Distribution areas of larvae and early juveniles of Trachidermis fasciatus were investigated in the northern Ariake Sound and the fluenting Kashima River waters during the period from February to May in 1994. The larvae (7~16 mm TL) appeared at stations near the coasts of the Ariake Sound where salinities ranged from 16 to 25 ppt and larvae and pelagic juveniles (13~20 mm TL) appeared in stations from the mouth of the Kashima River to 3 km upstream ranging from 3 to 25 ppt in salinities. The settled juveniles were collected at stations in the river where salinities were almost 0 during low tide.
Early larvae, pelagic juveniles, and settled juveniles hatched and reared in aquaria were exposed to various salinity conditions (0, 11, 23, and 34 ppt). Larvae most rapidly grew at 34 ppt, but settled juveniles more rapidly grew and their survival rates were higher in low salinities.