Estimating Population Size of the Crested Ibis Nipponia nippon Using Mark Resight Analysis

The Crested Ibis Nipponia nippon became extinct in the wild in Japan in 1981, but was reintroduced on Sado Island in 2008, where the population has reproduced successfully since 2012. A total of 157 ibises is estimated to reside on Sado Island. To facilitate monitoring of survival and mortality, all the released individuals were individually identified through different combinations of color rings, and a uniquely numbered ring. However, because they were overlooked or present in inaccessible nests, not all wild-hatched nestlings were ringed for identification. In addition, the detection rate of the marked individuals might decline with an increase in the population size owing to the limited monitoring effort. Ministry of the Environment (MOE) has determined the number of unmarked individuals by the maximum number of simultaneous observations, and the number of marked individuals has been counted by subtracting the number of individuals that have not been observed for six months. Under such circumstances, the population size could be underestimated. To address this shortcoming, we fitted a mark-resight logit-normal model for estimating unmarked individuals, and a poisson-log model with robust design for estimating total population size including marked individuals, using program MARK. We also compared estimated numbers with observed ones. There were 8.48 unmarked individuals since the first successful reproduction in 2012. The estimated number reached a peak of 30.23 in 2014, but has gradually decreased to 20.30. In all study periods, the estimated number of unmarked individuals was larger than observed ones. However, the estimated number of marked individuals was almost the same as the observed one, but the numbers published by MOE were overestimations. At the end of September 2015, there were approximately 109 released and 41 wild-hatched individuals on Sado Island. When evaluating the reintroduction program, we recommend that the population size of the Crested 51 山階鳥学誌 (J. Yamashina Inst. Ornithol.),48: 51‒63, 2017 ©Yamashina Institute for Ornithology Received 27 September 2016, Accepted 20 December 2016. 1 環境省佐渡自然保護官事務所,〒952‒0105 新潟県佐渡市新穂正明寺1277. E-mail: YUJI_OKAHISA@env.go.jp 2 新潟大学朱鷺・自然再生学研究センター,〒950‒2181 新潟県新潟市西区五十嵐2の町8050. 3 公益財団法人山階鳥類研究所,〒270‒1145 千葉県我孫子市高野山115. * 責任著者. 1 Sado Ranger Office, Kanto Regional Environment Office, Ministry of the Environment, Japan, Niibo-Syomyoji 1277, Sado, Niigata 952‒0105, Japan. E-mail: YUJI_OKAHISA@env.go.jp 2 Center for Toki and Ecological Restoration, Niigata University, Ikarashi Ninocho 8050, Nishi, Niigata, Niigata 950‒2181, Japan. 3 Yamashina Institute for Ornithology, Konoyama 115, Abiko, Chiba 270‒1145, Japan. * Corresponding author. Original Article(原著論文) 52 岡 久 雄 二・永 田 尚 志・尾 崎 清 明 Ibis should be estimated using mark-resight models. The result of this study will also contribute towards increasing accuracy in population estimates in other reintroduction programs.