Seasonal variation of Cesium-137 concentration in Asian black bear (Ursus thibetanus) and wild boar (Sus scrofa) in Fukushima Prefecture, Japan

Abstract

To elucidate and reduce the risk of radionuclide contamination in wildlife caused by the Tokyo Electric Power Company Fukushima Dai-ichi Nuclear Power Station accident, it is important to understand radionuclide variations in the wild animal population. Here, we used environmental monitoring data and muscle samples collected from Asian black bear (Ursus thibetanus) and wild boar (Sus scrofa) from May 2011 to March 2016 to examine seasonal variation in radiocesium (137Cs) concentrations in muscle tissues (hereafter, muscle 137Cs) of these important game species in Fukushima Prefecture. We measured muscle 137Cs of bears and wild boars killed by hunters or in animal control culls. First, using a linear mixed model (LMM), we tested for a positive relationship between muscle 137Cs and 137Cs in the soil at the site of capture (hereafter, soil 137Cs) estimated from a soil 137Cs deposition map produced by the Japan Atomic Energy Agency. In the LMM, muscle 137Cs was positively related to estimated soil 137Cs, which corroborates the results of previous studies. The LMM regression coefficients differed between the two species, with wild boar muscle 137Cs being higher than that of bears sampled at the same locations. We then employed a generalized additive mixed model (GAMM) to estimate seasonal variation in the muscle 137Cs of the target species. GAMM showed that muscle 137Cs varied seasonally and that this seasonal variation also differed between the two species. In bears, muscle 137Cs decreased from spring to early autumn, before increasing in winter. Wild boar muscle 137Cs remained low during spring and summer and was high during autumn and early spring. These patterns are likely influenced by differences in diet, habitat use, and physiology between these two species.

Relationship between radiocaesium in muscle and physicochemical fractions of radiocaesium in the stomach of wild boar

“…3). Seasonal variations were observed in the activity concentration of 137 Cs in the muscle tissue of wild boars in Fukushima, and the activity concentration of 137 Cs decreased from April to August, increased from September www.nature.com/scientificreports www.nature.com/scientificreports/ to November and remained high from December to March; 19 the trend in the activity concentration of 137 Cs in muscle in this study was roughly consistent with seasonal variations. Figure 4 shows changes in the activity concentrations of 137 Cs in the physicochemical fractions of the stomach contents.…”

“…In this study, we examined the relationship between the physicochemical fractions of 137 Cs in the environment and the bioavailability of 137 Cs in wild boars (Sus scrofa), which mainly inhabit the forests and countryside surrounding the FDNPP, and for which investigational data on the transfer of 137 Cs in the environment have been accumulated. Monitoring results in Fukushima Prefecture after the FDNPP accident showed that wild boar tended to have particularly higher activity concentrations of 137 Cs in muscle tissue than did other wildlife species 10,19 . Studies have also shown that wild boar captured in areas with similar levels of soil contamination exhibit extremely large inter-individual variations in the activity concentrations of 137 Cs 19 .…”

“…Monitoring results in Fukushima Prefecture after the FDNPP accident showed that wild boar tended to have particularly higher activity concentrations of 137 Cs in muscle tissue than did other wildlife species 10,19 . Studies have also shown that wild boar captured in areas with similar levels of soil contamination exhibit extremely large inter-individual variations in the activity concentrations of 137 Cs 19 . Studies conducted after the Chernobyl Nuclear Power Plant accident also reported that wild boar had high activity concentrations of 137 Cs, which enters the muscles after the digestion of food, and that the accumulation of 137 Cs occurred over extended periods [20][21][22][23] .…”

“…Regarding the activity concentration of radiocaesium in wild boar after the FDNPP accident, a monitoring survey conducted by the Fukushima Prefectural Government in 2017 detected animals in which the fresh mass (hereinafter, "FM") activity concentration of radiocaesium was several thousand to a ten thousand Bq kg -1 at more than 6 years after the accident 24 . In addition, seasonal changes have been reported in the activity concentration of 137 Cs in wild boar muscle; 19,25 however, the factors that affected these fluctuations in wild boar after the FDNPP accident remain unclear.…”

Radiocesium concentrations in wild boars captured within 20 km of the Fukushima Daiichi Nuclear Power Plant

“…Second, time trends and seasonal variations were still difficult to assess. Actually, seasonal change in radionuclide contamination in wild boar remains controversial 14,25 . Changes in food sources, eating habits, the natural environment, and human behavior may all affect radionuclide concentrations; therefore, continuous measurements are needed to determine how seasonal change affects the concentration of radiocesium in wild boars.…”