A HONGKONG University student doing a doctoral thesis on mangroves was dismayed when he returned to his site of study and found most of the area had been illegally chopped down. The site at Tsim Bei Tsui, near the Mai Po marshes, is designated as a protected area but villagers moved in and dredged it, apparently in preparation for oyster cultivation. It is illegal to change land use without permission from the Town Planning Board, which the villagers did not have and which they failed to obtain in an application made after the dredging. For Mr Gordon Maxwell, who is comparing mangroves in Hongkong, Brunei and Thailand, the dredging has turned his thesis on its ear. He had already found that the Tsim Bei Tsui site contained hardier species than those in Brunei and Thailand, making them suitable for re-planting, and that the species were different from those in the Mai Po marshes. ''I was completely knocked below the belt,'' he said. ''My research has been ripped apart and turned into a moonscape.'' Now, Mr Maxwell said, his work would be no more than a historical record of what had once been available and would have little value in terms of helping to develop mangroves around the region. ''The Philippines has managed to keep some spectacular pieces of mangrove despite the pressure on their economy, and so has Thailand. Why aren't we doing it here with all our money?'' he said. Mr Maxwell had been out of the territory for almost a year to do field work in Thailand and Brunei, and had no reason to expect the site would be touched. The villagers destroyed the mangrove stand last summer, even though it sits in a Site of Special Scientific Interest and no development or construction work has been permitted since town planning laws were amended in 1991. Their organisation, the Tang Yau Kung Tong, holds the lease to the land but had not used it for years. They asked the Town Planning Board to approve their development work in December, months after they had dredged it, but this was turned down in March and they now must go through a lengthy review and appeal process. If they lose, they could be made to re-instate the land to its original use - a difficult task in the case of mangroves - or face prosecution. In the meantime, the Yuen Long District Lands Office has persuaded them to open up some of the water channels in the mangrove to allow the few remaining trees, which are in poor condition, to get fresh water. District Lands Officer Mr Allan Hay said: ''The situation at the site has improved, although obviously the mangroves that were dredged up or whatever cannot be rehabilitated.'' Mangroves are highly productive ecosystems equivalent to rainforests and coral reefs. They serve as nursery grounds for fish, shrimps and crabs, protect the flora and fauna which are at the base of the food chain and clean the water that washes through them of pollutants.