原著論文: ORIGINAL PAPER

Settlement and upstream migration of the Japanese mitten crab Eriocheir japonica (de Haan)

Satoshi KOBAYASHI

Laboratory of Marine Biological Function, Division of Applied Biosciences, School of Agriculture, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan

小林　哲: モクズガニ Eriocheir japonica の稚ガニ期における着底と河川 遡上過程. Ecol. Civil. Eng. 7(1), 107-108, 2004.

Abstract: Upstream migration in the Japanese mitten crab Eriocheir japonica was investigated in the lower region of the Saigo River and adjacent seas in Fukuoka Prefecture, Japan. Megalopa larvae settled and metamorphosed to the crab stage in the upper tidal area in the river. The youngest crabs were nearly 2 mm in carapace width (CW). After attaining a minimum of 3.6 mm CW, the crabs began to migrate upstream and reached the freshwater area. Settlement of megalopa larvae occurred mainly in mid autumn (October) and late spring (May to June), while small numbers also settled in November and January - February. However, the majority of larvae that settled in winter did not survive and metamorphose to the crab stage due to low water temperatures (<10C). The young crabs that metamorphosed in autumn and winter remained in the tidal area and did not grow throughout winter. Upstream migration of the autumn settlers to the freshwater area was observed from March until June. They started to grow in March, and reached nearly 10 mm CW by June (about 8 - 9 months after settlement). In contrast. the larvae that settled in May to June grew very fast, reaching nearly 10 mm CW in September (about 4 months after settlement). Their upstream migration to the freshwater area began in July and ended by August. Thus, the growth and upstream migration of E. japonica seemed to be strongly influenced by water temperature in the estuary. This mode of settlement with two peaks in autumn and early summer reflected the temporal pattern of reproduction observed in the sea. With respect to engineering works in river estuaries and the release of the crab to rivers, it is recommended that we take into account the processes of settlement and migration of young crabs. We may also require information on the migratory processes of other diadromous crustaceans for their effective conservation and management.

Key words: Japanese mitten crab, Eriocheir japonica, settlement, upstream migration, megalopa larva.

摘要

通し回遊種モクズガニ Eriocheir japonica (de Haan) の稚ガニ期の着底と遡上の過程を明らかにするため, 福岡県宗像郡福間町の西郷川下流域と隣接する海域 (淡水域 3定点, 河川感潮域3定点, 海域 1定点) で 1997年7月〜1998年9月に月2回の割合で素手と手網による採集を行った. メガロパ幼生は河川感潮域の上部に着底しそこでカニに変態していた. 1齢稚ガニは甲幅2mm程度であり, 甲幅 3.6mm以上に成長したのちに 淡水域への遡上が開始された. メガロパ幼生の着底時期は, 秋(10月)と晩春(5〜6月) に2回のピークがみられたが, 11月や1〜2月にも少数確認された. 秋から初冬に定着した個体は 冬の間は成長せず感潮域に留まっていた. 水温が10℃以下に下がる冬に着底した幼生は, 変態できず生残できないことも明らかになった. 冬期感潮域に留まっていた稚ガニは, 淡水域への遡上を3月に開始し6月には終えた. また 3月 に水温が上昇すると成長を再開し, 6月にはほぼ甲幅 10mm に達した (着底後8〜9ヶ月後). 一方, 5〜6月に着底した個体は淡水域への遡上を7月に開始して8月には 終えており, 成長もはやく9月には甲幅10mmに達した (約4ヶ月後). このようにモクズガニの成長と遡上は河川感潮域の水温に強く影響されることが推察された. 福岡では海域で繁殖に参加するモクズガニに産卵時期の異なる2群があり, これを幼生の着底時期と比較すると, 秋から冬に繁殖するグループの産んだ幼生が秋から冬にメガロパとして着底し, 一方冬から春に繁殖するグループからの幼生が, 初夏に着底していると推定された. 河川感潮域での改修工事や堰の建設および稚ガニの放流事業は, 以上のようなモクズガニの着底と遡上のプロセスを考慮に入れて行うことが望まれる. 他の回遊性甲殻類についても同様に, 回遊のプロセスを明らかにし保護事業に活かす必要があるだろう.

Received 3 August 1998; Accepted 2 November 1998
* e-mail: mokuzuk@kais.kyoto-u.ac.jp