Nha Trang Bay (NTB), located off the south-east coast of Vietnam, is a famous tropical tourist site. In the past, the bay was characterised by flourishing coral reefs but, by the end of 2019, more than 90% of the corals were gone. In addition to chronic and complex anthropogenic effects, there was an extensive outbreak of crown-of-thorns starfish (Acanthaster planci), the main natural predator of reef-building corals in the Indo-Pacific, among the remnant coral communities in NTB. By 2019, the mean abundance of A. planci in NTB reached 4.2 starfish per 100 m2. Coral surveys conducted on 10 target sites over a 3-year period ending in 2019 revealed that mean coral cover decreased by 64.4% (coral loss varied from 43 to 95%). The greatest declines were in Acropora and Montipora, with 80.6 and 82.3% reductions in the coverage of these taxa respectively. The combination of nutrient enrichment and predator removal is thought to determine the extent of the outbreak of A. planci in the bay. The present trends in environmental conditions in NTB do not indicate a positive outcome for coral reef recovery in in this area in the near future.

A phase shift from a stony-coral community to an alternative state represented by octocorals and corallimorpharians was recorded on a coral reef at the Hon Noi Islands (offshore area of south-central Vietnam). For a seven-year period, average stony-coral cover decreased from 80% to 4.7%, diversity of reef-building corals decreased from 169 to 17 species, and in the shallow area of a reef (1–4 m depth), the cover of corallimorpharians (mainly Rhodactis indosinensis) and octocorals (mainly Clavularia sp.) increased to 62%. Mass scleractinian mortality could have resulted from an outbreak of crown-of-thorns starfish, Acanthaster sp., which developed in the surrounding area in the last decade. The only significant contributor to stony-coral recovery on the study reef to date was the blue coral, Heliopora coerulea. Given the serious coral decline on the upstream reefs as potential sources for coral recruitment and competitive advantages of corallimorpharians and octocorals over scleractinians, the prospects for rapid scleractinian recovery of the study site are poor.