What if coral reefs could be regrown?

Coral reefs are like busy underwater cities. Many fish live there.

But many reefs have been hurt. Could people help them grow back?

Coral reefs are home to huge numbers of fish and other sea animals, but many have been badly damaged. NOAA says that over the last 40 years, the Florida Keys have lost nearly 90 percent of the live corals that once covered their reefs.

So what if we could regrow coral reefs, the way people replant a forest? In this thought experiment you can follow three real paths: coral gardens, corals that cope better with heat, and the fish that come back to a restored reef.

NOAA reports that the world has already lost 30 to 50 percent of its coral reefs, and that nearly 90 percent of the live coral once dominating Florida Keys reefs has been lost over the last 40 years. Causes include warming and bleaching, disease, pollution, overfishing, storms, anchors and ship groundings.

This thought experiment asks what restoration can and cannot do. It follows three lines of evidence: coral propagation and outplanting, screening for thermal tolerance, and recovery of reef animal communities.

Ships and storms can break corals. Water that is too warm can make corals sick.

Broken pieces of coral roll around and often die.

Reefs are damaged in many ways. NOAA lists warming water that causes bleaching, disease, pollution, overfishing, storms, and boat anchors or ships that run aground on the reef.

When a ship grounds on a reef, it can leave hundreds or thousands of broken coral pieces scattered on the seafloor. Left alone, most would likely die, rolling around or buried under rubble. That is where people can help.

NOAA identifies local stressors (anchors, groundings, pollution, overfishing, storms, disease) and global ones (warming that causes bleaching, ocean acidification). Florida Keys reefs have faced all of these, and NOAA notes that although local restoration has succeeded at small scales, it has not kept up with the rate of decline.

Physical damage is where restoration began. After most groundings, NOAA's Office of Response and Restoration describes hundreds to thousands of fragments littering the seafloor, likely to perish unless rescued.

Divers collect broken coral pieces. They hang them on frames under the water, like clothes on a line.

The corals grow bigger. Then divers plant them back on the reef!

In a coral nursery, pieces of coral about four inches long hang from frames like clothes on a line or ornaments on a tree. Floating in the water, they get more water flowing past and are safer from coral-eating snails and worms.

Staghorn coral is one of the fastest growers: NOAA says it can grow up to eight inches a year. After about a year, divers can plant the corals back on the reef, or cut them into new pieces to grow even more. NOAA reported in 2014 that its partner nurseries were planting about 50,000 corals on reefs each year.

NOAA's 2014 account describes nursery fragments of about 4 inches hung on lines or PVC 'trees', improving water circulation and survival and avoiding predators such as fireworms and snails. Staghorn can grow up to 8 inches a year and elkhorn 4; after a year, colonies are outplanted or refragmented. Nurseries in Florida, Puerto Rico and the U.S. Virgin Islands then held about 50,000 corals and generated another 50,000 for transplanting each year.

NOAA's Mission: Iconic Reefs scales this up across seven Florida Keys reefs, aiming for 15 percent average coral cover in Phase 1 and 25 percent by the end of Phase 2.

Some corals grow new colonies from broken branches. So a small piece can grow into a big coral.

Plant-eating animals help too. They keep seaweed from covering the corals.

Restoration works especially well for staghorn and elkhorn corals, because in nature they make new colonies mostly from small branches that break off and reattach somewhere new. A nursery simply helps that happen faster and more safely.

Corals need space and light, so restoration teams also clear away algae that compete with them. NOAA's plan for the Florida Keys even adds algae-eating animals, such as long-spined sea urchins and Caribbean king crabs, to help keep the reef clean.

Staghorn and elkhorn reproduce predominantly by fragmentation, so propagation exploits their natural life history; fast growth makes them ideal nursery species, while slower species such as boulder and lobed star corals are harder to cultivate (NOAA).

Mission: Iconic Reefs pairs planting with site preparation: removing nuisance and invasive species, such as algae that prevent coral larvae from settling, and adding herbivores like long-spined sea urchins and Caribbean king crab. NOAA notes that about 25 percent coral cover is generally considered necessary to support a healthy ecosystem and protect reef structure.

In Indonesia, scientists listened to restored reefs. They heard lots of fish sounds: whoops, croaks and growls!

Regrowing reefs can help. But reefs also need the sea to stay healthy.

Does a regrown reef come back to life? In Indonesia, scientists recorded the sounds of restored reefs. They heard whoops, croaks, growls and other sounds, and found many more fish sounds on restored and healthy reefs than on damaged ones.

But restoration is not a complete fix. The study's lead author, Tim Lamont, said restoration can really work but is only part of the solution, alongside action on climate change and other threats. NOAA also says restoration in the Florida Keys has not yet kept up with how fast the reefs have declined.

Lamont, Simpson and colleagues (Journal of Applied Ecology, 2021) compared soundscapes from restored, healthy and degraded reefs in Indonesia using 2018–2019 recordings. Restored reefs were not identical to healthy ones, but their diversity of sounds was similar, and both had significantly more fish sounds than degraded reefs.

Lamont cautioned that without addressing wider threats, conditions would become more hostile and restoration eventually impossible. Scale is the other limit: NOAA notes local successes have not kept pace with decline, which is why Mission: Iconic Reefs targets ecosystem-level restoration.