Abstract
<title>Abstract</title> <p> Mass bleaching events caused by elevated thermal stress continue to drive widespread degradation of coral reef ecosystems. In the summer of 2024, unprecedented thermal stress (18°C-weeks) in subtropical Okinawa Island, Japan, caused the most severe mass bleaching event recorded on record, coinciding with the Fourth Global Coral Bleaching Event. We assessed the near-term impacts of this event on benthic and fish communities across shallow (1–6 m) and deeper (8–15 m) zones at seven sites across Okinawa Island. Surveys were conducted before (March–April 2024), during (August 2024), and after (January–March 2025) the bleaching event. Bleaching was extensive (69.8 ± 3.2%), resulting in an average 46% decline in hard coral cover. Bleaching severity and coral mortality were strongly mediated by depth and influenced by site characteristics. Benthic shifts were evident, driven by high mortality of <italic>Acropora</italic> and the proliferation of algae on dead coral skeletons, while coral genera previously identified as thermally sensitive (e.g., <italic>Pocillopora</italic> and <italic>Montipora</italic> ) showed resistance to bleaching. In contrast, short-term responses of fish communities were weak and limited to specific functional groups. Fish species richness and Shannon diversity remained stable despite coral loss, and only obligate corallivores showed declines in shallow reefs, whereas herbivores increased. These results provide the first integrated assessment of bleaching impacts on benthic and fish communities in Okinawa Island, highlighting depth-and taxon-specific coral vulnerability and the lagged nature of fish community responses. Sustained monitoring will remain essential to determine whether these patterns persist under continued warming. </p>