Habitat associations of settlement-stage crown-of-thorns starfish on Australia’s Great Barrier Reef

Abstract

Population irruptions of crown-of-thorns starfish (Acanthaster spp.) contribute greatly to the degradation of coral reefs throughout the Indo-Pacific. Effective management of these population irruptions is limited, in part, by incomplete knowledge of their early life history. Importantly, there are very limited data on the distribution and abundance of newly settled crown-of-thorns starfish (0 + starfish, in their first year since settlement). Extensive sampling was conducted around the circumference of three distinct mid-shelf reefs (at 1–18 m depths) in the central Great Barrier Reef (GBR), during active population irruptions, in May–June 2017, to quantify the occurrence and densities of settlement-stage starfish (2–65 mm diameter) and relate patterns of abundance to distinct habitat features at the scale of individual reefs. Overall, 140 settlement-stage starfish were detected across 1242 quadrats (1 m2). Settlement-stage starfish were recorded from 31 out of 42 sites (73.8%) at mean densities of 0–0.77 starfish m−2. Both estimated densities and the likelihood of occurrence of settlement-stage starfish within quadrats increased overall with the proportion of coral rubble (and dead intact corals), were greatest at intermediate depths (8–14 m), but decreased with the proportion of live hard coral. At the scale of individual reefs, settlement-stage starfish occurred most frequently in south-western and northern fore reef habitats. Our results suggest that settlement and/or early post-settlement survival of crown-of-thorns starfish is greatest in relatively shallow waters of obliquely exposed fore reef habitats where there is high cover of coral rubble. The specific occurrence of these habitat types (within spur and groove systems and rubble slips) provides an opportunity to concentrate searches and increase effective sampling of settlement-stage starfish, though these habitats are relatively widespread and unlikely to constrain the population replenishment or population irruptions of crown-of-thorns starfish on the GBR.