An exotic species of fish that eats mosquito larvae is being released in fish ponds in mosquito-breeding black spots. About 2,000 mosquito fish were released into abandoned ponds in Tin Shui Wai yesterday as the government stepped up its campaign to prevent the spread of dengue fever, which can be transmitted by the Aedes albopictus mosquito. The move came a day after the Food and Environmental Hygiene Department issued warnings that the mosquito could breed quickly in sunny weather that comes after prolonged rain. The area ovitrap index, used to monitor mosquito breeding, was 39.2 per cent in Tai Po North, 38.9 per cent in Fanling, 30.2 per cent in Lai King, and 26.4 per cent in Sheung Shui. An alert is issued when the index exceeds 20 per cent. Food and Environmental Hygiene Department officials said they plan to release the fish in ponds in the north New Territories, other fish farm areas and small reservoirs. 'What we need to pay attention to is to ensure that the ecology of the areas will not be affected,' a spokesman for the department said. Agriculture, Fisheries and Conservation officials said the fish would be introduced in confined environments and should not damage the ecology in areas where they are used. The fish, originating from Central America, were imported years ago under earlier anti-mosquito campaigns. Ecologists, however, fear that the fish might compete against some native species such as rice fish, or the rare Romer's Tree Frog. 'We would prefer such introductions did not take place though the mosquito fish are already widely abundant in the territory,' said Alan Leung Sze-lun, WWF senior conservation officer. He said the government should consider native fish species that eat mosquitoes.