A $620,000 Hong Kong University project on conservation of freshwater wetlands aims to save ''probably the most threatened type of wetland in Hong Kong'', says Dr David Dudgeon, partner in the project with Dr Richard Corlett. Most such wetlands - if not filled in and used as profitable container parks - were low-lying grassy areas, former fishponds or paddies, and would look to most people like marshy, muddy wasteland for most of the year, he said. Yet many would be home to unique plants or animals, particularly pitcher plants, a protected species in Hong Kong. The size of the area had little to do with its environmental value. For instance, the beleaguered, fingernail-size Romer's tree frog that had to be moved from Chek Lap Kok when the airport interfered with its home would not be interested in big areas as it liked small ponds that dried out for part of the year - ''whereasponds that dried up would not be much good for fish''. The three-year study is to produce a conservation management plan to point out which ponds are vital and which, if necessary, can be sacrificed. ''We want to be able to stick red flags in the map and say 'these [areas] are important','' Dr Dudgeon said. ''We do need to be able to pinpoint quickly where the important ones are.'' Another $620,000 project, to be carried out by Professor Wong Yuk-shan of Hong Kong University of Science and Technology and Dr Nora Tam of City Polytechnic, will do a similar job for Hong Kong's mangroves, to decide which are the most valuable stands. Mai Po marshes were the territory's best-known mangroves, but there were other groups around Sai Kung in the New Territories and Lantau, Professor Wong said. Part of the project would involve replanting mangroves on disused salt pans at Tai O, to compensate for their loss at Tung Chung due to the airport development, he said. The idea was to replant them in compartments and try to use them to treat waste from the housing developments planned there. The two researchers have done work on this use of mangroves in China. Both Professor Wong and Dr Dudgeon praised the Agriculture and Fisheries Department for backing ecological research, but both said $620,000 was the minimum required. Dr Dudgeon said: ''I'm encouraged that the Government is actually giving some money. But there is an unfortunate perception in Hong Kong that work with real animals and plants is cheap. ''People spend millions of dollars on experiments on rats, and I'm not saying they shouldn't, good for them. [But] the environment we have out there is the only one we've got. When it's gone we can't make another one. But there'll be plenty of time to get other rats.''