High water temperature causes mass coral bleaching, death in Con Dao: experts

Coral reefs in the Con Dao archipelago off the southern province of Ba Ria-Vung Tau have been bleaching and dying due to abnormally high water temperatures, according to the Con Dao National Park management.

The management and the Institute of Oceanography carried out a study at the end of May that discovered that sediments and being eaten by spiny starfishes were also affecting the corals.

Bleaching has occurred at 80-100% of reefs in places in the east such as Dam Tre Bay and Hon Cau, Hon Tai and Cuc Ga islets.

Some 15-20% of the coral there have died, with most losing their polyps and being covered in mosses.

In the western area, particularly Hon Tre Lon and Hon Tre Nho islets and Ong Cuong and Ong Dung beaches, 60 to 70% have been bleached and 10% have died.

Abnormal water temperatures in Con Dao have caused the corals to expel the colourful algae living in their tissues, leading to mass bleaching, according to Thai Minh Quang of the institute.

The ideal temperatures for corals’ survival range from 24°C to 30°, but it was 32°C at the time of the observation.

If the high water temperatures continue, they will lower the recovery rate of the reefs and cause a large number of them to die.

Besides, corals off Ong Cuong Beach and Hon Cau and Hon Tai islets have been eaten by spiny starfishes that also suppress their larvae from replenishing.

Those off Ong Cuong Beach have been affected by sediments, which can contain toxicants, pathogens and nutrients that impact coral health and kill them almost instantly.

Reports show that mass coral bleaching in Con Dao started in May, reached its peak around June 2-5 and might last until June 20.

Nguyen Khac Pho, director of the Con Dao National Park Management Board, said more observations and assessments would be critical for evaluating the impacts of mass bleaching and recovery rates of coral reefs, and deploying restoration solutions.

He said the park would postpone restoration programs since corals have a very low recovery rate after bleaching, and extracting larvae for restoration could also weaken adults.

Besides, socio-economic activities such as fishing, tourism and diving services around conservation areas in the park should be minimized for three months after bleaching, he said.

The park would also collect spiny starfish and promptly develop a plan to revive severely affected coral reefs, he said.

Coral reefs in Vietnam, especially Con Dao, have suffered bleaching periodically in the last 20 years, notably in 1998, 2000, 2002, 2010, 2016, and 2019.