抄録
A joint study was conducted for a period over one year on the ecology of the guppy, Lebistes reticulatus Peters, which was discovered in the autumn of 1970 to be breeding in highly polluted water of Yoshikawa River running through the heart of Chiba City neighbouring Tokyo. Being a tropical fish of the family Poecilidae native of South America, the guppy is known widely as a popular aquarium fish, and also as an efficient natural enemy of mosquitoes that breed in sewage waters. Previous observations in Japan have shown that the fish cannot survive outdoors over the freezing temperature during the winter season, but has established as wild colonies in sewage ditches in several hotspring areas, where the water temperature is maintained above 15℃ throughout the year. Yoshikawa is a small river of about 8km in length and 0.2/1,000 in the average slope. The upstream takes sewage water of public housing areas built in the outskirt of the city, and on the spot just before the river flows into the city a large quantity of underground water pumped up from natural gas wells is discharged continuously into the river. The well water has the temperature of about 30℃, and keeps the water temperature of its downstream not lower than 20℃ even during the winter season. The guppies were found to be breeding along the full length of the river during the summer season from July to October, but their distribution became restricted during the winter season to only the downstream of the natural gas well, or in the range where the water temperature was higher than 17℃. Seasonal observations of the chemical, physical and biological characters of the river water were carried out at three stations, at least four times a year. At the Station 2 located about 100m downstream of the inflow of the well-water, and at the Station 1 located 1.5km further downwards and in the business area of the city, the water temperature remained above 20℃ throughout the year and other chemical and biological characters were kept almost constant in these observations. The chlorine contents of samples collected from these stations were always very high due to the mixture of about the same amount of the well water showing the salinity as high as the sea water, but excellent reproduction of the guppies were seen in this zone. At the Station 3 located upstream of the well, the water temperature varied from the lowest of 11.5℃ during the winter season to the highest of 26.6℃ in the summer, and the guppies were found in these upstream zones only when the temperature was above 17.5℃. Results of both chemical and biological analysis of the water samples have shown that the river water was highly polluted, with BOD values from 6.5 to 20.7ppm and with the biological system of either beta-polysaprobic or alpha-mesosaprobic. The biological character of Station 1 and 2 remained fairly constant throughout the year, being beta-polysaprobic in the former and alpha-mesosaprobic in the latter, but that of the upstream zone at Station 3 was alpha-mesosaprobic during the summer season and beta-polysaprobic in winter in connection with the scanty rainfall of this season. It is a biologically interesting fact that a South American tropical fish is breeding enormously in a sewage ditch running through a modern city of the temperate zone, highly adapted to pollution of the water. In connection with the recent discovery by a group of Japanese scientists that the guppies are useful in the biological control of some mosquitoes and in the treatment of sewage waters, the ample supply of the fish from this natual breeding place located near Tokyo has made subsequent studies on this line much easier than before.
