Abstract
The mosquito fish (Gambusia affinis) that was transferred to Tokushima City, Tokushima Prefecture in 1969 was studied in relation to the control of mosquitoes. A number of the fish were collected at several ponds and ditches where the fish had increased in number and were repeatedly released into swamps, sewers, fallow rice fields, etc. They were established in 7 places in 1970, 36 in 1971, 76 in 1972 and 104 in 1973. In 27 of those places, both the medaka (Oryzias latipes) and mosquito larvae co-existed, but after the establishment of Gambusia in these places, no mosquito larvae were found. The population density of Oryzias became low and that of Gambusia increased. The water areas with high densities of Gambusia were still or had an inflow of household waste water causing eutrophy. When Gambusia and Oryzias were found together in a water area, the total number of fish was as large as 400-500 per 1 m2. The result from laboratory observations demonstrated that Gambusia prey on more mosquito larvae than Oryzias. If Gambusia were raised together with Oryzias in the same water tank, Gambusia attacked and eventually exterminated Oryzias. The feeding habits of Gambusia and Oryzias depend on the availability of food in the environment and are highly variable. It was shown that both species are involved in the competition for space and food in connection with their ecological niche. The above results indicated that Gambusia is more suitable for the control of mosquito larvae than Oryzias. In fact, it was observed that Gambusia became more predominant and more expansive in its habitat in the water areas examined.
