Substrate stabilisation and small structures in coral restoration: State of knowledge, and considerations for management and implementation

Abstract

Coral reef ecosystems are under increasing pressure from local and regional stressors and a changing climate. Current management focuses on reducing stressors to allow for natural recovery, but in many areas where coral reefs are damaged, natural recovery can be restricted, delayed or interrupted because of unstable, unconsolidated coral fragments, or rubble. Rubble fields are a natural component of coral reefs, but repeated or high-magnitude disturbances can prevent natural cementation and consolidation processes, so that coral recruits fail to survive. A suite of interventions have been used to target this issue globally, such as using mesh to stabilise rubble, removing the rubble to reveal hard substrate and deploying rocks or other hard substrates over the rubble to facilitate recruit survival. Small, modular structures can be used at multiple scales, with or without attached coral fragments, to create structural complexity and settlement surfaces. However, these can introduce foreign materials to the reef, and a limited understanding of natural recovery processes exists for the potential of this type of active intervention to successfully restore local coral reef structure. This review synthesises available knowledge about the ecological role of coral rubble, natural coral recolonisation and recovery rates and the potential benefits and risks associated with active interventions in this rapidly evolving field. Fundamental knowledge gaps include baseline levels of rubble, the structural complexity of reef habitats in space and time, natural rubble consolidation processes and the risks associated with each intervention method. Any restoration intervention needs to be underpinned by risk assessment, and the decision to repair rubble fields must arise from an understanding of when and where unconsolidated substrate and lack of structure impair natural reef recovery and ecological function. Monitoring is necessary to ascertain the success or failure of the intervention and impacts of potential risks, but there is a strong need to specify desired outcomes, the spatial and temporal context and indicators to be measured. With a focus on the Great Barrier Reef, we synthesise the techniques, successes and failures associated with rubble stabilisation and the use of small structures, review monitoring methods and indicators, and provide recommendations to ensure that we learn from past projects.

Selecting coral species for reef restoration

“…We demonstrate this approach with reef building corals found on the east coast of Australia, including the Great Barrier Reef, a region that has declined severely over the past few decades (De'ath et al, 2012;Hughes et al, 2017). Here, coral reef restoration activities are already underway to address the local-scale depletion of coral populations (Boström-Einarsson et al, 2020;Howlett et al, in review) and larger scale restoration interventions are in the research and development phase (Anthony et al 2020). Although the data and results are only illustrative at this stage, and further scrutiny is required before making formal restoration decisions at appropriate and manageable scales (McAfee et al, 2021), it provides a quantitative, reproducible and adaptable basis for selecting species for restoration projects and restoration research.…”

“…Restoration of reef corals is a relatively new field (Hein et al, 2021), and so there is little longterm knowledge of what makes species more or less amenable to the restoration process. A metaanalysis of coral restoration studies ranked the use of coral growth forms in restoration projects (Boström-Einarsson et al, 2020). While this ranking likely reflects a historical focus on coral gardening (i.e., fragmentation) as well as specific situations, such as the demise of branching Acropora species in the Caribbean, we nonetheless utilize this ranking as an index of species amenability to restoration.…”

“…Interventions in the coastal and marine realm can be direct, such as propagation and field deployment of habitat builders through seeds (e.g., seagrass in Virginia - Orth et al, 2020), propagules (e.g., oysters in South Australia - Vanderklift et al, 2020), early recruits (e.g., kelp -Fredriksen et al, 2020;coral -Randall et al, 2020) or parts of adult tissues (e.g., coral gardening -Rinkevich 2014). Indirect interventions, such as physical stabilization of degraded reef structures are also possible (Ceccarelli et al, 2020).…”

Photogrammetry as a tool to improve ecosystem restoration

“…Quantifying reef state is key in determining the type of intervention most likely to succeed (e.g., outplanting versus larval propagation). For instance, if the reef is mostly depleted, recruitment would drive recovery, and hence an intervention promoting recruitment would be best suited [30]. 2D maps might help practitioners decide where the best locations are to outplant or seed corals to maximize fitness.…”

Revealing the Systematic Nature of Coral Attachment to Reef Substrates

“…Fragments that successfully form these adult colonies can begin reproducing and contributing to reef populations from 1-2 years 4,13 . Successful attachment is inhibited by biological and environmental factors such as substrate type 14,15 , substrate mobility and water movement 8,16,17 . To date, fragment attachment has broadly been de ned as "the rst basal tissue" growing onto a substrate 18, with success quanti ed as the extent of basal tissue visible at the coralsubstrate interface 18,19 .…”