Use structures made from unnatural materials to restore / repair / create habitat for corals to encourage natural coral settlement
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Overall effectiveness category Awaiting assessment
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Number of studies: 10
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Supporting evidence from individual studies
A study in 1995–1998 in two artificial reefs in Florida, USA (Miller et al. 2001) found that three years after concrete blocks embedded with limerock were used to create habitat, stony corals, hydrocorals and octocorals had established on the unnatural substrates. At one site, three years after a ship grounding crater was filled with concrete blocks embedded with limerocks, seven types (species or genera) of coral were found at a density of 3 corals/m2. Porites astreoides was the most abundant (>15% of corals) at the site. Sixty percent of corals had settled on the embedded limerocks (25% of the structure), rather than the surrounding concrete (75%). At the other site, three years after a grounding crater was filled with limerock boulders, 11 types of coral were found, at a density of 4 corals/m2. Porites astreoides, Favia fragum and Agaricia sp. Were the most abundant, each constituting >15% of corals at the site. In October and November 1989, two ships grounded on reefs 6.5 km apart in the northern Florida Keys National Marine Sanctuary, leaving craters. In June–August 1995, at the 2.5 m-deep site, 40 concrete blocks embedded with limerocks were used to fill the crater and sealed with cement. At the other 10 m-deep site large limerock boulders were used to fill the crater. In summer 1998, three years after installation, juvenile coral recruits were mapped and measured on 17 concrete blocks and 17 limerock boulders. The proportion of corals on the embedded limerocks compared to surrounding concrete was measured on nine of the concrete blocks.
Study and other actions testedA replicated study in 1993–1995 at an artificial reef in Hoi Ha Wan, Hong Kong (Lam 2003) found that after pulverised fly-ash/cement blocks were used to create habitat, the number of stony coral recruits settling onto the blocks varied according to time immersed, block orientation, composition and species. A total of 387 Oulastrea crispata were recorded during the 24-month monitoring period (0–65/m2). More recruits settled on the top and reef-facing sides of the block compared to the sea-facing or bottom sides (data not reported). There was no difference in Oulastrea crispata recruitment on blocks comprising different pulverised fly-ash:cement mixes. Thirty Culicia japonica recruits were recorded during the monitoring period, with the density fluctuating (range 0–6/m2) and peaking after 24 months. More recruits were recorded on the reef-facing, top and bottom sides compared to the sea-facing (data not reported). More Culicia japonica settled on blocks comprising 3:1 pulverised fly-ash:cement mix (numbers not reported). In December 1993, a total of 176 smooth-sided cube blocks (0.15 m3) were randomly placed on top of an existing artificial reef 7 m deep. Blocks comprised different ratios of pulverised fly-ash:cement (0:1, 1:3, 1:1, 3:1). Coral recruits were counted approximately every three months for 24 months.
Study and other actions testedA site comparison study in 1995, and 1998–2001 at two damaged coral reefs in the Florida Keys National Marine Sanctuary, USA (Lirman & Miller 2003) found that using concrete armor or limerock boulders to repair the reefs led to natural settlement by corals with 70-80% of species the same as on nearby natural reefs, but the diameter of stony coral Porites asteroides colonies was lower, and density did not differ between restored and natural reefs. Six years after the artificial structures were installed, 80% of species recorded on concrete armor and 70% of species on limerock boulders were also found on the adjacent natural reefs. Average colony diameter of P. asteroides increased from 14 mm (concrete armor) and 18 mm (limerock boulder) in 1998 to 22 mm (concrete) and 23 mm (limerock) in 2001, but was smaller in 2001 than colonies on the adjacent natural reefs (adjacent to concrete 85 mm; adjacent to limerock: 34 mm). Average density of P. asteroids increased on the concrete armor reef from 2.1 colonies/m2 in 1998 to 4.5/m2 in 2001 whereas average density was unchanged on limerock boulders (1.4/m2 both years). Average density was not significantly different between either concrete armor or limerock boulders and their adjacent natural reefs (concrete armor: 4.5, adjacent reef: 5.4 colonies/m2; limerock boulders: 1.4; adjacent reef 0.9 colonies/m2). In 1995, six years after two ships (M/V Maitland and M/V Elpis) ran aground, artificial structures comprising 12 concrete armor blocks (Maitland site) and 16 limerock boulders (Elpis site) were installed to repair the damaged reef. The artificial reefs were monitored to record natural settlement by coral species. Density and diameter of P. asteroides were recorded in 1998 and 2001 and compared, in 2001, to P. asteroides colonies on natural reefs approximately 25 m away.
Study and other actions testedA replicated study in 2001–2004 at three artificial reefs in Singapore (Loh et al. 2006) found that after fibreglass/sand/calcium carbonate structures were used to create habitat, stony coral recruits settled, and at one site at a higher density compared to natural coral rubble substrate, although recruits were smaller. After 24–26 months, the average density of coral recruits across all sites ranged from 0.1 recruits/m2 to 4.8/m2. At one site after 23–31 months, coral density was higher (range: 6–11 recruits/m2) than the adjacent natural coral rubble (range: 4–10 recruits/m2). Although at that site the average size of recruits on the artificial structures grew between month 26 (1.0–1.5 cm) and 31 (2.0–2.5 cm), these were smaller than recruits on the natural substrate (2.5–3.0 cm for both months). Pocillopora damicornis was the dominant species at each site (50%, 79%,100%) with species from six other families also recorded (see paper for list). In October 2001, ninety-six 70 cm diameter 50 cm tall structures, comprising fibreglass mixed with sand and calcium carbonate, were installed at three sites. Structures were fixed to the seabed using 40 cm or 70 cm stakes. A random sample of 10 structures were monitored every 2-3 months for 24–26 months. In addition, from 23–31 months after installation, coral density and growth on five structures at one of the sites were compared to five 1 m2 plots on adjacent natural coral rubble. Each substrate structure cost US$130 (in 2006) and US$23 for six 40 cm stakes.
Study and other actions testedA site comparison study in 1999–2004 at an artificial and natural coral reef site in Bal Harbour, Florida, USA (Thanner et al. 2006) found that corals settled on an artificial reef made from concrete and limerock and, over time, the coral community more closely resembled the adjacent natural reef and stony coral coverage and density increased. The coral community on the artificial reef became more similar to the natural reefs during the first 3.5 years after the artificial reef was installed and then stabilized to a similarity of 45–58% (data presented as a Bray Curtis Index). Average cover of stony coral increased on the artificial reef to 1.35% after five years and was reported as similar to one of the natural reefs (0.70%). Density of stony corals increased from 0.21/m2 in year one to 25.29/m2 after five years. In May 1999, an artificial reef comprising a 46 × 23 m rectangle of 8,000 t of 0.9–1.5 m diameter limerock boulders surrounded by 179 prefabricated concrete and limerock modules (see paper for details). These modules were installed between two natural reefs, 3.1 km offshore, 20 m deep. Reefs were monitored every six months for five years from October 1999 using quadrats to record coral diversity and density.
Study and other actions testedA study in 2007 on artificial and natural reefs in Florida Keys National Marine Sanctuary, Florida, USA (Miller et al. 2009) reported that hard coral cover was similar on two older concrete and limestone artificial reefs compared to natural reefs but lower on two newer reefs. Percentage of hard coral cover on 12-year-old artificial reefs was similar to adjacent natural reference reefs (Maitland artificial: 5%, natural: 3%; Elpis artificial: 5%, natural: 4%) but newer reefs had lower hard coral cover than natural reefs (Iselin eight-year-old artificial: 2%, natural: 5%; Wellwood five-year-old artificial: 2%, natural: 8%). Results presented as a similarity index including all species recorded. The hard coral community on the 12-year-old artificial reefs was dominated by Porites asteroides. In 2007, four 10-metre-long line transect surveys were carried out on four concrete and limestone artificial reefs (two 12-, one eight-, and one five-years-old) and adjacent natural reefs. The percentage of hard coral cover was recorded.
Study and other actions testedA replicated, randomized, controlled study in 2005–2007 at three degraded coral reefs in northern Sulawesi, Indonesia (Ferse et al. 2013) found that concrete structures placed close to transplanted stony coral fragments had similar numbers of stony coral recruits to structures placed further away. The number of coral recruits was similar on concrete structures placed next to transplanted corals compared to structures placed away from corals in eight of nine comparisons (next to transplants: 0.02–0.28 corals/100cm2, away from transplants: 0.03–0.26 corals/100cm2), and higher in the ninth comparison (next to transplants: 0.58 corals/100cm2, away from transplants: 0.36 corals/100cm2). For limestone plates placed next to, or distant from, transplanted corals there were a similar number of recruits in 15 of 18 comparisons, more recruits in two comparisons, and fewer in one comparison (see paper for data). In July 2005–March 2006, six-thousand-one-hundred-and-sixty-four stony coral fragments (Acropora yongei, Pocillopora verrucosa, Acropora muricata, Isopora brueggemanni) were collected from donor colonies near three transplant sites. Two plots (10 × 10 m) at each of three sites, with each plot randomly assigned to either: concrete structures (25/plot) alternating in a ‘chessboard’ design with transplanted stony coral fragments attached to bamboo frames; or concrete structures only (25/plot). At all plots, six groups of three limestone settlement plates were also installed on metal frames. Coral recruits that settled on concrete structures were counted after 14–24 months. Recruits on limestone plates were counted every three months for 14–24 months. Plates were replaced every three months.
Study and other actions testedA site comparison study in 1997 and 2009–2010 at a fish farm and adjacent coral reefs in Setouchi Channel, Japan (Hata et al. 2013), found that corals that settled and began growing on suspended ropes had lower rates of bleaching but higher instances of infection than corals on natural reefs, and the community differed between the ropes and natural reefs. Three months after monitoring began, the percentage partial bleaching on rope-growing corals was lower (12%) than on corals growing on one of the disturbed reefs (46%), but similar to corals growing on the other disturbed (18%) and protected (12%) reefs. Rates of infection by flatworm Waminoa spp. were higher after nine months in rope-growing corals (4%) compared to corals growing on disturbed (0%, 1%) and protected (0%) reefs. Diversity of coral communities on the ropes was significantly higher than communities on the two disturbed sites, and either equaled or was higher than on the protected site (results presented as multivariate analyses, see paper for full species list). Coral responses to other threats (e.g. algae and sponge overgrowth) were not significantly different between rope-growing or naturally growing corals. In 1997, a tuna fish farm was established using floating cages suspended by rope 3 m deep, ~50 m above the seabed. In May and August 2009 and February 2010, surveys were carried out on the ropes and three adjacent coral reefs (two disturbed by outbreaks of crown-of-thorns starfish; one protected through management of crown-of-thorns starfish). Photographs were used to monitor diversity, bleaching, infection, and other threats.
Study and other actions testedA replicated, site comparison study in 2004 and 2014 at seven artificial reefs off Singapore (Ng et al. 2017) found that corals settled on fibreglass reefs, and the percentage of organisms that were stony corals increased over 10 years. Stony corals represented on average <1% of organisms on artificial reefs in 2004 and 2–42% (11% average) 10 years later. In 2014, stony coral colonies on average covered <1–32% of artificial reef surfaces and at three of seven sites 25–58% of corals were recorded with eggs (no eggs were recorded at the remaining sites). In the early 2000s, fibreglass artificial reefs were fixed with iron stakes to areas of sand and rubble at seven sites off Singapore’s southern offshore islands. The communities on the outer surfaces of all 84 artificial reefs were surveyed in 2004 and the 44 that remained in 2014. Thirty-five were surveyed in both years. In 2014, three fragments were taken from every adult coral colony ≥12 cm to look for eggs (to determine if the corals were reproductive).
Study and other actions testedA replicated, controlled study in 2004–2009 at a reef in the South Atlantic Bight, Georgia, USA (Gleason et al. 2018) found that using concrete paving slabs led to higher recruitment of temperate soft coral Oculina arbuscula but a higher mortality rate than the natural reef substrate. After almost five years, the average number of coral recruits was higher on concrete paving slabs (17/plot) than on the natural reef (2/plot). The maximum number recorded during one survey was 85 (concrete) and 3 (natural)/plot. Mortality (deaths/plot) was higher at the end of the study for recruits on the concrete paving slabs (5) than on the natural reef (0.25). In June 2004, twenty 30 × 30 cm plots were marked on a hard-bottom reef comprising relict scallop shells on rocky substrate, 20 m deep. Concrete paving slabs (30 × 30 × 5 cm) were placed, unsecured, into 10 plot areas. The remaining plots were left as natural substrate. Twenty surveys were carried out periodically from June 2004June 2009 to record coral recruitment and mortality using photographs.
Study and other actions tested
Where has this evidence come from?
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This Action forms part of the Action Synopsis:
Coral ConservationRelated Actions
| Actions | Effectiveness | Studies | Category | |
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Repurpose obsolete offshore structures to act as structures for restoring coral reefs Action Link |
Awaiting assessment | 2 |
Synopsis Link |
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Use structures made from unnatural materials to restore / repair / create habitat for corals to encourage natural coral settlement Action Link |
Awaiting assessment | 10 |
Synopsis Link |
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Modify existing man-made structures to encourage natural coral settlement Action Link |
No evidence found (no assessment) | 0 |
Synopsis Link |
Coral Conservation - Published 2024
Coral synopsis
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