Written by Sara Gagliardi
Edited by Bobbie Renfro
The loss of stony coral populations in the last decades has increasingly frightened researchers. These habitat-forming animals are subject to increasing environmental pressure due to climate change, as well as to human-driven disturbances. As a result, coral reefs are suffering from increasing frequency, intensity and severity of bleaching and disease outbreaks. Furthermore, supply of coral larvae, and their settlement, recruitment and survival is compromised because of the repeated disturbances. Therefore, the natural recovery of coral populations after catastrophic events including mass bleaching and severe storms is unlikely, and even impossible in many locations. In order to conserve coral reef ecosystems, habitat protection and restoration projects are rising worldwide.
In their review, Boström-Einarsson et al. assembled case studies of active coral restoration using published peer-reviewed scientific articles, grey literature (e.g., scientific reports, technical summaries), online descriptions (e.g., blogs, online videos), and survey practitioners reports. Indirect coral restoration projects (e.g., predator removal, disease control), passive restoration interventions (e.g., control against dynamite fishing, water quality improvement), and many artificial reefs (e.g., those used for fisheries enhancement) were not taken into account. Methods of direct coral restoration reported here aimed to reintroduce corals (e.g., coral fragment transplantation, larval enhancement) or augment the coral assemblages (e.g., substrate stabilization, algal removal). Ten main coral restoration methods and techniques were identified by Boström-Einarsson et al. in their review.
Where, how, and why is coral restoration occurring ?
The majority of projects are conducted in the USA (mainly Florida and Hawaii), Philippines, Indonesia and Thailand, however 56 countries are at least somewhat involved in these practices. The majority of projects involve coral fragmentation or transplantation of coral fragments (68%) and focus in the domain of scientific research (improvement restoration approach and answering ecological questions). The size of restored areas varied between peer-reviewed literature projects, conducted on a smaller spatial scale (median 300 m2) in comparison to the survey respondents’ reports (median 500 m2), and in the grey literature projects size is 47 m2. Grey literature and peer-reviewed projects have a duration of monitoring of 12 months, however coral restoration practitioners tended to report longer projects (median 24 months).
Coral restoration involves 229 species from 72 coral genera, primarily focusing on fast-growing branching corals. The top five species used in coral restoration are Acropora cervicornis, Pocillopora damicornis, Stylophora pistillata, A. palmata and Porites cylindrica. The importance of the coral genus Acropora sp., involving a third of projects and prioritising A. cervicornis and A. palmata, is due to the importance of these species as reef-forming corals and to the mass mortality they underwent because of bleaching and diseases. Indeed, the conservation status of A. cervicornis and A. palmata is listed as threatened, according to the United States Endangered Species Act (71 FR 26852), and as endangered, according to the International Union for Conservation of Nature Red List of Endangered Species (IUCN 2018).
Methods for coral restoration
Coral fragments are directly transplanted (Fig. 1) from a donor to a recipient reef. This method is mainly used to prevent the destruction or disturbance of colonies due to a planned construction activity. Studies on direct transplantation report 64% survival and primarily involve fast-growing branching corals. Furthermore, an assumption of enhanced colonisation (i.e., the ability of transplanted corals to attract other reef species) exists, but experimental designs to test the hypothesis are rare.
Because of the continuous harvesting of coral fragments being detrimental for the donor reef, a more sustainable model was developed. Using the method of coral gardening, coral recruits or small fragments are raised in intermediate nurseries (Fig. 2) prior to outplanting on the restoration sites. Because of the sensitivity of early life stages of coral fragments, the nursery stage protects the corals from damaging conditions. In addition, nurseries provide a reservoir for new coral fragments that can be broken into smaller pieces and grew again in the nursery, thus avoiding collections from the natural reef. On average, 66% of corals survived the outplanting phase. To improve growth and survival rates of coral fragments, the nursery phase can be carried out in-situ (field-based) or ex-situ (land-based), depending on the local conditions.
Micro-fragmentation is the technique that allows massive and encrusting corals to be outplanted and achieve larger, reproductive sizes faster. Because of their thicker skeleton, massive corals are cutted with a diamond blade saw into small fragments that are then mounted on tiles. Due to their slow growth rate, after 12 months fragments can be either further divided to generate new micro-fragments, or outplanted in a restoration site. After outplanting, micro-fragments fuse together with the reef substrate or dead coral skeleton (re-skinning).
Genetic diversity in asexual propagation
Coral gardening uses clonal processes to create new coral fragments, thus eliminating the creation of new genotypes and inhibiting the increase of resilience to existing and future stresses. Indeed, the lack of genetic diversity avoids the expression of phenotypes that may be successful in resisting environmental and human-driven pressures. Therefore, projects of assisted fertilization and nursery stocks from larvae of brooding corals are in development as valuable tools for the maintenance of the genetic diversity in coral gardening.
Larval enhancement methods aim to increase coral fertilization, larval survival and recruitment. Two main types of methods exist. The first improves the post-settlement survival rate by collecting gametes and raring embryos ex-situ to subsequently settle them on an artificial structure. The second larval enhancement technique collects gametes and rares embryos and larvae to subsequently release them directly on the reef in enclosures, in order to retain them over a targeted substrate during the settlements period. By doing so, the speed of outplanting is increased compared to the transplantation methods.
The creation or addition of substratum such as artificial reefs aims to place artificial structures on the seabed to mimic the characteristics of a natural reef or to increase the potential habitat for the reef fauna, fisheries production, recreational diving opportunities, or the prevention of trawling. This technique is generally combined with coral transplantation.
The stabilization of substratum is the direct physical restoration of damaged substratum that has been affected by storms, ship grounding or fishing. Methods are generally followed by coral transplantation, and mostly involve stabilizing rubbles, and the most common method is to install mesh netting to prevent their movement.
Substratum enhancement with electricity
The enhancement of substratum with electricity aims to mimic the chemical and physical properties of the reef limestone. On an artificial substrate, the precipitation of calcium and magnesium is encouraged, in order to increase the calcification of coral polyps and boost the colony growth and resilience to stressors. However, results are inconsistent, with different outcomes that may vary even between congeneric coral species.
Boström-Einarsson L, Babcock RC, Bayraktarov E, Ceccarelli D, Cook N, Ferse SCA, Hancock B, Harrison P, Hein M, Shaver E, Smith A, Suggett D, Stewart-Sinclair PJ, Vardi T, McLeod IM. 2020. Coral restoration – A systematic review of current methods, successes, failures and future directions. PLOS ONE 15:e0226631.
Cover image: Successful coral restoration in Florida (photo courteously of I.CARE)