How to regrow a coral reef

Coral reefs could disappear by the end of the century, warns broadcaster David Attenborough1.  However, it might not be too late to save them.  Some scientists are exploring ways to regrow damaged coral reefs.

What’s a coral reef?

You’ve probably seen photos of the amazing, statuesque shapes that coral colonies grow into, from platforms to branches to domes.  These colonies are made of many tiny coral polyps, which are creatures related to jellyfish.  Each polyp houses algae that give the coral both energy from the sunlight and colour2.

Each polyp has a small skeleton, usually just millimetres across, made from limestone taken from the seawater3.  New polyps grow on top of the skeletons of old polyps, making the coral colonies expand year on year4.

Coral colonies provide hiding places and homes for fish and many other forms of life.  The entire ecosystem of corals, animals and plants is called a coral reef.  Astonishingly, although coral reefs cover less than 1% of the ocean floor, they support over 25% of marine life5.

Coral colonies make many different shapes as polyps grow on top of old skeletons.  Photo credit: Holobionics (CC BY-SA 4.0).

How do people damage coral reefs?

Coral bleaching

Much of the heat trapped by climate change goes into the oceans, warming them.  When the water around a coral reef gets too hot, the algae that live inside the corals start producing toxins.  The polyps evict their algae to survive.  Since the algae gives coral its colour, the coral goes white.  This is known as coral bleaching.

Bleached branching coral (foreground) and healthy coral (background).  Image credit: Acropora (CC BY 3.0).

Bleached coral doesn’t die immediately.  It can still feed itself by catching prey with its tentacles6.  However, bleached coral is less able to reproduce and can die if the algae doesn’t return in a few weeks or months7.

Bleaching is happening more often now: the average gap between bleaching events has decreased from 25-30 years before the 1980s to just 6 years now.  That doesn’t give coral reefs time to recover fully: even the fastest growing corals need 10-15 years to recover8.


Climate change has another effect: seawater becomes more acidic because there’s more carbon dioxide in it.  It’s enough to start dissolving the hard shells of coral – much like your teeth might get holes if you drink too many acidic drinks9.  By the end of the century, coral reefs might be dissolving faster than they can grow10.

Many others

Taking too many fish harms coral reef habitats.  Some particularly harmful fishing methods, such as dynamite blasting and net trawling, can physically damage the coral structures.  Some types of fish eat seaweed: if they are overfished, the seaweed grows over the coral.  Pollution from nearby land can also harm coral reef ecosystems.  Tourists might take pieces of coral for souvenirs or go on ‘feeding the fish’ diving tours that can train the fish to act aggressively towards divers.

Why bother protecting the reefs?

The beauty of coral reefs is only one benefit.  Heathy coral reefs provide food for many people, bring money in through tourism and protect coasts from erosion by absorbing the energy of storm waves.  The incredible genetic diversity that they hold could inspire medical research.

How much are these goods and services worth each year?  Estimates vary from $30 billion11 to $172 billion12 to $375 billion13.  One thing is clear: we’d be worse off without coral reefs.

Some protective measures are already in place – Fiji has recently announced that much of its Great Sea Reef will become a protected site14.

But when the damage has already been done, can coral reefs be restored or are they gone for good?  Here are some ways that damaged coral reefs can be revived.

Coral gardening

When the water cools down after a bleaching event, new colonies can grow when coral eggs arrive on the currents from healthy neighbouring colonies.  However, corals might only release eggs once a year, so it can take a long time for bleached areas to recover.

‘Coral gardening’ solves this problem.  In Fiji, local fishers have been taught by biologists to pick out young corals that are growing in overcrowded places.  They attach the corals to wires or small cement discs and take them to raised platforms with plenty of space and light.  Eventually, the coral grows big enough to be planted onto bare areas of the reef.

BBC Earth’s feature on the project, below, shows how it works.

The starter fragments of coral could also come from large, healthy colonies that stay colourful even when other colonies around them are bleached.  This would help the spread of coral that will be able to cope with the warmer seas of the future.

The Fijian project has replanted over 14,000 corals of over 25 different species into the reef since 200615.  Visitors who come to see the restoration efforts and go on snorkelling tours have brought thousands of dollars to local villages.

The success of the coral gardening does appear to rely heavily on establishing no-fishing zones.  It’s also not clear whether the projects, initially funded by a charitable grant, are yet financially independent.

Coral gardening has also successfully increased populations of the critically endangered staghorn coral in the Caribbean16.  Other places where coral gardening is used include the Philippines17, Australia’s Great Barrier Reef (where coral is planted onto the reef at a very young stage rather than spending much time in a nursery)18 and Thailand19.

Micro-fragmentation for speed

In the Florida Keys, biologist Dr David Vaughan leads a team hoping to restore the local coral reefs.  He initially used techniques similar to other coral gardening projects: repairing damaged reefs by transplanting coral pieces a few centimetres in size.

After accidentally shattering a coral he was trying to grow in the laboratory, Vaughan found that the fragments grew back to their original size in just two weeks.  Experimentation showed that tiny coral fragments could grow 25 to 40 times faster than was thought possible!

Soon, Vaughan was using this ‘micro-fragmentation’ strategy to grow corals that normally take years to develop.  Some large dome-shaped species of coral are particularly important because they live for centuries and protect coastlines from strong waves20.  Vaughan glued four tiny fragments of this coral to a ceramic tile and let them grow and fuse into each other.  In just 9 months, this technique produced a coral dome that would otherwise take 15 to 25 years to grow!  It’s not clear, however, whether the fused coral domes are as strong as the natural slow-growing domes.

Micro-fragmentation gives a speed boost to coral restoration efforts, which could help reefs to recover before the next bleaching event hits.

The downside is that micro-fragmentation could create a monoculture, where all corals grown from one parent coral are genetically identical21.  If a heatwave or disease hits the reef and the restored corals don’t have the genes they need to cope, then all of the restored corals could die at once.  Farming has a similar problem: in the 1950s, the main commercial variety of banana, Gros Michel, was almost wiped out by a fungal disease.  Nowadays, 99% of bananas exported to developed countries are the relatively bland Cavendish variety, which could itself fall prey to a new fungal disease22.

Vaughan might have an answer to this problem.  In the laboratory, the water in the tanks can be made warmer and more acidic to simulate the effects of climate change.  The coral varieties that cope with this might have a better chance of surviving on reefs in the future.

Watch The Atlantic’s feature on the work below.  Vaughan’s enthusiasm is infectious!

Artificial coral reefs

Restoring existing reefs might help in the short term, but what about when those locations become too hot and bleaching occurs nearly every year?  Is it possible to set up an entirely new reef in a cooler location that could cope better with climate change?

The idea of artificial coral reefs has been around for some time: place rocks or other man-made structures into the sea, and they will attract ocean wildlife and form the basis of a new ecosystem.  Ships and other structures have been purposely sunk to attract fish and bring money to the area from divers and fishers23.

Not all materials are suitable.  An artificial reef in Florida made from used tyres had to be removed, because the tyres were breaking loose and damaging existing coral24.

However, some artificial reefs have attracted astonishing numbers of fish.  An artificial reef off the coast of Japan supported a weight of fish 20 times greater than natural reefs of the same size.  Oil and gas rigs near California had a weight of fish per square metre of sea floor 27 times higher than natural rocky reefs25.  Researchers think that the new habitats actually increase the overall fish population, rather than just attracting fish away from their existing homes.  This could be good news for people who rely on fish as a food source, since 90% of fisheries around the world are either fully fished or overfished26.

The shape of artificial reef structures determines how well it can support fish27.  The Reef Ball Foundation has used this principle to develop concrete balls with holes of different sizes.  The hollows attract fish and coral can be transplanted directly onto the surface.  The concrete blocks are simple to make and are stable; the Reef Ball Foundation claims that they stayed in place even during the 2004 Boxing Day Tsunami28.

Reef balls ready to be placed into Lake Pontchartrain.  Image credit: Louisiana Sea Grant College Program, Louisiana State University (CC BY 2.0).

One particularly good material for coral to grow on is ‘Biorock’.  These are metal structures to which a low electrical current is applied (it is not dangerous to fish or divers).  The electricity attracts minerals from the seawater, which gradually build up a rock-like material.

Corals grow well on Biorock, because it is the same material that they would naturally attach to.  Biorock reefs were even found to cope better with bleaching: in the Maldives in 1998, more than 95% of coral on natural reefs died due to bleaching, while only 20-40% of corals on the electrified reefs died29.

However, if the electric current is removed, then the new reef loses its protection and could become overgrown with seaweed.  Biorock may therefore not be a good way of preserving coral reefs in the long term, but it could still be useful for attracting tourists or fish.

A Biorock reef in Indonesia.  Image credit: Marionpinta (CC BY -SA 3.0).

Will coral restoration work?

While coral gardening, microfragmentation and artificial reefs all have benefits, they don’t tackle the root problems.  As long as it makes economic sense to overfish reefs, burn fossil fuels and allow reefs to be polluted, these problems will continue to threaten coral reefs.

The good news is that pollution and overfishing can be tackled relatively quickly, compared to tackling climate change.  Doing so could give coral reefs more resilience to cope with warmer waters30.

Simply having the technology to revive coral reefs doesn’t mean that coral is safe.  While the benefits of coral reefs are widespread, the organisations doing the work often find it difficult to get funding.

Part of the problem is that healthy coral reefs are, to some extent, a public good31.  That means that while many people would benefit from restored coral reefs, most of those people can still get those benefits even if they don’t pay for the restoration.  Businesses will find it difficult to get enough income to justify doing the work, leaving charities or governments to step in.

It makes more business sense to provide coral reef restoration services when it’s clear who will benefit and when those people are willing to pay.  This could apply to tourism in particular – for example, hotels and diving schools might decide to invest in an artificial reef to attract more tourists.

Perhaps the key to saving coral is discovering viable business models for coral reef restoration.

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  1. Dowell (2017), RadioTimes, David Attenborough to end Blue Planet II with a stark warning on coral reefs
  2. Buchheim, Coral Reef Bleaching
  3. NOAA (2017), What is a coral reef made of?
  4. There are also soft corals that don’t have skeletons.
  5. Coral Reef Alliance, Coral Reef Biodiversity
  6. Florida Keys National Marine Sanctuary, Corals get their food from algae living in their tissues or by capturing and digesting prey; microdocs (2012), Coral Bleaching
  7. Buchheim, Coral Reef Bleaching
  8. Unfortunately the original study is locked behind a paywall, but you can read about its findings here: Hannam (2018), The Sydney Morning Herald, ‘Rather startling’: Study finds rapid increase in frequency of coral bleaching
  9. Guardian (2016), Ocean acidification slowing coral reef growth, study confirms
  10. Pacific Marine Environmental Laboratory, What is Ocean Acidification?
  11. WWF, Coral reefs: importance
  12. EurekAlert! (2009), What are coral reef services worth? $130,000 to $1.2 million per hectare, per year: experts
  13. Costanza et al. (1997), The Value of the World’s Ecosystem Services and Natural Capital, Table 2
  14. UN Environment, Fiji’s Great Sea Reef nominated as Ramsar Site at start of International Year of the Reef
  15. Reef Resilience Network (2014), Fiji – Ecological Restoration
  16. Gaworecki (2017), Pacific Standard, Behind the restorative effects of coral gardening
  17. Shaish et al. (2008), Fixed and suspended coral nurseries in the Philippines: Establishing the first step in the “gardening concept” of reef restoration
  18. The Japan Times (2017), Coral transplant experiment raises survival hopes for Australia’s Great Barrier Reef
  19. COREsea, Coral restoration
  20. National Ocean Service, What are brain corals?
  21. Smith (2016), Reviving the reef
  22. Stergiopoulos, Drenth and Kema (2016), The banana as we know it is in imminent danger
  23. National Ocean Service, What is an artificial reef?
  24. The Guardian (2015), Florida retrieving 700,000 tires after failed bid to create artificial reef
  25. The Economist (2014), Watery dwellings and Occidental College (2014), Oil platforms among top marine fish habitats, study shows
  26. Neslen (2016), Global fish production approaching sustainable limit, UN warns
  27. Perkol-Finkel et al. (2006), Can artificial reefs mimic natural reef communities?  The roles of structural features and age; Sherman et al. (2002), Artificial reef design: void space, complexity, and attractants
  28. Reef Ball Foundation, What is a reef ball?
  29. Biorock International Coral Reef Restoration, Have mineral accretion results been documented?
  30. Waters (2016), There’s still hope for the Great Barrier Reef
  31. Investopedia, Public good

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