Abstract
Upstream migration in the Japanese mitten crab Eriocheir japonica was investigated in the lower region of the Saigo River and the 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 (< 10°C). 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.
