Reconstruction of the migratory history of the catadromous fourspine sculpin Cottus kazika from the Monobe and Shimanto River, Kochi prefecture in southern Shikoku, using otolith Sr/Ca concentration ratios

カマキリ当歳魚の遡上履歴を推定する目的で，耳石輪紋の日周性ならびに耳石Sr/Ca比と塩分濃度との関係について調べた。その結果，本種の耳石輪紋は日周輪であること，および耳石Sr/Ca比は海水中で顕著に上昇することを確認した。これらの結果をもとに，高知県四万十川および物部川で採集したカマキリ当歳魚の遡上時期を推定したところ，海水から淡水への移行期は，いずれの河川においても3月上旬と推定された。The fourspine sculpin (Cottus kazika) is a catadromous member of the family Cottidae that is endemic to Japan. Our objective was to evaluate the utility of using sagittal otolith Sr/Ca concentration ratios to document the movement of juvenile sculpin between the estuarine areas and freshwater of the Shimanto and Monobe Rivers in southern Shikoku. We confirmed the daily periodicity of increment formation and the correlation between the otolith Sr/Ca ratio and ambient salinity at laboratory condition. Otolith profiles suggested that juveniles tended to move from seawater to freshwater at the beginning of March in both river systems