Westcott DA et al. (2020), Relative efficacy of three approaches to mitiga...

ECB-ART-49714

Sci Rep 2020 Jul 28;101:12594. doi: 10.1038/s41598-020-69466-1.

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

Westcott DA , Fletcher CS , Kroon FJ , Babcock RC , Plagányi EE , Pratchett MS , Bonin MC .

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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.

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	Figure 1. Site locations for manual control of the Pacific Crown-of-Thorns Starfish (COTS), Acanthaster cf. solaris. Manual control of COTS took place at a total of 52 sites at 21 reefs in three different types of spatial zoning in the Cairns sector of the Great Barrier Reef from July 2013 to December 2017 (light blueâ=âtake zones, dark blueâ=âlimited take, greenâ=âno take). Data on COTS abundance from the COTS control program, and coral cover from the Great Barrier Reef Marine Park Authorityâs Reef Health Impact Surveys, were collected at each of the 52 sites during this period. Insert shows location of study area in Australia. The spatial layers to create the map were obtained from the Great Barrier Reef Marine Park Authority under a Creative Commons Attribution 4.0 licence (CC BY) (https://www.gbrmpa.gov.au/about-us/resources-and-publications/spatial-data-information-services).
	Figure 2. The effect of number of control voyages on the density of Pacific Crown-of-Thorns Starfish (COTS), Acanthaster cf. solaris. Manual control of COTS took place at a total of 52 sites at 21 reefs in three different types of spatial zoning in the Cairns sector of the Great Barrier Reef from July 2013 to December 2017. The density of COTS encountered during a voyage at a site declined as a function of the number of voyages that had previously visited that site. This decline was initially rapid and after roughly five voyages COTS densities fluctuated while remaining low. Note, beyond 22 visits, sample sizes decline substantially with just six sites visited 23 or more times; variation in COTS densities increases dramatically as a result. Dashed lineâ=âthe ecologically sustainable threshold for COTS outbreaks27, i.e. the density of COTS that can be sustained before coral cover is lost, Solid barâ=âmedian, boxâ=âquartiles, whiskersâ=âextremes, circlesâ=âoutliers. Sample sizes given above the x-axis.
	Figure 3. The effect of number of control voyages on the density of Pacific Crown-of-Thorns Starfish (COTS), Acanthaster cf. solaris, within four size classes. Manual control of COTS took place at a total of 52 sites at 21 reefs in three different types of spatial zoning in the Cairns sector of the Great Barrier Reef from July 2013 to December 2017. The densities of the three largest size classes (>â15â25Â cm,â>â25â40Â cm, andâ>â40Â cm diameter) show a sharp decline during the first four voyages, while densities of the smallest size class (<â15Â cm diameter) show a slight and longer-term reduction but remain relatively high. Dashed lineâ=âthe ecologically sustainable threshold for COTS outbreaks27, i.e. the density of COTS that can be sustained before coralÂ cover is lost, Solid barâ=âmedian, boxâ=âquartiles, whiskersâ=âextremes, circlesâ=âoutliers. Sample sizes can be found in Fig.Â 2.
	Figure 4. The effect of number of control voyages on the percentage change in hard coral cover to the end of 2017. Manual control of COTS took place at a total of 52 sites at 21 reefs in three different types of spatial zoning in the Cairns sector of the Great Barrier Reef from July 2013 to December 2017. The proportional change in hard coral cover at these 52 sites over the 4.5Â year control period was significantly and positively related to the number of voyages that visited these sites. That is, coral cover was not just maintained but actually increased as the number of voyages increased. The horizontal dotted line represents no change in coral cover, theÂ solid line the regression equation (R2â=â0.19, F1, 50â=â11.99, Pâ<â0.0011) and the dashed lines are the 95% confidence limits. Colours indicate zoning: light blueâ=âtake zones, dark blueâ=âlimited take, greenâ=âno take.
	Figure 5. Median densities of COTS culled haâ1 when control began at site zoned with different levels of protection. Colours indicate zoning: light blueâ=âtake zones, dark blueâ=âlimited take, greenâ=âno take. Solid barâ=âmedian, boxâ=âquartiles, whiskersâ=âextremes, circlesâ=âoutliers.

References [+] :

Babcock, Assessing Different Causes of Crown-of-Thorns Starfish Outbreaks and Appropriate Responses for Management on the Great Barrier Reef. 2016, Pubmed, Echinobase