Use natural materials to restore/repair/create habitat for corals to encourage natural coral settlement

Overall effectiveness category Awaiting assessment
Number of studies: 4
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Background information and definitions

Man-made reefs provide a solution to the pressure of human activity by expanding the available habitat on which corals can naturally settle and colonize (Abelson & Schlesinger 2002). Using natural material to restore or create habitat for corals to settle on can provide a more sustainable option than using unnatural materials. Natural materials can be coral rock/rubble, limestone rock, or calcium carbonate substrate such as giant clam Hippopus and Tridacna shells (Neo et al. 2015). They can also be ‘living’ materials such as coral outcrops (sometimes known as ‘bommies’), comprising habitat-forming species of coral (e.g., Porites spp.) that can provide a substrate for other corals to colonize. Using natural material to construct reefs can enable corals to settle, particularly if the material being used is similar to nearby substrate. In addition, natural materials can offer an opportunity to design a reef which closely resembles the natural surroundings.

Here we focus on the creation of reef structures using natural materials to encourage subsequent settlement by wild coral from existing populations in the vicinity. Other similar actions include Use structures made from unnatural materials to restore / repair / create habitat for corals to encourage natural coral settlement; Stabilize damaged or broken coral reef substrate. Actions relating to transplanting or cultivating coral species on natural substrates are covered in Transplant nursery-grown corals onto natural substrate; Transplant wild-grown corals onto natural substrate; Cultivate coral fragments in an artificial nursery located in a natural habitat; Cultivate coral larvae in an artificial nursery located in a natural habitat.

^{Neo M.L., Eckman W., Vicentuan K., Teo S.L.-M., Todd P.A. (2015) The Ecological Significance of Giant Clams in Coral Reef Ecosystems. Biological Conservation, 181, 111-123. https://doi.org/10.1016/j.biocon.2014.11.004}

^{Abelson A. & Shlesinger Y. (2002) Comparison of the development of coral and fish communities on rock-aggregated artificial reefs in Eilat, Red Sea. ICES Journal of Marine Science 59, S122-S126. https://doi.org/10.1006/jmsc.2002.1210}

Study locations

Key messages

Four studies evaluated the effects of restoring / repairing / creating habitat for corals using natural material to encourage coral settlement. Two were in Indonesia and one study was in each of Israeland one was in Australia.

COMMUNITY RESPONSE (1 STUDY)

Richness/diversity (1 study): One site comparison study in Israel found that large rocks placed in an orderly pattern had a lower diversity of coral species than natural reef patches.

POPULATION RESPONSE (2 STUDIES)

Abundance/Cover (4 studies): Three of four studies (two replicated including one controlled, and one site comparison) in Israel, Indonesia, and Australia found that using piles of rocks to create reefs led to higher numbers of corals colonizing when rocks were randomly aggregated compared to orderly, in different patterns or compared to bare rubble. The fourth study found that repositioned coral columns (‘bommies’) retained live coral tissue and were colonized by other coral species.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

A site comparison study in 1989–1998 of two man-made reefs in the Gulf of Aqaba, near Eilat, Israel (Abelson & Shlesinger 2002), found that a reef comprising randomly aggregated piles of smaller rocks had a greater number of coral species than one comprising orderly aggregated piles of larger rocks, and the orderly aggregated reef had a lower number of coral species than the nearby natural reef. Species richness was higher on a reef with randomly aggregated piles of small rocks (33 species) than one with orderly aggregated piles of larger rocks (25 species) after 8.3 years. The average number of coral species and number of individuals were significantly lower on the orderly aggregated reef (8 species, 17 individuals) compared to a natural reef located 100 m away (18 species, 58 individuals) after seven years. Two artificial reefs, constructed using limestone rocks to imitate the substrate on the nearby natural reef, were deployed in December 1989, one hundred meters south of a Coral Reserve. One reef comprised randomly aggregated piles of rocks (area: 4.9 m², average rock diameter 18.9 cm) and the other orderly aggregated piles of rocks (area: 12 m², average rock diameter 49.5 cm). Coral species were visually recorded on the two artificial reefs every 4–6 months for four years and eight months, then with a single survey eight years and four months after deployment. Comparison between the orderly aggregated and natural reef was made during a single transect survey in 1996.
Study and other actions tested
Referenced paper
Abelson A. & Shlesinger Y. (2002) Comparison of the development of coral and fish communities on rock-aggregated artificial reefs in Eilat, Red Sea. ICES Journal of Marine Science, 59, S122-S126.
A replicated, controlled study in 2000–2003 at nine coral rubble sites in the Komodo National Park, Indonesia (Fox et al. 2005) found that using rock piles to create reefs led to higher numbers of stony coral recruits and greater area covered by coral than sites left as bare rubble. The average number of stony corals increased during the study period from 1–21/m² (six months after rock pile installation) to 1–42/m² (three years after installation) (data not statistically tested). The average area covered by corals increased from 0 -19 cm²/m² (six months after installation) to 14–1262 cm²/m² (three years after installation). There was no detectable increase in coral numbers or coverage on the bare rubble control site. In spring 2000, piles of limestone and lithic sandstone rocks (0.5–2.0 m³) were placed inside three or four 10 m² areas of coral rubble substrate at each of nine sites. Rock piles were 70–90 cm high and placed 2–4 m apart. Surveys were carried out every 6 months until May 2002 then a final survey in March 2003. Coral recruits were counted, and area covered by coral was measured using 1 m² quadrats.
Study and other actions tested
Referenced paper
Fox H., Mous P.J., Pet J.S., Muljadi A.H. & Caldwell R.L. (2005) Experimental assessment of coral reef rehabilitation following blast fishing. Conservation Biology, 19, 98-107.
A study in 2002–2003 at four coral rubble sites in the Komodo National Park, Indonesia (Fox et al. 2005), reported that stony corals settled on rocks piled in different patterns whereas none settled on areas of bare rubble. Six–twelve months after rock piles were installed, average coral numbers were 7/m² (4–14/m²) and the average size of corals was 8 cm² (3-11 cm²). Data were not statistically tested. In March–September 2002, rock piles each ~140 m³ and comprising limestone and lithic sandstone were installed in different patterns at four sites with >1000m² of coral rubble substrate. Site 1: rocks completely covered the site ~75 cm high; site 2: rock piles 1–2 m³ were placed every 2–3 m; site 3: spurs ~75 cm high, 2 m wide were placed every 2–3 m parallel to the prevailing current; site 4: spurs ~75 cm high, 2 m wide were placed every 2–3 m perpendicular to the prevailing current. Sites were surveyed once in March 2003 (6–12 months after rocks were installed). Coral recruits were counted and measured using 1 m² quadrats. An area of bare rubble adjacent to each site was surveyed for comparison.
Study and other actions tested
Referenced paper
Fox H., Mous P.J., Pet J.S., Muljadi A.H. & Caldwell R.L. (2005) Experimental assessment of coral reef rehabilitation following blast fishing. Conservation Biology, 19, 98-107.
A replicated study in 2017–2018 off Whitsunday Island, Great Barrier Reef, Australia (McLeod et al. 2019) found that following the repositioning of displaced column-shaped coral outcrops (‘bommies’) of stony coral Porites spp. colonies, some live tissue was retained, and other coral species colonized them. Sixteen months after bommies were repositioned, coverage of original live tissue ranged from 0–20% (average 6%) with 16 of the 22 bommies surveyed still retaining some live tissue. Thirteen of the 22 bommies were colonized by other corals including species of Pocillopora, Cyphastrea, Favia, Favites, Goniastrea, Psammocora and Hydnophor). Eight bommies had at least one coral recruit, four had at least two, and one had six. Recruits ranged from 3–15 cm in diameter. In March 2017, a cyclone dislodged bommies of Porites spp. colonies (1–3 m diameter) and deposited them on the intertidal zone. In June 2017 heavy machinery was used to roll the bommies back into the subtidal region along with 100 m³ of dead coral rubble. Divers surveyed coral bommies in October 2018, recording live tissue coverage (%) and identifying coral species recruited onto the bommie. The costs (reported in 2019) to reposition dead coral rubble were ~AUS$30,000 (it is not reported whether this included the bommie repositioning).
Study and other actions tested
Referenced paper
McLeod I.M., Williamson D.H., Taylor S., Srinivasan M., Read M., Boxer C., Mattocks N. & Ceccarelli D.M. (2019) Bommies away! Logistics and early effects of repositioning 400 tonnes of displaced coral colonies following cyclone impacts on the Great Barrier Reef. Ecological Management & Restoration, 20, 262-265.

Please cite as:

Thornton A., Morgan, W.H., Bladon E.K., Smith R.K. & Sutherland W.J. (2024) Coral Conservation: Global evidence for the effects of actions. Conservation Evidence Series Synopsis. University of Cambridge, Cambridge, UK.

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This Action forms part of the Action Synopsis:

Coral Conservation

Related Actions

	Actions	Effectiveness	Studies	Category
	Use natural materials to restore/repair/create habitat for corals to encourage natural coral settlement Action Link	Awaiting assessment	4	Synopsis Link
	Stabilize damaged or broken coral reef substrate or remove unconsolidated rubble Action Link	Awaiting assessment	6	Synopsis Link