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

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 (Cs-137) concentrations in muscle tissues (hereafter, muscle Cs-137) of these important game species in Fukushima Prefecture. We measured muscle Cs-137 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 Cs-137 and Cs-137 in the soil at the site of capture (hereafter, soil 137 Cs) estimated from a soil Cs-137 deposition map produced by the Japan Atomic Energy Agency. In the LMM, muscle Cs-137 was positively related to estimated soil 137 Cs, which corroborates the results of previous studies. The LMM regression coefficients differed between the two species, with wild boar muscle Cs-137 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 Cs-137 of the target species. GAMM showed that muscle Cs-137 varied seasonally and that this seasonal variation also differed between the two species. In bears, muscle Cs-137 decreased from spring to early autumn, before increasing in winter. Wild boar muscle Cs-137 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.