Bayesian Estimation of Population Size in Raccoon (procyon Lotor) Using State-space Model Based on Removal Sampling

27 Mar Bayesian Estimation of Population Size in Raccoon (procyon Lotor) Using State-space Model Based on Removal Sampling

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Abstract

千葉県内でアライグマ（Procyon lotor）の高密度地域において行われた捕獲記録を用い，除去法計算過程を状態空間モデルとして構成した階層ベイズモデルによる個体数推定（ベイズ除去法）を行うとともに，CPUEから生息密度へ換算する係数の推定を行った．また，この係数を用い，千葉県内の2012年度の個体数推定を行った．捕獲は，千葉県いすみ市塩田川流域（35.1 km2）において，2012年6月22日～2013年3月23日に100台の箱ワナ（平均近傍距離301 m）を用いて実施された．捕獲の結果，オス成獣53頭，メス成獣29頭，幼獣55頭の計137頭が捕獲された．ベイズ除去法による推定の結果，捕獲開始前の生息数はオス成獣が89頭，メス成獣が103頭，幼獣が130頭，計322頭と推定された．捕獲期間の3か月間で，メス成獣および幼獣の76%以上を除去することができたが，オス成獣は生息密度を維持しており，捕獲開始直後に優位オスが除去されたのち，隣接地域からの放浪個体が移入することが推測された．

English Abstract

Raccoon population size (Procyon lotor) and the conversion factor from CPUE to raccoon density were estimated using Bayesian removal sampling methods based on a state-space model with capture records from a high density area in Chiba prefecture, Japan. Raccoons had been captured via pest control in the Shiota River basin (35.1 km2) in Isumi City, Chiba prefecture, from June 22, 2012 to March 23, 2013 with 100 cage traps (mean nearest distance was 301 m). Fifty three adult males, 29 adult females and 55 juveniles (total 137 raccoons) were captured via pest control. The population size of each age-sex class was estimated as 89 adult males, 103 adult females and 130 juveniles (total 322 raccoons). Although more than 76% of adult females and juveniles could be removed using 100 cage traps in the first three months of the pest control, the density of adult males was maintained throughout the whole control period. We suggest that unchanged density of male raccoons can be maintained by the dispersal of young male raccoons from their tal areas, and of nomadic adult male raccoons from adjacent areas.