Coral bleaching impacts cortisol levels of coral reef fishes

Abstract Mass coral bleaching events, driven by increasingly frequent and intense marine heatwaves, pose a growing threat to coral reefs. While declines in reef fish populations are often associated with bleaching-induced coral mortality, physiological mechanisms underpinning these declines remain poorly understood. We investigated hormonal and energetic responses in three Pomacentrus species with varying dependency on live corals on the northern Great Barrier Reef prior to (2021 and 2023) and during (2024) the Fourth Global Coral Bleaching Event. Baseline cortisol and lipid levels (proxies for stress and energy stores, respectively) were measured alongside fine-scale benthic surveys that quantified local habitat composition and the severity of coral bleaching. The obligate coral dweller P. moluccensis exhibited significantly elevated baseline cortisol during the 2024 bleaching event, with concentrations ~ 2.2-fold higher than in 2023, and cortisol concentration positively related to local bleaching severity. Similarly, the rubble-associated P. chrysurus displayed elevated cortisol during the bleaching event, with concentrations ~ 1.9-fold higher than in 2023, although cortisol levels were unrelated to bleaching extent. In contrast, the facultative coral-dwelling P. amboinensis showed no change in baseline cortisol concentration across sampling points or with bleaching extent. Lipid content increased across all three species from 2021 to 2023, with a further significant rise in P. moluccensis during bleaching, that may reflect stress-induced metabolic shifts and/or opportunistic feeding on organic matter from bleached corals. Collectively, these results highlight species-specific physiological responses to bleaching, shaped by coral dependence, and provide a mechanistic insight into differential vulnerability of reef fishes to climate-driven disturbance.