The slow science of coral nurseries

Forty minutes after sunrise, a pair of divers slip into the water above a reef off a quiet island. Their first job is not to explore. It is to check on the equivalent of a garden: rows of rope and metal frames hung in open water, each strung with tiny fragments of living coral.

This is a coral nursery, and it represents one of the most hopeful and most debated ideas in marine conservation.

Why nurseries exist

Reefs cover less than one percent of the ocean floor yet support roughly a quarter of marine species. Rising sea temperatures cause bleaching, where corals expel the algae that feed them. Storms, disease and pollution add further damage.

Corals regrow naturally, but slowly, often only a few centimetres a year. After a severe event, a reef may need decades to recover, if conditions allow it at all.

How the gardening works

Divers collect small fragments from healthy colonies, sometimes ones that survived a bleaching event. They attach these to nursery structures where the pieces are protected from predators and grow quickly.

After several months, the fragments are carried to damaged reef sites and fixed in place with cement or clips. Some programmes now use micro-fragmenting, cutting corals into very small pieces that grow faster.

• Choose fragments from heat-tolerant colonies when possible.
• Mix several species and genotypes for resilience.
• Monitor survival for years, not weeks.

What the evidence says

Survival rates vary widely, from highly successful projects to ones where most transplants were lost to the next heatwave. Researchers stress that restored patches do best where the underlying causes of decline, such as pollution and overfishing, are also addressed.

The cost question

Nursery work is labour intensive and expensive per square metre compared with protecting an intact reef. Critics argue the money would be better spent on protection and climate policy, while supporters point out that some reefs are too damaged to wait.

Nobody involved thinks nurseries will save the world's reefs by themselves. They think a reef lost is a reef lost.

Looking ahead

New work focuses on selective breeding for heat tolerance and on assisted spawning, which could scale restoration far beyond what divers can plant by hand. For now, the scaffolds sway in the current, and the people tending them keep counting.