Assessing protective shading and lowering of coral nurseries during a mass bleaching event on the great barrier reef

Abstract Tropical coral reefs are increasingly threatened by more frequent and severe global stressors induced by Anthropogenic climate change. Restoration-based interventions, such as in situ coral propagation and out-planting, are increasingly being adopted to enhance natural recovery. However, these interventions face the same global stressors affecting natural reefs, and hence also need protection. Shading or relocating corals can reduce the severity of coral bleaching, but how such protective interventions are best suited to coral nurseries on the Great Barrier Reef (GBR) remains untested. We therefore conducted a manipulative field experiment factorially crossing two treatments, adding shade structures and lowering of nurseries, to test the efficacy of decreasing solar irradiance during an ocean heatwave to improve coral-bleaching outcomes at two sites on Opal Reef, GBR in 2024. Metrics of coral health (paling and mortality) were monitored for 138 days. The nursery-lowering treatment (from 4 to 7 m) statistically improved bleaching outcomes (less paling and higher survival) at one site, but not the other. Overall, shading nurseries did not reduce coral paling or mortality, suggesting that irradiance may not have been a primary regulator of bleaching severity, or that thermal stress was not severe enough wherein shading would mitigate bleaching. Our results suggest practitioners should first consider lowering coral nurseries where possible ahead of predicted bleaching conditions as this may provide a low-cost low-effort benefit. Consideration of nursery stock importance, practitioner context, and irradiance projections will further help assess the risk/reward of additional shading. We recommend further research could evaluate different shading regimes (e.g., time and length of shading and percentage shade) on nursery corals under higher irradiance conditions.