Long-term changes in the susceptibility of corals to thermal stress around Phuket, Thailand

The bleaching susceptibility of 28 coral taxa around southern Phuket was examined in four natural major bleaching events, in 1991, 1995, 2010, and 2016. Surveys were conducted by line intercept and belt transect methods. All coral colonies were identified to genus or species-level and their pigmentation status was assessed as: (1) fully pigmented (i.e. no bleaching), (2) pale (loss of colour), (3) fully bleached, and (4) recently dead as a result of bleaching-induced mortality. Bleaching and mortality indices were calculated to compare bleaching susceptibility among coral taxa. In 2016 some of the formerly bleaching susceptible coral taxa (e.g. Acropora, Montipora, Echinopora, and Pocillopora damicornis) showed far greater tolerance to elevated sea water temperature than in previous years. In P. damicornis the higher bleaching resistance encompassed all sizes from juveniles (<5cm) to adults (>30cm). In contrast, some of the formerly bleaching-resistant corals (e.g. the massive Porites, Goniastrea, Dipsastraea, and Favites) became more susceptible to bleaching over repeated thermal stress events. Our results support the hypothesis that some of the fast-growing branching corals (Acropora, Montipora, and Pocillopora) may have life-history traits that lead to more rapid adaptation to a changed environment than certain growing massive species.