Evaluation on naturally-recruited/stocked Japanese eel distribution based on otolith stable isotope ratios.

2021 Fiscal Year Final Research Report

• Project/Area Number: 18H02225
• Research Category: Grant-in-Aid for Scientific Research (B)
• Allocation Type: Single-year Grants
• Section: 一般
• Review Section: Basic Section 39060:Conservation of biological resources-related
• Research Institution: Chuo University
• Principal Investigator: Kaifu Kenzo 中央大学, 法学部, 教授 (30615258)
• Co-Investigator(Kenkyū-buntansha): 白井 厚太朗 東京大学, 大気海洋研究所, 准教授 (70463908)
• Project Period (FY): 2018-04-01 – 2022-03-31
• Keywords: ニホンウナギ / 放流 / 耳石 / 安定同位体比

Outline of Final Research Achievements

Carbon and oxygen otolith stable isotope ratios were measured for Japanese eels that were captured in Japanese rivers. Those eels were classified into naturally-recruited/farmed-and-stocked individuals based on a discrimination model we developed. Along with the previously reported data, we obtained ratios of naturally-recruited/farmed-and-stocked individuals for 21 water systems in 17 prefectures (876 individuals). Ratio of farmed-and-stocked individual were from 41.4% in total with a range from 0.0 to 100.0%. Farmed-and-stocked individuals were dominant in Japan Sea coast where few glass eels recruit. The ratio of farmed-and-stocked individuals tended to be greater in waters that have dams in their lower reaches, even though the water systems are flowing into the Pacific Ocean or the Seto Inland Sea.

Free Research Field

保全生態学

Academic Significance and Societal Importance of the Research Achievements

環境省は2013年にニホンウナギを絶滅危惧IB類に区分した。この時の評価に用いられた主なデータは、日本の内水面ウナギ漁獲量である。しかしながら放流個体が再生産に参加する確証はないため、ニホンウナギの個体群動態を正確に理解するにあたり、放流個体の存在は一種のコンタミとなっている。本研究により、水系ごとに大きな違いはあるものの、日本の内水面漁場に生息するニホンウナギのうち、放流ウナギがかなりの割合を占めることが確認された。本研究で試行したように、今後は天然遡上個体が優占する水域でデータを得ることにより、より正確にニホンウナギの個体群動態を把握できるようになる。