Enhancing Coral Survival on Deployment Devices With Microrefugia

Carly J Randall,Andrew J Heyward,Andrew P Negri,Christine Giuliano

doi:10.3389/fmars.2021.662263

Carly J Randall, Andrew J Heyward + Show 2 more

Open Access

https://doi.org/10.3389/fmars.2021.662263

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Abstract

Surviving after settlement through the first year of life is a recognised bottleneck in up-scaling reef coral restoration. Incorporating spatial refugia in settlement devices has the potential to alleviate some hazards experienced by young recruits, such as predation and accidental grazing, and can increase the likelihood of survival to size-escape thresholds. Yet optimising the design of microrefugia is challenging due to the complexity of physical and biological processes that occur at fine spatial scales around a recruit. Here, we investigated the effects of microhabitat features on the survival ofAcropora tenuisspat in a year-long experimental field deployment of two types of artificial settlement devices—grooved-tiles and lattice-grids—onto three replicate racks on a shallow, central mid-shelf reef of the Great Barrier Reef. Spat survival across device types averaged between 2 and 39% and about half of all devices had at least one surviving coral after a year. While the larvae settled across all micro-habitats available on the devices, there was strong post-settlement selection for corals on the lower edges, lower surfaces, and in the grooves, with 100% mortality of recruits on upper surfaces, nearly all within the first 6 months of deployment. The device type that conferred the highest average survival (39%) was a tile with wide grooves (4 mm) cut all the way through, which significantly improved survival success over flat and comparatively featureless control tiles (13%). We hypothesise that the wide grooves provided protection from accidental grazing while also minimising sediment accumulation and allowing higher levels of light and water flow to reach the recruits than featureless control devices. We conclude that incorporating design features into deployment devices such as wide slits has the potential to substantially increase post-deployment survival success of restored corals.

Highlights

Coral populations are declining globally (Gardner et al, 2003; Bruno and Selig, 2007; De’ath et al, 2012; Hughes et al, 2017), stimulating widespread efforts to mitigate further losses, enhance the recovery of existing populations, and potentially increase reef resilience through coral restoration programs (Boström-Einarsson et al, 2018, 2020; Bay et al, 2019)
No spat survived on any upper surface of either lattice-grid type and were excluded from the comparison of survival success among habitat types
Survival was significantly higher for spat on ‘lower inner’ compared with ‘lower outer’ habitats across lattice devices (GLMM: z = −1.97, p = 0.049)

Summary

Introduction

Coral populations are declining globally (Gardner et al, 2003; Bruno and Selig, 2007; De’ath et al, 2012; Hughes et al, 2017), stimulating widespread efforts to mitigate further losses, enhance the recovery of existing populations, and potentially increase reef resilience through coral restoration programs (Boström-Einarsson et al, 2018, 2020; Bay et al, 2019). High post-settlement mortality in corals can be caused by accidental grazing by fishes (Baria et al, 2010; Penin et al, 2010, 2011; Trapon et al, 2013; Gallagher and Doropoulos, 2017), competition with other benthic organisms (Box and Mumby, 2007; Hughes et al, 2007; Vermeij and Sandin, 2008; Vermeij et al, 2009), sedimentation (Sato, 1985; Babcock and Smith, 2002; Jones et al, 2015; but see Trapon et al, 2013), and direct corallivory (Gallagher and Doropoulos, 2017). High variability in post-settlement growth rates among species and growth morphologies (Miller, 2014; Suzuki et al, 2018) means that what may work for one species or growth morphology may not work for another

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