1967 年 17 巻 5 号 p. 206-213
Regular surveys were made in 1965-1966 of the guppy population, which had been acclimatized in a short stream of polluted hot spring water in Togura-Kamiyamada spa (N 36°29', E 138°05') about ten years ago. Water temperature of the habitat was maintained at about 20℃ even in winter. These guppies were not cold-resistant. They showed so rapid growth and so great fecundity that they could attain an extreme high level in number (more than 2000/m^2). In summer they began to disperse to irrigation streams and rice fields, and the population number of the guppies in the short stream remarkably decreased. The guppies were euryphageous and they ate even attached bacteria and deposits of the stream. Four characters of tail figure were observed in the male fish. No tendency of shortening in tail length of the male in the natural conditions was observed.