Nearly 57 per cent of the sea area around Kien Giang Province's Phu Quoc Island is affected by coral bleaching.
Experts said the bleaching is caused by increased water temperature, water pollution, use of chemicals and coral exploitation.
The bleaching was revealed in research recently conducted by the Institute of Oceanography in central coastal Nha Trang Province.
Phu Quoc Sea Conservation Zone managers have taken measures to protect the reefs, including raising people's awareness, increased security and surveillance, scientific investigations and observation of water quality and ecosystems.
Phu Quoc, in the Gulf of Thailand, is Viet Nam's largest island, with a total area of 574sq km. The island receives about 280,000 tourists each year and is forecast to have 2 million tourists by 2020.
Within the zone, the west of Hon Thom and Ganh Dau communes are affected by the highest coverage of coral bleaching, with around 90 per cent. In the core areas of Hon Vong, Gam Ghi and Hon Xuong, 20-40 per cent are affected.
During the past three years, authorities have confiscated two tonnes of coral illegally removed, including many rare species. Coral exploitation has been difficult to control.
The zone has a water surface area of over 26,800ha, of which 3,000ha are under strict management, nearly 13,600ha are reserved for ecosystem recovery and over 10,300ha are planned for further development.
The area is located in the northeast, southeast and south of the An Thoi Archipelago under Phu Quoc District, with 6,800ha stretching to Ham Ninh and Bai Thom communes reserved for seagrass conservation and over 9,700ha for coral conservation stretching over Hon Thom Commune.
There are currently 108 species of corals, 135 species of coral reef fish, 3 types of migrating fish, 132 types of molluscs and 6 types of marine mammals living in Phu Quoc sea area.