Can we rebuild coral reefs? The promising (and weird) world of coral reef restoration 

Here’s what you need to know about coral reef restoration: 

Corals are cool. But the reefs face danger. A warming Ocean causes corals to bleach more regularly. Some estimates say we have already lost 50% of the world’s coral reefs.  

While we work to reduce the emissions of greenhouse gases, to keep our world from warming, we can also look to support the recovery and rebuilding of impacted coral reef systems. So today we ask: how can we restore our coral reefs (and how is a coral reef like a struggling orchestra)? 

Why should we care about coral reefs? 

Anyone that has had the privilege of diving on a coral reef will tell you how special these places are.  

Reefs cover less than 0.1% of our Ocean floor but support 20-30% of marine species. We have tried to estimate the economic value coral reefs bring, but it’s a difficult area, and economists can’t agree on the price tag.  

The range of $30 billion to over $300 billion puts coral reef value somewhere between “tremendously valuable” to “astronomically precious”. Include the goods and services provided by coral reefs and the estimated figure is $2.7 trillion. Trillion with a T.  

Coral reefs are excellent shields – healthy reefs can absorb 97% of wave energy. This protects our coasts, on which many of us live. Think of the most famous reef in the world – the Great (what?) Reef for example. 

So, we have to keep our corals around. The question is: how are we going to?  Let’s take a look at the three most common coral reef restoration methods. 

What is coral gardening? 

The most used method of reef restoration is coral gardening.  

Fragments of healthy coral are taken from an existing coral reef and placed in a nursery. This nursery is set up for the baby coral to thrive. When the corals reach a big enough size, they are ‘planted’ back onto the reef.  

This is a very accessible, increasingly cheap way of tackling coral decline. Costs are estimated to drop from $150-$400 per coral to <$10 per coral with improving techniques. It doesn’t require expensive equipment, and is a very visible, practical way to engage communities.  

Does coral gardening work long-term?  

Not sure. Short-term results? Pretty promising. Two large projects (Coral Reef Foundation in Florida and CARMABI in Curaçao) claim over 80% survival after one and three years respectively.  

However, it isn’t all sunshine and coral roses. These figures aren’t peer-reviewed (cross-examined by other scientists) and likely reflect best-case scenarios for certain coral species.  

A more accurate long-term figure is likely 30-50%, and although it does increase coral cover, does not comprehensively improve reef health.  

A healthy coral reef is diverse.  

Gardening projects, however, tend to focus on fast-growing genus like Acropora, ignoring slower growing (but just as important) species.  This results in ‘restored’ reefs that are low in biodiversity.

It’s like trying to have an orchestra with only violins. It is technically music, and possible to even be good, but lacks the depth and the magic of the interplay between instruments that brings it to life.

(One of our marine scientists favourite orchestral pieces is the Planet Earth II Suite: the layering of the song as different instruments come in make your soul soar. What other piece can boast having sleigh bells?? Listen here.) 

This coral restoration method is also limited in scalability – can it be used to make a big difference?

Coral gardening is like trying to replant the Amazon by using window boxes. It’s cost effective, and great for fast-growing corals. BUT it produces reefs with low genetic diversity (making them vulnerable to disease) and low species diversity.  

Gardening alone isn’t going to save our coral reefs.  

Can cutting corals into tiny bits help? 

Microfragmentation is chopping up coral colonies into little pieces. The fragments are placed next to each other, and will grow out, to form larger colonies.

The key advantage here is in the species this method targets, such as star or brain coral. 

Where, with coral gardening, we are predominantly working with fast-growing corals, this is for the slow-growing corals that are key to reef building, and for whom other methods won’t be effective. These are the bass section in our orchestra: there are less of them, and they are slower, but still crucial to the symphony.  

Studies have suggested that this method of coral restoration can accelerate the growth of massive colonies by 10-15%.  

However, this is limited to massive species and carries the same dangers of limited genetic diversity as gardening, if few donor colonies are used.

As coral reproduction is strongly linked to size, smashing colonies into little bits certainly impacts their reproductive capacity in the short term. Currently, we don’t know how much or how long that effect lasts. 

While this method is an excellent boost for the big boys on the reef, it’s not a reef-wide solution. If it’s used with more conventional gardening, you can help specific species of corals grow more successfully. But how can we support the entire reef system, in all its complexity and diversity? 

How do corals reproduce? 

Coral reproduction is weird. A few nights of the year, all the corals on the reef will release their eggs and sperm to mix in the Ocean currents. These are called coral spawning events. 

The fertilised eggs will be Ocean floaters until they find a spot to settle. Most species settle within two weeks, but some can take as long as to 2 months.  

Can we increase the amount of coral larvae settling?  

There’s growing appreciation of the different ways coral larvae decides where to settle. We now know that the sounds produced by a healthy reef act as a draw for young corals (find out more about the sounds of a coral reef here). Similarly, we are now realising that young corals “smell” their way to their new home.  

What can we do with this information? We can advertise reefs in a way young corals can understand. Speakers playing the noises of healthy reefs, and a newly engineered gel releasing chemical cues replicating a healthy reef are some examples.  

These solutions increase coral settlement, helping the reef rebuild itself. This is like advertising for more players in the orchestra, looking to bring in new talent. But what if we take that further?  

How do you help corals have more babies?  

The Ocean is becoming a lot less of a love nest as it warms. The success rates of coral fertilisation drop with rising temperature and acidity.  

So, how can we help the corals? By collecting the eggs and sperm during spawning events and taking them back to the lab. There, they have the best chance at fertilisation, and the larvae can be reared until they are ready to settle. Then, they can be released back to the reef.  

We can protect the coral at their most vulnerable stage of life.  

In the wild, less than 1% of coral larvae will make it to settlement. Of those that do, up to 90% won’t survive the first few months. The proportion of larvae to survive to a juvenile coral is minute, somewhere between 0.001 – 0.1%.  

Through assisted reproduction, the success rates are still low, but much higher than the wild. Some studies have shown survival rates to a year to be 0.1-1%. That might seem small, but it’s at least ten times better than the chances for a wild coral larvae. Others show an increase in coral cover after nearly three years. Even more promising? Drop the young corals in, rather than manually fixing them to the reef (a seeding approach), survival rates after a year can reach a whopping 9.6% while the costs remain low.

Here’s the real magic: these methods keep the gene pool diverse and interesting.    

We’ve already discussed fragment-growing methods like coral gardening and micro-fragmentation. But unlike fragment growing (which is basically coral cloning), assisted reproduction gives us reefs with genetic variety – think coral cousins rather than identical twins. And that variety? It makes reefs more resilient long term.  

This would be the equivalent of sponsoring a musical training programme, nurturing the next generation to guarantee the success of our orchestra.  

Where’s the catch?  

All the data here comes from projects with scientists doting on every need of the corals. Basically: If we were to strip back the money and the monitoring, the survival rates of corals will probably take a hit.  

Assisted reproduction works with the natural reefs, which is its strength as it maintains diversity and avoids the risks of disrupting the ecosystem with new species. It’s also a weakness, as some reefs have lost too many sexually mature corals to rebuild themselves.  

While it may not be the most efficient way to resurrect a reef, assisted reproduction could make the difference on degraded reefs needing a boost.  

Are artificial reefs the answer? 

Like corals, people are great builders. Like us, corals need a good foundation to build on. Some of the most fun coral reef projects focus around providing those foundations, through concrete blocks and 3D printed units.  

These foundations can encourage our polyp pals (AKA: coral babies) to settle down and make their home. We can build a new concert hall for our orchestra.  

These structures provide habitat immediately for non-coral animals to use as well (little fishies can hide and sponges and algae can grow).  

We have also found that running electricity through a concrete foundation helps coral growth (now that’s current science).  

Artificial coral reefs are a good initial boost.  

Plus, it works quickly and can be scaled up easily. Like coral gardening, artificial reef building is accessible enough for local communities to get their hands wet. And there’s nothing like a concrete reef structure to make conservation visible *literally* and raise the profile of reef protection efforts. 

But we don’t have a silver bullet here. There is concern that they could act as an ‘ecological trap’. They are fish magnets, not factories – they concentrate, not create.

Natural reefs and their residents settle and thrive not just because of a hard surface, but because there are good water conditions, plenty of food for their inhabitants and their populations are balanced.  

A reef in a poor location could end up negatively impacting the local fish. Imagine a new housing estate, but with no water and no shops. And acid rain. And bears. This is not a good housing estate.  

For artificial coral reefs to work they must be designed AND located with care and understanding. Don’t build a concert hall with terrible acoustics, no electricity and no public transport links. You need to know the area you are building in.  

Can we make super corals to survive climate change? 

None of the approaches so far tackle the root issue. The main threat to coral reefs is that the Ocean is changing faster than they can cope with.  

Could the answer then lie in us accelerating their adaption, selecting the more heat-resistant corals as evolution does, but faster?  

Our orchestra can experiment with new instruments and compositions to make a new sound.  

What is a super coral? 

Through selective breeding (choosing corals that can take the heat and breeding them) and microbial manipulation (like giving corals a probiotic yogurt, with beneficial bacteria and other tiny friends), we may be able to create ‘super corals’.  

When it works, it works brilliantly. Some lab studies showed that selectively bred corals could handle Ocean temperatures 1.5 degrees warmer than their non-selected colleagues.  

It’s an approach that directly addresses the main threat to corals, temperature rise, and could produce corals able to thrive in the predicted conditions of the Ocean in 2100. 

But temperature isn’t the only thing at work. These super corals have shown decreased resistance to Ocean acidification, the co-conspirator to Ocean warming. Think of a triathlete that can swim *like a fish* but also cycles like one. One-trick ponies aren’t what we’re going for when it comes to building healthy coral reefs.  

Our high performer corals also put a lot of energy into being super, so have less to put into growth and reproduction.  

Another simple hesitation is the amount we don’t know. How could the super corals fit in? Will they outcompete naturally evolving corals? Disrupt ecological balances we don’t yet understand? Will our new music find an audience? 

Despite these challenges, assisted evolution remains a promising way for reef conservation in a rapidly warming world.  

As one researcher memorably put it: “We’re not playing God with corals; we’re simply giving evolution a helping hand when we’ve rather inconsiderately moved the finish line.”  

But if we are making new music, maybe we need a different orchestra set up. 

Is coral reef restoration the way to go to save reefs?  

New approaches and ways of thinking suggest that we should embrace our changing world.

We may not be able to ‘restore’ our reefs to the way they were, as our world is not the same as it was. Instead of spending time, money, and effort trying to build the reefs that used to exist, we could help build a reef that can thrive in the future conditions of the Ocean.  

To play to the tune of the future, maybe we need more woodwind and brass. Think more jazz improvisation than classical recital.  

This could offer a more pragmatic approach, acknowledging that full restoration is not feasible in the long term. It focuses on ecosystem function rather than maintaining the old reefs and could integrate the use of ‘super corals’.  

But this comes with the issues of the unknown.  

Ecosystems are notorious for their chain reactions. Tweak one thing, and something you thought completely unrelated is affected.  Bring wolves back to Yellowstone? Suddenly the rivers change course. Remove tiger sharks from an area? Watch the dugongs reduce the seagrass to mud flats since they don’t have to watch their backs. 

How would the new ecosystem function and effect the life around it? What if the new saxophonist doesn’t get on well with the trombone players? What if the audience don’t like it? 

So how are we doing with coral reef restoration? 

One estimate calculates that less than 0.1% of degraded reef area is under active restoration. Most projects are small-scale (100m² or less) and short-term, with monitoring lasting less than 18 months.  

Restoring just 10% of degraded reefs could cost billions.  

The reality is most restoration projects are in convenient places, not where the corals will thrive. This renders most restoration projects vulnerable. Some can be completely lost after promising growth. 

But reef restoration is a stark reminder – humanity can act.  

Coral bleaching is among the most visual representations of our changing climate. But the time, effort and care that is devoted to restoring coral reefs around the world shows the desire to protect our natural world.  

For us to have healthy coral reefs, to have our orchestra really sing, we need to combine approaches. We can’t focus only on strings or on bringing in the young talent. We need to support the whole orchestra so we can enjoy the music.  

As corals have been a poster child for the degradation of our world, so too could they be the success story. Every young coral nurtured today could be the foundation of a healthy future reef, different to yesterday’s maybe, but no less important for our blue planet.