Journal of Fisheries Technology, 3 2, , , , 2011 The Development of Artificial Spawning Grounds for Ayu, Plecoglossus altivelis

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1 Journal of Fisheries Technology, 3 2, , , , 2011 The Development of Artificial Spawning Grounds for Ayu, Plecoglossus altivelis altivelis Masayoshi KONDO, Koichi IZUMIKAWA, Katashi OSAKADA, Kiyoto OHTSUKI and Naoki SASADA On the Yoshii River upstream of the Tomata Dam, we developed four artificial spawning grounds at different times or with different methods, comprehensively investigated changes in the physical environment, transition of eggs, downstream transport of fish fry, etc., and examined techniques for developing artificial spawning grounds. As a result, we found that the gravel supplying method was more effective than the plowing method. However, when there was no place suitable for spawning in the area, it appeared that even spawning grounds developed with the plowing method were utilized effectively. We concluded that the gravel supplying method for reinforcing a natural spawning ground could be utilized more effectively when it was developed at the early stage of spawning. Since it is difficult to change a stream bed gradient with any method during spawning ground development, it is necessary to select places that have a stream bed gradient and flow velocity that are gentle enough so that gravel that is suitable for spawning will not be washed away Plecoglossus altivelis altivelis 1 2, Okayama Prefectural Technology Center for Agriculture, Forestry, and Fisheries, Research Institute for Fisheries Science, 35, Kashino, Ushimado, Setouchishi, Okayama, , Japan masayoshi_kondou3@pref.okayama.lg.jp 137

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7 であったこの産卵場間は岩や産卵に不適なサイズの石で構成されておことから 7 月から 8 月にかけてアユの分布は広範囲とり

点付近に移動し成熟個体の割合が高くなることが分か産卵場でふ化したものと考えられ人工産卵場の有効性ったを確認できた流下仔魚の採集数が増加したのは

とすると 2008 年 9 月上旬以降が仔魚調査結果を表 3 に示した採集した流下仔魚の合計産卵盛期となり人工産卵場の造成時期に問題はなかっ図 8

6 日の分布は奥津湖合流点から上流 100 m の間に 53.5 は産卵場直上が 18 尾直下が 21,503 尾でそれぞれののアユが見られ, 婚姻色の割合は 75.7 であったこの産卵場間は岩や産卵に不適なサイズの石で構成されておことから 7 月から 8 月にかけてアユの分布は広範囲とりアユが産卵出来る状態にはなく潜水時にも産卵がなるものの水温の降下とともに奥津湖と吉井川の合流観察されなかったことからほとんどの流下仔魚が人工点付近に移動し成熟個体の割合が高くなることが分か産卵場でふ化したものと考えられ人工産卵場の有効性ったを確認できた流下仔魚の採集数が増加したのは 2008 年 9 月 16 日以降で箱岩橋の平均水温を基にふ化まで流下仔魚調査採集時間を 10 分換算した時間帯別流下の積算水温をとすると 2008 年 9 月上旬以降が仔魚調査結果を表 3 に示した採集した流下仔魚の合計産卵盛期となり人工産卵場の造成時期に問題はなかっ図 8 No. 1 産卵場の物理環境の変化左河床材料中付着泥右河床の状態図 9 No. 2 産卵場の物理環境の変化左河床材料中付着泥右河床の状態図 10 No. 3 産卵場の物理環境の変化左河床材料中付着泥右河床の状態図 11 No. 4 産卵場の物理環境の変化左河床材料中付着泥右河床の状態 142

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9 1 1993, 23 4 pp ,,, T. Nagai, T. Tamura, Y. Iida, and T. Yoneji 2004 Differences in Susceptibility to Flavobacterium psychrophirum among Three Stocks of Ayu Plecoglossus altivelis, Fish Pathology,, , ,,, ,,, , 4 23 pp ,,, ,,, pp ,,,

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