Relative efficacy of three approaches to mitigate Crown-of-Thorns Starfish outbreaks on Australia’s Great Barrier Reef

Abstract

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...

Large-scale interventions may delay decline of the Great Barrier Reef

“…Open Sci. 8: 201296 CoTS control at currently planned levels reduce sediment and nutrient run-off [9] existing CoTS control programme [13,19] over a 20-year timescale, gradually restrict plume footprints (manifested as reduced coral growth rates and enhanced CoTS recruitment) on the inner reef. In the model, this was equivalent to reducing the flood impacts (but not direct wave impacts) of cyclones by one cyclone category [17].…”

“…The implementation of baseline CoTS control in the model closely followed the approach used by control vessels currently operating on the GBR [14,19] (assumption 6a). Control firstly targeted priority reefs and then moved onto other reefs as allowed by vessel capacity.…”

Homing behaviour by destructive crown-of-thorns starfish is triggered by local availability of coral prey

“…For individuals homing within high relief reef habitat, this indicates that the starfish will be most detectable by divers on the reef surface during the afternoon/evening and during the night, as opposed to the morning. On the GBR, the most direct and assured way to minimize local densities of crown-of-thorns starfish and associated coral loss is through recurrent culling at fixed locations [28]. Equivalent manual control programs have however had mixed success throughout the Indo-Pacific [56,57] and effectiveness of manual control is critically dependent on detectability of crown-of-thorns starfish [58].…”

“…Despite recent instances of mass-coral bleaching [22] and increasing disturbances on the GBR [23], population irruptions of crown-of-thorns starfish remain one of the major causes of coral loss on the GBR [24,25]. Accordingly, to alleviate cumulative anthropogenic pressures impacting coral reef ecosystems, there are concerted efforts to manage starfish irruptions on the GBR, both directly and indirectly [26][27][28]. The effectiveness of direct control measures is reliant on improved understanding of the spatial and temporal dynamics of starfish populations, and especially cryptic and emergent behavioural dynamics which determine their susceptibility to, and the efficiency of, in situ culling by divers [29].…”

Knowledge Gaps in the Biology, Ecology, and Management of the Pacific Crown-of-Thorns Sea StarAcanthastersp. on Australia’s Great Barrier Reef

“…Despite their ecological importance and the corresponding research effort, some important unanswered questions remain about the biology, ecology, and management of COTS (e.g., Pratchett et al, 2017). Most importantly, there is persistent controversy regarding the causes of population irruptions of COTS and the efficacy of corresponding management actions that should be taken (Kenchington, 1987;Babcock et al, 2016a;Hoey et al, 2016;Pratchett and Cumming, 2019;Westcott et al, 2020).…”