Coral health status before and after the tourism halt caused by the COVID-19 pandemic in Koh Tao (Thailand)

Blue tourism is often the primary income for small tropical islands, but uncontrolled over tourism can cause irreversible detrimental effects to coral reefs. Among impacts of tourism growth, recreational diving can physically damage corals through direct contact or sediment resuspension, while nutrient enrichment, marine litter and increased sediment loads into the sea can reduce corals' immunocompetence and increase their susceptibility to disease. This study assessed the effects of decreased human activity during the COVID-19 pandemic on Koh Tao' coral reefs, (Thailand), comparing the coral health status in the pre- (2016-2019), during- (2021), and post-pandemic (2022-2024) periods. Underwater surveys were conducted from 2016 to 2024 across five bays by considering seven main coral health status categories and multiple subcategories. Results showed a general worsening of coral health conditions from the pre- to the post-pandemic period; diversity declined, with three coral families disappearing in one of the sites, while an overall increase in Acroporidae and severe reduction in Fungiidae were observed everywhere. Bleaching and White Syndrome were the primary disorders affecting corals, while Skeletal Eroding Band, Yellow, Brown, and Black Band diseases appear to develop as secondary infections. The combined effects of extreme temperature and intense rainfall together with decennial unregulated coastal development have caused a dramatic decline in reef health and biodiversity. This suggests that a brief tourism halt was insufficient to reverse chronic disturbances caused by climatic and anthropogenic stressors. Coordinated conservation actions are urgently needed to protect coral reefs in Koh Tao and in small tropical islands with high levels of unregulated tourism in general.