Chandler JF et al. (2023), Increasing densities of Pacific crown-of-thorns...

ECB-ART-52663

Sci Rep 2023 Nov 07;131:19306. doi: 10.1038/s41598-023-46749-x.

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Increasing densities of Pacific crown-of-thorns starfish (Acanthaster cf. solaris) at Lizard Island, northern Great Barrier Reef, resolved using a novel survey method.

Chandler JF , Burn D , Caballes CF , Doll PC , Kwong SLT , Lang BJ , Pacey KI , Pratchett MS .

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Recurrent population irruptions of Pacific crown-of-thorns starfish (CoTS, Acanthaster cf. solaris) are among the foremost causes of coral mortality on Australia's Great Barrier Reef (GBR). Early intervention during the initiation of new population irruptions represents the best opportunity to effectively manage this threat. However, current survey methods are not sufficiently sensitive to detect changes in CoTS densities during the early onset of population irruptions. Using scooter-assisted large area diver-based (SALAD) surveys, this study revealed increasing densities of CoTS at Lizard Island from 2019 to 2022. Inferred densities of adult CoTS (which account for distinct sets of observed feeding scars where starfish were not detected) increased from 4.90 ha-1 (± 0.85 SE) in 2019 to 17.71 ha-1 (± 2.3 SE) in 2022. A wide range of size classes were recorded suggesting that recruitment over several years is contributing to increasing densities. Importantly, the sustained density increases reported here denote that renewed CoTS population irruptions may soon become fully established at Lizard Island and more broadly in the northern GBR, especially without early intervention through effective population management.

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	Figure 1. ‘SALAD surveys’ conducted at Lizard Island. (a) Lizard Island’s location in the Northern Great Barrier Reef, also shown is the putative ‘Initiation Box’ (dashed box), (b) Lizard Island area with survey locations marked in red, (c) SALAD survey conducted with a Seascooter, (d) Methodology: searches are conducted at two depths simultaneously, with GPS units towed along the surface in floating housings.
	Figure 2. Inter-annual variation in CoTS densities at Lizard Island from 2019 to 2022. (a) Recorded density estimates based on starfish counts, (b) inferred density estimates (accounting for additional, distinct sets of feeding scars where starfish were not detected). Each grey point represents a density estimate for an individual SALAD survey. Boxes and whiskers display 25–75th and 5–95th percentile ranges, respectively. Diamonds and thick lines indicate mean and median values, respectively. Dashed line indicates the ‘outbreak threshold’ commonly used by monitoring agencies51.
	Figure 3. Inter-annual variation in size frequency distribution of adult CoTS recorded at Lizard Island between 2019 and 2022. Dashed line indicates the mean CoTS diameter (mm) for each survey year.