The Department of Public Health has instructed all districts in the State to take up biological methods such as introducing larvivorous fishes to control mosquito breeding, especially the malaria vector.

The fish are to be introduced in all unused wells in rural and semi-urban areas besides in freshwater such as stagnant ponds and slow-moving streams before the mosquito breeding season in order to maximise its impact.

R. Damodharan, Deputy Director of Health Services, told The Hindu here on Monday that Coimbatore district had already begun rearing Gambusia affinis , a species that originated from the U.S. that can adapt to local water conditions. Distribution of the fish began in the city on Sunday through the primary health centres.

While indigenous larvivorous species such as haplochilus and barbus were also available, he said these species had low tolerance to changes in temperature and oxygen levels.

Each well must have at least a dozen fish, with the male to female ratio being in the ratio of 1:3 in order to effectively control mosquito breeding. Care must be taken to ensure chlorination was not done in the water bodies where the fish were being introduced.