Spatial Behavior and Long-Term Decline of Radiocesium in Green Pheasants (Phasianus versicolor) in Fukushima: A Left-Censored Regression Approach

Hisashi Komatsu , Kousuke Kanda, Kiemi Murakami

Long-term monitoring of radiocesium (137Cs) in wildlife is important for assessing environmental recovery after the Fukushima Daiichi Nuclear Power Plant accident. We integrated GPS telemetry data with muscle 137Cs concentrations in green pheasants (Phasianus versicolor) to evaluate whether observed declines in radiocesium are consistent with reduced local environmental availability. Winter telemetry indicated highly restricted movement, with 95% KDE home ranges of 1.0–7.5 ha, suggesting that muscle 137Cs concentrations primarily reflect local contamination conditions. Using monitoring data from 2011–2024, the primary left-censored lognormal regression model (n = 370) estimated a significant temporal decline in muscle 137Cs concentrations. This trend was supported by supplementary mixed models examining detection probability (n = 379) and concentrations among detected individuals (n = 206). These results suggest that radiocesium concentrations in green pheasants in Fukushima are declining in a manner consistent with reduced local environmental availability.