How restoration can help coral reefs

KOH TAO, Thailand — Record-high sea temperatures are bleaching coral reefs worldwide and putting a new focus on attempts to restore these key marine ecosystems.

Here is an overview of how coral restoration is being done around the world:

Restoration begins with obtaining coral, sometimes by breaking it off from a healthy reef. These pieces can be broken into smaller bits in a process called microfragmentation.

Each piece can become a new coral.

Another option involves collecting "fragments of opportunity" -- coral pieces broken off by natural causes such as storms.

Conservationists can also propagate from egg bundles collected during reef spawning, though this is perhaps the most difficult approach.

Spawning is brief, generally happening just once a year, and is affected by many factors, including the lunar phase and water temperature.

Coral microfragments generally go into a "nursery" until they grow sturdy enough to be transplanted to an existing reef or an artificial structure.

Fragments of opportunity are treated similarly. If big enough, they can be transplanted directly to natural or artificial reefs.

Bundles of coral eggs and sperm collected during spawning will develop into larvae that can then be settled onto reefs or, more commonly, grown on artificial foundations before being transplanted to their final homes.

Other techniques are used to bolster coral restoration, including mineral accretion technology.

This involves sending a low-voltage electrical current through seawater to encourage minerals to dissolve and crystallise on artificial reef structures, speeding up coral growth.

The technique has had mixed results, with some studies reporting better growth and more resilient corals, but others finding no significant benefits.

Other interventions include substrate stabilisation, which shores up reef foundations, and algae removal.

Restoration projects heavily favour quick-growing branching corals.

The delicate branches of these corals are more susceptible to becoming fragments of opportunity, and are also easier to microfragment than massive or encrusting corals.

Their fast-growing nature gives restoration projects quicker results, though focusing on