Marine heatwaves disrupt the culling of Crown-of-Thorns Starfish on the Great Barrier Reef

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Abstract Crown-of-Thorns Starfish (CoTS) predation poses an enduring threat to coral health on the Great Barrier Reef (GBR), driving substantial investment in research and control efforts. However, how CoTS respond to heat stress in situ remains uncertain, limiting our ability to optimise control efforts. We analysed 33 years of CoTS control records, including pre-culling surveys (manta tows) and cull data, with satellite-derived thermal stress metrics to investigate how heat stress influences CoTS detections and culling effectiveness patterns across the GBR. CoTS detections on pre-culling surveys declined significantly with cumulative heat stress, decreasing by 22% per °C-week increase in Degree Heating Weeks (DHW) to a maximum absolute decrease of 95% at 12°C-weeks. Cull data showed 9–14% fewer CoTS culled per dive per °C-week depending on size. These patterns underpin a seasonal response: CoTS culling efficiency peaks in winter and declines in summer, with deeper and longer summer troughs under higher annual heat stress. Culling is triggered when pre-culling surveys meet thresholds for CoTS feeding scars or abundance. We find the false-positive rate, cases where no CoTS are later encountered, is higher (50%) when culls are triggered by scars than by sightings (36%), indicating scars are a less reliable indicator of CoTS presence. Finally, while coral and CoTS populations often exhibit a dynamic balance on controlled reefs, this equilibrium deteriorates when heat stress exceeds ~6 °C-weeks, leading to accelerated coral loss compounded by CoTS presence. This highlights the need for adaptive management strategies that incorporate seasonal and thermal shifts in CoTS behaviour, especially as ocean warming intensifies.

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