Tracking coral bleaching and Chaetodon octofasciatus dynamics in the Gulf of Thailand during the fourth global mass bleaching event

Abstract The fourth global coral bleaching event in 2024 impacted tropical coral reefs worldwide. Focusing on a data-poor region of the Gulf of Thailand, we assessed the responses of corals and the obligate corallivore Chaetodon octofasciatus (Bloch, 1787) to this extreme thermal stress through near-continuous benthic surveys and visual censuses across three reef sites in Koh Phangan from February to November 2024. Sea surface temperatures were on average 0.54 °C higher than in 2023, with cumulative heat stress exceeding 15 degree heating weeks by June. This triggered severe bleaching, affecting almost 45% of coral colonies and reducing hard live coral cover by 19% relative to February, followed by partial recovery by October. Despite these losses, coral cover remained higher than regional averages. C. octofasciatus exhibited rapid life-stage-specific responses. Adult abundance remained stable during bleaching but declined post-bleaching compared to pre-bleaching. Individual mass decreased during bleaching and recovered thereafter. Juvenile settlement was rare, occurring exclusively on Acropora and Pocillopora and never on fully bleached colonies. By providing one of the first assessments of the 2024 global bleaching event from a data-poor region, this study emphasizes the importance of integrated benthic–fish monitoring to anticipate species-specific responses to climate change in tropical reef systems.