Though a large number of rainbow trout had been stocked in Japan, after the first introduction from the United States in 1877, establishment of natural population is seldom witnessed.
In the three tributaries of upper Tama River drainage, rainbow trout believed to be naturally spawned (60 to 173mm in total length) and spawning redds were recognized from 1976 to 1984. A study was made on the analysis of environment of the streams and origin of rainbow trout caught there to determine the factors which made natural spawning possible. Since such small fish nor eggs were stocked there, they were believed to be definitely the result of natural spawning.
Rainbow trout believed to be naturally spawned and spawning redds were not recognized in the ordinary fishing area (catchable fish stocked once just before opening of season), but recognized only in or near the special fishing area (catchable fish stocked daily or weekly throughout season). Though almost all of the fish stocked into ordinary area are removed within several days of opening of season, there are recognized surviving fish in the special area even after close of season. So, rainbow trout believed to be naturally spawned might owe their origin to the survived parent fish stocked into special area.
The rainbow trout are stocked into most of the streams in Japan through the same way as done for the ordinary area of the Tama River. So one of the important reasons of scarce establishment of natural rainbow trout population in Japan is supposed to be removal of stocked fish by high angling pressure.