Abstract
Age composition, growth, sex ratio and gonad development of the steelhead trout and the Kamchatkan trout in the North Pacific were examined based on the data which were collected through Japanese research operations from 1972 through 1982.Most of the fish sampled spent one or two years in the sea.Females tend to occur predominantly in the western waters of the range in all months.It is considered that the greater proportion of the specimens in the western waters is S.mykiss and in the eastern waters is S.gairdneri based on the analysis of the temporal and spatial density.Therefore, it is suggested that there are significant differences in anadromy by sex between both species.The differentiation of mature and immature forms of the fish was estimated from the seasonal change in the gonad weight distribution.This indicated that high concentrations of immature fish occur widely in the central North Pacific in summer.Based on the obtained data, migration patterns of S.gairdneri and S.mykiss in the North Pacific are discussed.
