Detectability of Crown-of-Thorns Starfish and Consequences for Culling or Removal

Population irruptions of crown-of-thorns starfish (CoTS; Acanthaster spp.) represent a perennial threat to Indo-Pacific coral reefs, often causing extensive coral loss and contributing to reef degradation. Therefore, extensive efforts are being made to contain population irruptions of CoTS either by culling or removing individual starfish across large reef areas. However, the efficacy and effectiveness of these management strategies are inherently constrained by limited detectability, even among adult starfish. This study adds to the limited quantitative data on the detectability of CoTS based on two independent intensive experimental studies conducted on Australia's Great Barrier Reef. During depletive sampling (where all CoTS detected were removed prior to re-surveying the same area) over 3 days at Lizard Island, a total of 96 (out of 132) CoTS were recorded during initial surveys, and the average detectability across 34 transects where CoTS were recorded was estimated to be 78.4% (±13.4 SE). Estimated detectability declined to 64.4% (±11.22 SE) on day 2, suggesting that the remaining CoTS were more cryptic. During mark-recapture studies at Rib Reef, the total sample population was estimated to comprise 411 individuals, of which 266 (64.7%) were sighted during initial (day-time) surveys, while 322 (78.3%) were sighted at night. Average detectability across all surveys was estimated to be 75.9% (±0.05 SE). Our findings reaffirm that the detectability of adult CoTS is limited, which will inherently constrain the effectiveness of culling and removal.