Repurpose obsolete offshore structures to act as structures for restoring coral reefs

Overall effectiveness category Awaiting assessment
Number of studies: 2
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Background information and definitions

Man-made offshore structures, such as oil rigs and aquaculture boxes, provide hard surfaces that may allow coral larvae to settle in areas where there is otherwise a lack of suitable substrate. Once these structures are no longer used for their intended commercial purpose they can be removed from the marine environment or be made into artificial reefs. If being repurposed for biodiversity, they can either be left in the same location (standing or toppled) or can be moved to a new location to increase the likelihood of natural colonisation by corals and fish communities. Programmes that encourage the repurposing of obsolete structures as artificial reefs, such as ‘Rigs-to-Reefs' in the Gulf of Mexico, operate under the premise that the structures will provide benefits for nature by providing new habitats, and benefits for businesses by reducing the costs for decommissioning and removing obsolete equipment (Macreadie et al. 2011).

Other similar actions include Use structures made from unnatural materials to create habitat to encourage coral settlement (where a structure has specifically been made as a reef) and Modify existing man-made structures to create habitat to encourage coral settlement (where a structure was created for another purpose but has been modified to allow colonization by corals or has been colonized in its original state).

^{Macreadie, P.I., Fowler, A.M. & Booth, D.J. (2011) Macreadie, P.I., Fowler, A.M. & Booth, D.J. (2011) Rigs-to-reefs: will the deep sea benefit from artificial habitat? Frontiers in Ecology and the Environment, 9, 455–461. https://doi.org/10.1890/100112.}

Study locations

Key messages

Two studies evaluated the effects of repurposing obsolete offshore structures to restore coral reefs. One study was in Japanand one in the Gulf of Mexico.

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (2 STUDIES)

Abundance/Cover (2 studies): One study in Japan found that concrete aquaculture boxes had higher coral cover than the surrounding reef. One replicated, site comparison study in the Gulf of Mexico found that toppled oil rig platforms had similar overall stony coral density to rigs left standing, but density of species varied between rigs.

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 replicated study in 1996–2003 at an aquaculture site at Miako Island, Okinawa, Japan (Omori et al. 2006) reported that coral cover was higher inside empty aquaculture boxes compared to the surrounding reef. After seven years, coral cover inside five boxes originally designed to be used for rearing top-shell snails Trochus niloticus was 90% compared to 20% on the surrounding reef (data not statistically tested). By 2003, twenty-six species had colonized the base of the boxes, dominated by stony coral Acropora spp. that had grown to 40-65 cm in diameter. In 1996, five concrete aquaculture boxes (2.1x2.1x0.6 m) in shallow water (depth not specified) were left empty to enable coral to grow on the base. The box bases were made from plastic lattice reinforced with quartz sand-coated fibreglass to which the corals could attach. Monitoring frequency and other methods are not reported.
Study and other actions tested
Referenced paper
Omori M., Kubo H., Kajiwara K., Matsumoto H. & Watanuki A. (2006) Rapid recruitment of corals on top shell snail aquaculture structures. Coral Reefs, 25, 280-280.
A replicated, site comparison study (years not given) on seven decommissioned oil rig platforms in the Gulf of Mexico (Sammarco et al. 2014) found that toppled platforms did not have greater overall density of stony corals than standing platforms, but densities of three of four stony coral species varied between toppled and standing platforms. There was no significant difference between the average density of all corals on toppled oil platforms (90 corals/10 m²) and standing platforms (20 corals/10 m²). However, on average, Madracis decactis and Tubastraea coccinea densities were higher on toppled (Madracis decactis: 0.4 corals/10 m²; Tubastraea coccinea: 28 corals/10 m²) than standing platforms (Madracis decactis: 0.3 corals/10 m²; Tubastraea coccinea: 19 corals/10 m²). In contrast, Phyllangia americana density was lower on toppled (1 coral/10 m²) than standing platforms (4 corals/10 m²). There was no difference in Oculina diffusa density between toppled (2 corals/10 m²) and standing platforms (2 corals/10 m²). Surveys for stony corals were carried out on two standing oil platforms deployed 15–30 years prior (sea level to maximum depth of 101 m and 113 m) and five obsolete oil platforms cut at the base and toppled 13–20 years prior (minimum depth: 23–30 m; maximum: 48–195 m). Monitoring was carried out using photos and videos taken by remotely operated vehicles along two to four vertical and two horizontal struts/platform (20 m to a maximum of 110 m depth).
Study and other actions tested
Referenced paper
Sammarco P.W., Lirette A., Tung Y.F., Boland G.S., Genazzio M. & Sinclair J. (2014) Coral communities on artificial reefs in the Gulf of Mexico: Standing vs. toppled oil platforms. ICES Journal of Marine Science, 71, 417-426.

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.

Where has this evidence come from?

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This Action forms part of the Action Synopsis:

Coral Conservation

Related Actions

Actions	Effectiveness	Studies	Category
Repurpose obsolete offshore structures to act as structures for restoring coral reefs Action Link	Awaiting assessment	2	Synopsis Link
Modify existing man-made structures to encourage natural coral settlement Action Link	No evidence found (no assessment)	0	Synopsis Link
Use settlement tiles made from unnatural materials to encourage natural coral settlement Action Link	Awaiting assessment	16	Synopsis Link