Coral restoration methods

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The family of interventions that put coral back on a reef, which fall into two groups with very different properties. Asexual propagation — coral gardening — takes fragments, grows them on in-water nurseries of frames, trees or ropes, and outplants them; it is fast, reliable and produces genetically identical stock, which means it does not add genetic diversity and can outplant a clone that will bleach at the same threshold as its parent. Sexual propagation collects gametes at the annual mass spawning, rears larvae, and settles them on the reef, which is slower and less reliable but produces new genotypes. Around both sit the assisted-evolution approaches: selective breeding for heat tolerance, inoculation with tolerant symbionts, microbiome manipulation, and cryopreservation of gametes as insurance. The honest framing is arithmetic. Restoration operates at the scale of hectares and hundreds of thousands of fragments; reef loss operates at the scale of hundreds of thousands of square kilometres. Restoration is a way of holding genotypes, structures and local sites through a bad period. It is not a substitute for ending the thermal stress, and the summer of 2023 in Florida demonstrated why: outplanted coral dies in the same heat as wild coral.

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