Monitoring of radioactive cesium in wild boars captured inside the difficult-to-return zone in Fukushima Prefecture over a 5-year period

Following the Fukushima Daiichi Nuclear Power Plant accident in 2011, tissue samples from wild boar ( Sus scrofa ) outside the evacuation zone (difficult-to-return zone, DRZ) tended to show high activity concentrations of cesium-137 ( 137 Cs). Understanding the 137 Cs dynamics of wild boar populations inside the DRZ is necessary because they affect 137 Cs dynamics and wild boar management in areas outside the DRZ. Since few detailed, long-term studies have been conducted inside the DRZ, we measured 137 Cs activity concentrations in 221 wild boar muscle samples obtained from wild boar caught inside the DRZ and surrounding areas over a 5-year period. Our results showed that the 137 Cs activity concentration in wild boar from inside the DRZ were higher than those in wild boar outside this zone. No significant difference was observed between muscle and soil 137 Cs levels, but significant correlations were observed between muscle 137 Cs activity concentrations and body length and body weight in the low-activity-concentration season, but not between all seasons and the high-activity-concentration seasons. It is considered that the size effects observed during the low-activity-concentration season may be due to factors related to metabolism and changes in food habit. This is the first long-term survey of 137 Cs in wild boar inside the DRZ.