/kg FM after correction from freeze-dried values. External DRZ comparison samples were collected as part of a separate prefectural monitoring program.
Evidence fitness
This paper provides the highest-quality long-term monitoring data available for 137Cs in wild boar in the Fukushima DRZ, characterizing both the magnitude and temporal trend of contamination. It documents that contamination persists at levels orders of magnitude above the 100 Bq/kg FM standard for at least five years post-accident and shows seasonal variation within that trend. It does not address stable cesium, does not characterize human health risk from consumption, and is specific to animals captured in the highest-contamination restricted zones in Japan.
Limitations
- DRZ samples reflect the most extreme post-accident contamination scenario; findings are not representative of wild boar or other game meat in regions without nuclear accident deposition.
- Body condition, age, and home range of individual animals were not fully characterized, limiting interpretation of size-effect findings.
- Temporal coverage (6 years post-accident) is insufficient to characterize the long-term trajectory toward baseline.
- No data on 134Cs; only 137Cs reported.
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