Observations on Gambusia affinis introduced into Tokushima as a natural enemy of mosquitoes

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Abstract
The top minnow, Gambusia affinis (Baird et Girard, 1853), is a viviporous poeciliid fish native of the southern United States, and has been introduced into a number of countries mainly with the purpose of malaria control. In Japan, the fish is known to be imported in 1916 via Hawaii and Taiwan, and has been found recently to be breeding widely in polluted waters in and around the city of Tokyo. The present paper deals with the results of observations on Gambusia introduced from Tokyo to Tokushima City of Shikoku Island in April 1969. Tokushima is a city situated on the delta of Yoshino River, where tremendous numbers of mosquitoes were breeding every year in swamps, ditches and rice paddies difficult to be drained. Surveys were carried out on the larval breeding places, and on the seasonal prevalence of adults with light traps set at 6 stations. Of 14 species of mosquitoes found in this area, Culex pipiens pallens was predominant and most widely distributed, while Anopheles sinensis and Culex tritaeniorhynchus were also found abundantly in some swamp areas. In some salt marshes near the sea coast, tremendous breeding of Aedes dorsalis was observed in the rainy seasons. Under such environmental conditions, it was considered by us that the introduction of Gambusia might be an effective measure for the control of mosquitoes, and two shipments of about 400 fishes collected from sewage ditches in the Tokyo International Airport were made to Tokushima by air cargo in April and May, 1969. The fishes were released and bred first in certain ponds and pools, and were further distributed to other ponds, swamps and ditches from the autumn of 1969. Especially successful and effective was the culture of Gambusia in the salt water marshes where Aedes dorsalis had been breeding; from August 1970,one year after the first batch of some 500 fishes were released, they became visible in high population densities all over the swamps, and thereafter complete eradication of the mosquito larvae could be achieved from these areas. Field and laboratory studies were made with Gambusia thus successfully established in large number of pools in and around this city. The results are described in this report on its distribution, the trend of increase in the population densities, the competition with the indigenous fishes such as Oryzias latipes, the capacity as a predator of mosquito larvae, the correlation of the number of youngs borne with the body size of the mother fishes, the intervals of the births, the resistance to the cold temperature and to various poisonous substances in water, etc.

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