Coral reefs have lost between thirty and fifty percent of their global cover over the last half-century, and the decline is accelerating. Warmer seas cause the corals to expel the symbiotic algae that feed them, a process known as bleaching that can become fatal within weeks if the heat persists. Pollution, overfishing, and destructive coastal development have reduced the resilience of the reefs that remain. For many conservationists the question is no longer whether reefs can be protected in place but whether damaged reefs can be actively rebuilt - and if so, how quickly.

A. The simplest form of reef restoration is fragmentation. A living coral colony, when broken, readily grows new shoots from the fragments. Volunteers working off the Florida Keys have cut thousands of small pieces from healthy donor colonies, mounted them on concrete disks, and attached the disks to the reef with marine epoxy. Once a fragment has fused with the substrate, it can grow at close to its natural rate, and after five years the restored patch is often indistinguishable from the surrounding reef. The technique is cheap and requires no laboratory equipment, but it relies on donor colonies remaining healthy, and it does nothing to introduce genetic variation into a population that has been reduced by bleaching.

B. To address those limitations, researchers have turned to larval propagation. Most reef-building corals release their eggs and sperm during a brief synchronised spawning period, typically within a few nights of the full moon in spring or autumn. A team led by the Australian biologist Priya Narayan has learned to collect large quantities of this gamete "slick" with floating nets, fertilise the eggs under controlled conditions, and rear the resulting larvae on submerged screens for a few weeks before settling them onto prepared substrates. The advantage of this approach is that it produces genetically diverse offspring, which can then be transplanted to damaged reefs in numbers that fragmentation cannot match.

C. A more controversial line of research tries to make corals more heat-tolerant. Narayan's laboratory has experimented with assisted evolution, breeding colonies that survived a major bleaching event with one another in the hope of concentrating the genetic traits responsible for their resistance. In parallel, researchers in Hawaii have exposed coral fragments to gradually increasing temperatures to induce what is called hardening, a phenomenon in which an organism's existing physiology becomes more resistant to stress after a controlled exposure. Both approaches have produced measurable improvements, but sceptics argue that a reef adapted to current maximum temperatures will not necessarily cope with the higher temperatures expected within twenty years.

D. A separate problem is the community of species that a reef depends on. Coral is the structural engineer, but the ecosystem also includes fish, sea urchins, and countless invertebrates whose grazing controls the seaweed that otherwise overwhelms young colonies. Restoration projects that focus on coral alone often fail because an algae-dominated reef does not allow coral larvae to settle. Experimental work in the Caribbean has shown that releasing juvenile long-spined sea urchins, a species devastated by disease in the 1980s, can tip the balance back towards coral. The long-spined urchin eats algae aggressively and, having declined to about one percent of its historical density, has only recently begun to recover.

E. Physical protection can matter as much as biological intervention. Reefs protect coastlines from wave energy, and their removal exposes beaches and estuaries to erosion that degrades the remaining reef in a vicious cycle. Engineered breakwaters, if placed carefully, can reduce wave impact at the same time as they provide hard substrate for coral to colonise. A project in the Philippines has combined concrete breakwater structures with transplanted corals, and after eight years the resulting structure has trapped sediment, reduced coastal erosion, and been colonised by a modest reef community.

F. Not every restoration effort succeeds, and the failures are instructive. Narayan's team has documented several projects in which transplanted corals grew successfully for two or three years and then died in a later bleaching event, leaving the site worse off than if nothing had been done. The lesson, on her account, is that restoration must be matched to the local threat environment. Sites where bleaching is unavoidable within a few years are not good candidates for labour-intensive planting; they may be better served by protecting existing populations and reducing local stressors such as sewage and sediment runoff.

G. The scale of the challenge is daunting. A large coral reef measures millions of square kilometres, while a typical restoration project covers a few hectares. The French oceanographer Louis Marchand has calculated that rebuilding even ten percent of the reefs lost since 1980 would require more coral fragments than have ever been produced by human effort, with current techniques. His conclusion is that restoration, however skilful, cannot substitute for reducing the underlying pressures of climate change and pollution. It may, however, buy time for corals in particular locations, protecting cultural, economic, and ecological values that would otherwise be lost while the larger transitions play out.