In order to clarify the ecology with the river ascension of the larval and early juvenile Japanese sea bass Lateolabrax japonicus, larva net sampling and environmental observations were made in the Chikugo River estuary of Ariake Bay. A total of 3297 fish (SL, 10-24mm) was collected with 274 tows in 1981-1983.
Preliminary notes on the ascending mechanism of the sea bass are given. In the vertical research across the river, the ascending sea bass larvae were concentrated near the bank, and the catch on flood tide is larger than on ebb tide both in the middle and near the bank of the river. The catch in the surface layer is more than that at the bottom (the ratio, 1:0.55).
Comparative examination of yearly abundance and mean length was made, including results in mid 10-day of March during 1979-1983. There was a 10-fold difference in yearly abundance around the river mouth area. The range of yearly variation in the mean body length was more than 2mm.
Ages and spawning season were determined from daily growth increments in the otoliths. One increment was formed each day after hatching. Spawning season extended from November to the next February, and the peak spawning was in January. The mean hatching date slightly changed each year. Estimated standard length was almost the same for sea bass of the same age (100 days after hatching). The sample shrinkage by preservation and percentage of water contents was also examined.