Population outbreaks of Crown-of-Thorns Starfish (COTS; Acanthaster spp.) are a major contributor to loss of hard coral throughout the Indo-Pacific. On Australia's Great Barrier Reef (GBR), management interventions have evolved over four COTS outbreaks to include: (1) manual COTS control, (2) Marine Protected Area (MPA) zoning, and, (3) water quality improvement. Here we evaluate the contribution of these three approaches to managing population outbreaks of COTS to minimize coral loss. Strategic manual control at sites reduced COTS numbers, including larger, more fecund and damaging individuals. Sustained reduction in COTS densities and improvements in hard coral cover at a site were achieved through repeated control visits. MPAs influenced initial COTS densities but only marginally influenced final hard coral cover following COTS control. Water quality improvement programs have achieved only marginal reductions in river nutrient loads delivered to the GBR and the study region. This, a subsequent COTS outbreak, and declining coral cover across the region suggest their contributions are negligible. These findings support manual control as the most direct, and only effective, means of reducing COTS densities and improving hard coral cover currently available at a site. We provide recommendations for improving control program effectiveness with application to supporting reef resilience across the Indo-Pacific. Crown-of-Thorns Starfish (COTS; Acanthaster spp.) are among the largest and most efficient coral predators, and COTS population irruptions (often termed 'outbreaks') are a major contributor to coral loss throughout the Indo-Pacific region 1. To address increasing and elevated densities of COTS, manual control programs, mainly involving lethal injections in situ 2 or hand collections and disposal on shore 3 , have been implemented since the 1960s. By 2014 some 84 manual control programs had been conducted across the Indo-Pacific, killing or removing at least 17 million starfish at an estimated cost of US$15-44 million 1. Such programs have generally had limited success in either suppressing COTS densities or preventing coral loss 3-5. Several authors have suggested that successful manual control is unlikely with large scale or severe COTS population outbreaks 6-9. Given that single-injection methods for culling individual COTS are now close to 100% effective 10 , significant improvements in manual control programs are likely to be found in the strategic and effective deployment of in-water resources to manage COTS impacts on coral reefs 11,12. Despite this, there is little information available on the efficacy of manual COTS control programs nor on their performance relative to other measures implemented to manage population outbreaks of COTS to minimize coral loss. The Great Barrier Reef Marine Park (GBR) is one of the world's largest coral reef ecosystems and a globally recognized world heritage area. Despite its iconic status, the GBR is subject to a range of anthropogenic threats, and, like tropical coral r...

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On the iconic Great Barrier Reef (GBR), the cumulative impacts of tropical cyclones, marine heatwaves and regular outbreaks of coral-eating crown-of-thorns starfish (CoTS) have severely depleted coral cover. Climate change will further exacerbate this situation over the coming decades unless effective interventions are implemented. Evaluating the efficacy of alternative interventions in a complex system experiencing major cumulative impacts can only be achieved through a systems modelling approach. We have evaluated combinations of interventions using a coral reef meta-community model. The model consisted of a dynamic network of 3753 reefs supporting communities of corals and CoTS connected through ocean larval dispersal, and exposed to changing regimes of tropical cyclones, flood plumes, marine heatwaves and ocean acidification. Interventions included reducing flood plume impacts, expanding control of CoTS populations, stabilizing coral rubble, managing solar radiation and introducing heat-tolerant coral strains. Without intervention, all climate scenarios resulted in precipitous declines in GBR coral cover over the next 50 years. The most effective strategies in delaying decline were combinations that protected coral from both predation (CoTS control) and thermal stress (solar radiation management) deployed at large scale. Successful implementation could expand opportunities for climate action, natural adaptation and socioeconomic adjustment by at least one to two decades.