Remove problematic species

Overall effectiveness category Awaiting assessment
Number of studies: 7
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Background information and definitions

The effect of soft corals, macroalgae, and other problematic species on coral growth is mixed. However, degraded coral reefs can transition from hard (stony) coral to soft coral or algae-dominated systems (Fox et al. 2003; Souter et al. 2021). These systems can inhibit growth of stony coral and lack the complex structure that reef-building stony corals provide, which is essential to support high biodiversity and ecosystem services (Fox et al. 2003; Souter et al. 2021). In addition, the quantity of algae on a coral reef system is an indicator of stress in coral reefs and is associated with a global decline in stony corals (Souter et al. 2021).

This action specifically relates to the effect on stony corals of removing problematic species from around transplant or restoration sites. The action refers to nuisance macroalgae species and not the beneficial algae (zooxanthellae) that form part of the coral itself. Studies describing transplanting wild-grown coral onto natural substrates or nursery-grown corals onto natural or artificial substrates are covered in Transplant nursery-grown corals onto natural substrate; Transplant nursery-grown coral fragments onto artificial substrate; Transplant wild-grown corals onto natural substrate; and Transplant wild-grown coral onto artificial substrate.

^{Fox H.E., Pet J.S., Dahuri R. & Caldwell R.L. (2003) Recovery in rubble fields: long-term impacts of blast fishing, Marine Pollution Bulletin, 46, 1024-1031 https://doi.org/10.1016/S0025-326X(03)00246-7.}

^{Souter D., Planes S., Wicquart J., Logan M., Obura D. & Staub F. (Eds.) (2021) Status of Coral Reefs of the World: 2020 Global Coral Reef Monitoring Network (GCRMN) Available from https://gcrmn.net/2020-report/}

Study locations

Key messages

Seven studies evaluated the effects of removing problematic species. Two studies were in the USA and one study was in each of Indonesia, the Philippines, the British Virgin Islands, Belize, Menorca (Spain).

COMMUNITY RESPONSE (0 STUDIES)

POPULATION RESPONSE (6 STUDIES)

Abundance/Cover (3 studies): Two of three replicated studies (including one randomized, controlled, one controlled, and one before-and-after) in Indonesia, the Philippines, and Menorca, found that repeated removal of problematic soft coral led to an increased number of stony coral colonies, and removing nuisance algae led to a higher number of juvenile corals. One study, found that using topshell snails to control nuisance algae around transplanted coral did not lead to an increase in coral recruitment.
Survival (4 studies): One of four replicated studies (including one randomized, controlled) in the Philippines, the British Virgin Islands, Belize, and the USA found that removing nuisance algae and zoanthids from stony coral fragments immediately after transplanting led to greater survival than fragments on sites cleared monthly or not at all. Two studies found that clearing nuisance algae from around transplanted fragments did not lead to higher survival for elkhorn coral and led to a lower survival rate for mustard hill coralcompared to areas without clearance. One study found that using topshell snails to control nuisance algae around the transplant site did not result in higher survival of transplanted coral compared to areas without topshell snails.
Condition (4 studies): One of four replicated studies (including one randomized, controlled) in the British Virgin Islands,Belize, and the USA, found that clearing macroalgae from around transplanted fragments of elkhorn coral led to higher live tissue growth compared to fragments transplanted without algae clearance, and one study found removing nuisance algae from the transplant site led to a lower bleaching rate for one of two transplanted species, but no effect on growth of either. One study found that % tissue loss after 12 months was lower on corals where excavating sponges were removed, and the resultant cavity filled. One study found mixed effects on growth of transplanted stony coral fragments when algae and zoanthids were removed from around the transplant site immediately, monthly, or not removed.

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, before-and-after study in 1999–2000 at a coral rubble site in Komodo National Park, Indonesia (Fox et al. 2003) found that repeated clearance of problematic soft coral from hard coral rubble led to an increase in the number of stony coral colonies, but soft coral increased in areas cleared only once. The average number of stony coral colonies increased in the repeatedly-cleared plots, from 2.94/m² when soft coral was first cleared to 7.15/m² five months later. In plots where problematic soft coral was only cleared once, coverage of soft coral increased to 95–100% five months after the initial clearance. In November 1999, problematic soft coral colonies were cleared from fifteen 1 × 1 m plots on areas of coral rubble left by blast fishing. Eight of the 15 plots were re-cleared of soft coral every alternate month; the remaining seven plots were left to be re-colonized by soft coral. Plots were surveyed and photographed, and stony coral colonies were counted when the soft coral was first cleared and again five months later.
Study and other actions tested
Referenced paper
Fox H.E., Pet J.S., Dahuri R. & Caldwell R.L. (2003) Recovery in rubble fields: Long-term impacts of blast fishing. Marine Pollution Bulletin, 46, 1024-1031.
A replicated, controlled study in 2007 near a coral reef in Bolinao, northwestern Phillipines (Villanueva et al. 2010) found that adding topshell snails Trochus niloticus to control algal growth on artificial reef structures (some with transplanted stony coral fragments attached) did not lead to an increase in coral recruitment or survival rate of fragments compared to structures without topshells. Five months after transplanting, there was no significant difference in the average density of coral spat (settled larvae) on structures with (8 – 30 spat/m²) and without topshells (16–22/m²), or survival rate for fragments (data not reported). Overall, survival rate ranged from 51% (Acropora muricata) to 97% (Montipora digitata). A total of 2,189 coral spat were recorded; 85% pocilloporids, 8% poritids, 4% acroporids, and 6% unidentifiable. In January 2007, forty-two concrete pallet balls (1.2 m diameter, 0.9 m high) (see paper for full design) were placed 4-8 m deep on sandy substrate 3-5 m from a natural coral reef. Ten topshells were added to each of 21 of the balls. Each pallet ball also had zero, 25 (5/species) or 50 (10/species) nursery reared stony coral Pocillopora damicornis, Acropora muricata, Porites cylindrica, Montipora digitata, and Echinopora lamellosa fragments attached. Coral spat was counted on each pallet ball after approximately five months. Survival was recorded after six months.
Study and other actions tested
Referenced paper
Villanueva R.D., Edwards A.J. & Bell J.D. (2010) Enhancement of grazing gastropod populations as a coral reef restoration tool: Predation effects and related applied implications. Restoration Ecology, 18, 803-809.
A replicated, controlled study in 2008–2009 at two coral reef sites near Guana Island, British Virgin Islands (Forrester et al. 2011) found that removing macroalgae from the transplant site for storm-generated fragments of elkhorn Acropora palmata coral led to a higher increase in live tissue growth but no difference in survival compared to fragments transplanted without algae removal. One year after transplanting, the increase in live tissue surface area was higher on fragments where algae had been removed (160%) than fragments transplanted without algae clearance (68%). Survival of fragments after one year did not vary significantly (algae cleared: 52%; algae not cleared: 60% survival). In July–August 2008, a total of 237 storm-generated fragments of elkhorn coral were collected from a coral reef and prepared for transplantation either at the collection site or another site 0.4–3.6 km away. Fragments were attached to the reef substrate, or dead elkhorn coral skeletons, using cable ties, marine epoxy or cement and ensuring live tissue was in contact with the substrate. Once attached, macroalgae was scraped away from a circle of 20 cm radius around 117 of the 237 fragments. Growth (surface area of live tissue) was measured after two and 12 months, and survival was recorded after 12 months using photographs.
Study and other actions tested
Referenced paper
Forrester G.E., O'Connell-Rodwell C., Baily P., Forrester L.M., Giovannini S., Harmon L., Karis R., Krumholz J., Rodwell T. & Jarecki L. (2011) Evaluating methods for transplanting endangered Elkhorn Corals in the Virgin Islands. Restoration Ecology, 19, 299-306.
A replicated, controlled study in 2003–2004 of 16 patch reefs in Belize (McClanahan et al. 2011) found that removing algae from transplant sites for massive starlet Siderastrea siderea and mustard hill Porites astreoides corals had mixed effects on bleaching and survival rates, and no effect on growth. Eighteen months after transplanting, average bleaching rates for massive starlet coral were lower at sites with algae removed (0.7%) than without (1%), and there was no effect on growth or survival rates (data not reported). For mustard hill coral, sites with algae clearance had lower average survival rates (85%) than those without (90%), and there was no effect on growth or bleaching rates (data not reported). In January 2003, and monthly thereafter, algae were removed from eight of 16 patch reefs (each 25–50 m²) using hedge clippers and wire brushes. Algae were left intact on the other eight reefs. Shortly after initial algae removal, six ‘fist-sized’ massive starlet and mustard hill corals were collected from 1–3 km away and attached to each of the 16 reefs using masonry cement. Bleached corals were counted monthly, and surviving corals measured every three months, until August 2004.
Study and other actions tested
Referenced paper
McClanahan T.R., Huntington B.E. & Cokos B. (2011) Coral responses to macroalgal reduction and fisheries closure on Caribbean patch reefs. Marine Ecology Progress Series, 437, 89-102.
A randomized, replicated study in 2008 at two sites in Menorca, Balearic Islands, NW Mediterranean (Linares et al. 2012), found that removing nuisance turf algae from near temperate soft coral Eunicella singularis colonies led to a higher number of juvenile coral than areas without algae removal. Three months after turf algae was removed, the average number of juvenile soft coral was higher in areas where turf algae had been removed (Cap Roig: 14.7/m²; Na Ponsa: 2.2/m²) compared to areas without algae removal (Cap Roig: 1.5/m²; Na Ponsa 0/m²). In April 2008, forty 40 × 40 cm quadrats were randomly marked 15–20 m deep at two sites (Cap Roig and Na Ponsa). Turf algae was removed from within 20 quadrats at each site and 20 were left undisturbed. Three months later, in July 2008, quadrats were inspected in-situ and Eunicella singularis recruits (~3–5 mm high) were counted.
Study and other actions tested
Referenced paper
Linares C., Cebrian E. & Coma R. (2012) Effects of turf algae on recruitment and juvenile survival of gorgonian corals. Marine Ecology Progress Series, 452, 81-88.
A replicated, controlled, before-and after-study in 2012 at a coral reef in the USA (Halperin et al. 2016) found that removing the excavating-sponge Cliona delitrix from colonies of stony coral Montastrea cavernosa and filling the cavity (using cement or epoxy) led to some reduction in dead tissue compared to corals with sponges remaining but no reduction in the rate of sponge recurrence. After six months, there was no statistical difference in % dead tissue between corals with sponges removed (6%) than with sponges remaining (20%) but after 12 months, % tissue loss was lower on corals with sponges removed (4%) than with sponges remaining (33%). There was no difference in % dead tissue between cavity-filling material (cement: 6 and 12 months 11%; epoxy 6 months 1%, 12 months -2%). There was no statistical difference in the rate of sponge recurrence after six months (overall: 14%; cement: 18%; epoxy: 9%) or 12 months (overall: 36%; cement: 45%; epoxy 27%). In July 2012, excavating sponges Cliona delitrix, were removed from 22 of 33 stony coral Montastrea cavernosa colonies using a hammer, chisel and steel wool; the remaining 11 colonies had no sponges removed. Cavities created by sponge removal on the 22 corals were filled with either cement (11/22) or epoxy (11/22). Percentage dead tissue, and % sponge recurrence were recorded using photographs six and 12 months after sponge removal.
Study and other actions tested
Referenced paper
Halperin A.A., Chaves-Fonnegra A. & Gilliam D.S. (2016) Effects of excavating-sponge removal on coral growth. Journal of the Marine Biological Association of the United Kingdom, 32, 61-69.
A replicated, randomized, controlled study in 2016–2017 at a coral reef in Florida, USA (Lustic et al. 2020) found that removing algae, along with zoanthids Palythoa caribaeorum, from around colonies of three stony coral species immediately after transplanting resulted in greater overall survival compared to those with algae and zoanthids removed monthly or not removed, and there were mixed effects on growth. After 17–18 months, overall survival for three coral species combined was greater at transplant sites where algae and zoanthids were initially removed than at sites where algae and zoanthids were removed monthly or not removed (data reported as statistical model results). Removing algae and zoanthids initially or monthly led to greater increases in volume of staghorn coral Acropora cervicornis colonies (990–1,409%), greater losses in surface area of great star coral Montastraea cavernosa colonies (-25 to -30%) and similar losses in surface area of mountainous star coral Orbicella faveolate colonies (-4 to -12%) compared to colonies without algae and zoanthids removed (staghorn: 570%; great star: -8%; mountainous star: -12%). In March 2016, forty-five nursery-grown colonies of each of three coral species (staghorn: 66–575 cm³; great star: 45–120 cm²; mountainous star: 38–130 cm²) were transplanted onto hard substrate on a reef. One of each of three treatments was applied to each colony: algae and zoanthids Palythoa caribaeorum removed from a circle of 25-cm radius at the time of transplanting or at monthly intervals, or algae not removed (15 colonies/species/treatment). Colony survival and growth (volume or surface area of live tissue) were recorded after 3, 6, 9, 13 and 17–18 months.
Study and other actions tested
Referenced paper
Lustic C., Maxwell K., Bartels E., Reckenbeil B., Utset E., Schopmeyer S., Zink I. & Lirman D. (2020) The impacts of competitive interactions on coral colonies after transplantation: A multispecies experiment from the Florida Keys, US. Bulletin of Marine Science, 96, 805-818.

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
Transplant nursery-grown coral fragments onto artificial substrate Action Link	Awaiting assessment	15	Synopsis Link
Change transplant attachment method Action Link	Awaiting assessment	3	Synopsis Link
Transplant wild-grown coral onto artificial substrate Action Link	Awaiting assessment	24	Synopsis Link