VISITING academics have joined local conservation groups in calling for steps to ensure that the mangrove marshland at Mai Po is protected from development. The World Wide Fund for Nature, which runs the reserve, and Friends of the Earth, said developers were being allowed to encroach on to the reserve. International experts also fear parts of the 330-hectare Site of Special Scientific Interest may be developed after 1997. Fourteen environmentalists and researchers are spending two weeks at the New Territory reserve, following a conference at Hong Kong University of Science and Technology. Reserve manager Lew Young said the marshland was shrinking because local landowners were ignoring regulations and filling in ponds vital to prawn life. ''Technically, they are not allowed to do it, but, once the ponds are filled in, the Government may make them plant trees on it. That doesn't help the marshland,'' he said. Lisa Hopkinson, of Friends of the Earth, said the Government needed a conservation policy for the territory instead of allowing piecemeal development. World Wide Fund for Nature conservation officer Ken Chu Wing-hing said that, despite restrictions on land usage imposed in 1991, government action against breaches was very slow. All conservation groups want more government protection to stop the reserve, the home to 250,000 birds each year and vital prawn and fish breading grounds, being chipped away. China has already reclaimed about 100 hectares of marshland for industrial redevelopment on its side of the border. Although the Hong Kong Government has agreed, in principle, to list Mai Po under the Ramsar Convention on Wetlands of International Importance, it has to do so before 1997. If not, China could technically claim the land at Mai Po for development. Australian conservationist David Vance said it would be a catastrophe if the marshland was not properly protected. ''If you get rid of the mangroves, the world wouldn't fall apart,'' he said. ''But, if concrete jungles were built everywhere, the world wouldn't be a very nice place. I can only hope that the Chinese protect the reserve.'' Sarah Wu Po-chu, Principal Assistant Secretary for Planning, Environment and Lands, said the Government was doing all it could. ''I think China is environmentally conscious enough to recognise the reserve,'' she said. ''We are entitled to take enforcement action against landowners in the territory, and we hope that, with additional staff, we could do it more speedily.''