| Coral tipping (replacing overturned corals after mechanical damage) |
Enhancing survival of overturned corals |
• No extraneous materials needed |
• Small scale |
• Large Porites colonies thrown onto land by a storm and replaced into subtidal reef. Positive outcomes: abundant recruitment and increase in fish abundance using the colonies. Negative outcomes: months between impact and intervention killed most of the coral tissue, high cost and machinery required, original coral tissue died [103]. Considered a temporary measure to precede the use of cement [83]. |
| • Negligible to no material costs |
• Must occur rapidly after disturbance |
| • Promotes natural processes of attachment and survival |
| • Ineffective in naturally high-energy environments |
| • Retains existing habitat structure |
| • If colonies are large (e.g. Porites), heavy machinery may be required |
| • Subject to movement during storms |
| • Manual activity, potentially high labour cost |
| Coral reattachment |
Use of cement to attach individual storm-blown colonies to enhance survival |
• Negligible to no material costs |
• Small scale |
• Positive outcomes: Successful attachment and low mortality of colonies, species composition similar to pre-disturbance [82]. |
| • Promotes natural processes of attachment and survival |
• Must occur rapidly after disturbance |
| • Must be preceded by rubble removal (see below) |
| • Use of original coral assemblage |
| • Some machinery required (cement mixer) |
| • Depends on rapid setting of cement |
| • Subject to movement during storms |
| • Manual activity, potentially high labour cost |
| Rubble removal on reef slopes and flats |
Exposing solid substrate underneath, to encourage settlement of sessile organisms |
• Does not impact reef aesthetics |
• Small scale |
• Removal of rubble after ship grounding. Positive outcomes: Successful removal of rubble after a number of attempts and engineering problems. Exposed bare consolidated substratum for coral reattachment (S3 Appendix). |
| • Allows attachment and settlement of corals onto exposed solid substrate |
• Potential negative impact at rubble disposal site if offshore |
| • Death of organisms living in/on rubble |
| • Does not add structural complexity |
| • High cost |
| Metal stakes |
Provision of settlement substrate |
• Cheap materials, readily sourced locally |
• Small scale |
• No literature to assess outcomes |
| • Limited potential to trap and stabilise unconsolidated substrata |
| • Becomes inconspicuous relatively quickly (gaining aesthetic appeal) |
| • Unknown how microbiome may be affected by materials, and how this might affect recolonisation success |
| • Quick and easy deployment–does not require complex machinery |
| • May act as habitat for unwanted organisms |
| • Introduction of foreign material |
| Metal stakes and plastic mesh netting |
Substrate stabilisation and provision of settlement substrate |
• Cheap materials, can be sourced locally |
• Small scale |
• Positive outcomes: Increase in fish biomass, coral recruit size and coral recruit survival (63% vs 6%) after two years Absent outcomes: Non-significant increase in coral cover. Negative outcomes: plastic netting still visible after 5 years [84] (S3 Appendix). |
| • Microbiome may be affected by materials, potentially limiting recolonisation success |
| • May become inconspicuous (gaining aesthetic appeal) |
| • Quick and easy deployment–does not require complex machinery |
• May act as habitat for unwanted organisms |
| • Likely restricted to relatively sheltered areas for deployment success and long-term stability |
| • Corals known to settle on both stakes and netting |
| • Risk of burial by surrounding rubble during storms due to low profile |
| • Prevents movement of loose rubble |
| • Introduction of foreign material |
| • Use of plastic for netting can introduce debris once breakdown begins |
| Inject chemicals (usually cement) to bond unconsolidated substrates |
Substrate stabilisation |
• Often cheap materials, readily sourced locally |
• Diffuse deployment (potential to contaminate non-degraded areas) |
• No literature to assess outcomes |
| • Can be deployed over moderately large areas (10–100 m2) with little expertise |
• Difficult to do underwater. |
| • Unknown toxicity of chemicals to rubble biota and other organisms |
| • Likely restricted to relatively sheltered areas for success |
| • Speeds the consolidation of rubble fields towards suitable settlement substrata |
| 3D frames (e.g. MARRS Reef Stars) |
Substrate stabilisation and providing habitat structure |
• Modular, ready scope to scale (quick and easy deployment–does not require complex machinery) |
• Potential refuge of corallivores, hindering coral recruit survival |
• Positive outcomes: MARRS Reef Stars resulted in increase in coral cover from 10% to over 50% after three years [99] (S3 Appendix). |
| • May require further ecosystem modification to establish (e.g. damselfish/corallivore removal) |
| • Can trap unconsolidated rubble from adjoining degraded reef areas |
| • Reef stars must be sourced from supplier and involves cost for bespoke fabrications (under patent). |
| • Can provide improved growing conditions for coral (higher than surrounding benthos) |
| • Addition of structures may incur high permitting risk |
| • Unknown resistance to high hydrodynamic energy |
| • Becomes inconspicuous relatively quickly (gaining aesthetic appeal)• |
| • Adding plastic, epoxy and steel to the marine environment |
| • Microbiome may be affected by materials, potentially limiting |
| • recolonisation success |
| • May act as habitat for unwanted organisms |
| • Provide/facilitate refuge for fish and invertebrates |
| • May serve as fish attracting devices, drawing fish from natural habitats |
| • Can be fixed in place or temporary for removal |
| • Visible for several months, reducing aesthetic appeal until the coral covers the frames |
| • Installation can allow for strong community engagement |
| BioRockTM; mesh frames (with or without electrical current) |
Substrate stabilisation and providing habitat structure |
• Same as for 3D frames; also: |
• Same as for 3D frames; also: |
• Positive outcomes: Increased attachment rates, survival and / or growth of coral fragments [97, 108–112], densities of reef associated fishes 6 times greater [113]. Negative outcomes: Decreased growth of fragments [114]. Absent outcomes: No change in growth rates [100]. |
| • Potential for facilitating/increasing levels of cementation within the rubble bed |
• Requires source of power adding costs and logistical challenges |
| • Current required for many months for good accretion |
| • Eventually incorporated into the reef framework |
| SECORE Tetrapods |
Providing structure for coral recruitment |
• Relatively inexpensive materials, readily sourced locally |
• Small size reduces scalability |
• Positive outcomes: 5 to 18-fold reduction in out planting costs compared to direct methods. Negative outcomes: low survivorship of coral recruits, rapid colonization by algae [98]. |
| • Need to be wedged into complex reef structure; role in |
| • rubble is unclear |
| • Can be deployed by divers |
• Introduction of foreign material |
| • May resemble consolidated reef substrate with aesthetic appeal |
| • Colonization by undesired organisms |
| • Eventually incorporated into the reef framework |
• High labour (diver) costs |
| Natural or concrete-fabricated structures: Reef BallsTM; Subcon reef modules; boulders, pipes and large objects. |
Substrate stabilisation and providing habitat structure |
• Modular design facilitates scalability |
• Larger scale habitat engineering may incur high permitting risk |
• Positive outcomes: Reef BallsTM provide shoreline protection and lead to increased fish abundance [115, 116]. Negative outcomes: Low coral recruitment [116]. |
| • Often relatively inexpensive materials, readily sourced locally (except Reef BallsTM) |
• Ecological (and climatic/biogeochemical) impacts of different grades of concrete |
| • Positive outcomes: Subcon modules were colonised by invertebrate and fish fauna similar to a nearby shipwreck in 20 months. Negative outcomes: The modules were rapidly colonised by algae [106]. |
| • Can create habitat structure at scale easily |
• Patented structures must be sourced from supplier and involves cost for bespoke fabrications (under patent). |
| • Promotes biodiversity, and can withstand some physical stress as scale increases |
| • Installations increasingly permanent as scale increases |
| • Positive outcomes: Tubular pipes completely overgrown with Porites colonies in 12 years. Negative outcomes: The Porites-dominated community replaced assemblages originally composed of Acropora thickets [117]. |
| • Provide/facilitate refuge for fish and invertebrates |
| • May resemble consolidated reef substrate with aesthetic appeal |
• Almost always requires heavy machinery |
| • Introduction of foreign material |
| • Eventually incorporated into the reef framework, depending on size |
• High risk of sedimentation onto colonised substrate in areas of degraded reef |
• Positive outcomes: Rock piles resulted in increase in fish communities similar to those of healthy reefs, hard coral cover from 0% to 44.5% over 14 years [69], (S3 Appendix). |
| • Reef BallsTM moulds can be bought from the company for different sized structures and fabricated on site using locally sourced cement plus admixtures |
• Microbiome may be affected by materials, potentially limiting recolonisation success |
| • May act as habitat for unwanted organisms |
| • May serve as fish attracting devices, drawing fish from natural habitats |
| • Sustainability issues around concrete production and transportation |
| Gabion cages/baskets/reef bags |
Substrate stabilisation and providing habitat structure |
• Mostly the same as for ‘natural or concrete-fabricated structures’–accessible and relatively low cost |
• Mostly the same as for ‘natural or concrete-fabricated structures’ except: |
• Positive outcomes: Reef bags stable, CCA recruitment, increased fish abundance, some coral recruitment after 7 months [104]. |
| • May require heavy machinery |
| • Filled with existing natural materials (e.g. reef rubble primed for coral recruitment) |
| • Eventually incorporated into the reef framework |
| • Can be constructed in situ by divers |
| • Provide shoreline protection if designed and positioned correctly |
