Bimodal distribution of coral bleaching prevalence is consistent with state transition dynamics in thermal stress response: five years of standardised monitoring in Japan

Coral bleaching is conventionally modelled as a continuous response to cumulative thermal stress, yet the distributional structure of site-level bleaching prevalence has rarely been examined. Here we analyse five years (fiscal years 2020–2024) of standardised bleaching surveys from Japan’s Monitoring Site 1000 program, encompassing 2,288 observations across 585 survey points at 26 sites in the Ryukyu Archipelago and adjacent waters. We document three principal findings. First, bleaching prevalence is bimodally distributed in all five years: the intermediate range (20–80%) remains stable at 21–27% of observations, while inter-annual variation is driven by redistribution between low and high domains — confirmed by Hartigan’s dip test (all p < 0.001) and beta mixture modelling (ΔBIC = 9.0–113.9). Second, the 2022 and 2024 bleaching events are qualitatively distinct: 2022 was a partial mass bleaching (positively skewed, selective), while 2024 was comprehensive (symmetric, median 60.0%). Third, a simple threshold metric (days above 30°C) outperformed Degree Heating Weeks in discriminating bleaching across all severity levels (GEE-based AUC: 0.877 vs 0.624 at ≥50% prevalence, p < 0.001; 0.830 vs 0.633 at >0%, p = 0.007), indicating that metric structure and the ecological severity threshold defining the outcome are inseparable design considerations.