Enhancing coral reproduction and recruitment on recovering reefs
Methods to help re-establish corals in degraded areas are being assessed. The techniques comprise various combinations of:
- larval seeding – enhancing the reproductive success, or the rate of coral larval settlement and growth, on reefs
- fragmentation – similar to plants, corals can be broken into small fragments, with each section growing into an adult coral. As coral fragments grow, they, in turn, can be repeatedly fragmented as required.
Coral seeding - sexual reproduction
Coral seeding aims to speed the return of coral cover to a disturbed or damaged reef by increasing the number of available coral larvae for natural settlement, particularly where the reef has a low larval supply (e.g. following a large-scale bleaching event). The larvae could either come from wild populations – collected and relocated from other reefs – or could be cultured through mass-production aquaculture.
This technique has been successfully trialled on small scales, mostly using the larvae of a single species, spawned and reared in an aquarium setting. Natural wild coral larval slicks – collected from annual mass spawning – have also been successfully relocated to reef surfaces.
Harvesting wild coral larval slicks is likely to have a minimal negative impact on Reef ecology, as the mortality rate of the larvae in a slick is typically high. Compared with re-seeding just a few species, it offers the potential to capture a diverse range of species and allow the re-establishment of reef communities.
While coral spawn naturally travels long distances between reefs, relocating both wild and cultured corals would require measures to ensure the introduced coral did not harm the local population. More research is needed into when and where slicks form, and whether the wild larvae can be cultured, moved and re-distributed without negative effects on receiving populations.
Further research is also needed to:
- improve forecasting of future larval supply and recruitment rates
- quantify larval connectivity
- determine quantities required to have an impact under different scenarios
- devise methods to scale-up larval seeding for dispersal over larger areas.
It is likely this method would be used in conjunction with some form of reef surface conditioning, to increase the subsequent success of released larvae to settle onto the reef.
Photo by Marie Roman
Coral seeding using settlement devices
A promising method of efficient dispersal is the use of small, re-seeding units (such as the tetrapods developed by SECORE) that allow corals raised in aquaculture – or collected from wild spawn slicks – to be placed on reefs with minimal handling. This significantly reduces time spent re-locating and planting cultured corals. Millions of these units could be deployed to accelerate recovery at large scales (see diagram above).
Harvesting coral fragments - asexual reproduction
Coral fragments can be harvested from areas of high coral cover, or by using fragments broken off in weather events, to be transplanted to high need areas such as degraded reefs or coral colonies under threat. For this method to be successful, the donor reefs need to be healthy enough to withstand harvesting, and the receiving reefs needs to have conditions favourable for coral growth and reef establishment.
Additional treatments with beneficial microbes and techniques to increase resilience may also be applied.
Success is likely to depend on the size and health of the fragments, the methods of transport and attachment, and external factors such as the environmental conditions following the transplantation. Often fragments that have already been dislodged through a disturbance – ‘corals of opportunity’ – are used.
Harvested coral fragments could be treated with microbes or 'hardened' to increase the resilience of resultant reefs.
Fragmentation is an asexual propagation method. If used on a large scale, care would be needed to ensure genetic diversity was not unacceptably reduced.
This method is labour-intensive and would require automation to be able to make an impact at scale.
Photo by Christian Miller