Successful Coral Sowing Reef Regeneration Trial in Curaçao
The loss of coral reefs around the world through bleaching, pollution, destructive fishing practices and other environmental hazards, has prompted scientists and conservationists to look for ways of regenerating damaged reefs.
Until recently, most of these projects involved coral transplantation – collecting coral fragments from damaged reefs and allowing them to mature in a coral nursery, before re-attaching them to the damaged reef. Other initiatives include the manufacture and sinking of concrete balls or ‘reef buds’ to encourage new coral growth.
Both processes are costly, time-consuming, and can only cover a limited area of reef at any time. However, new research has shown that coral grown in laboratory conditions and then ‘seeded’ back into degraded reefs may greatly expand the scope of regeneration. One such project in Australia involved capturing sperm and eggs released during a coral spawning event, mixing them to create larvae in a laboratory, and then seeding them back onto the reef once the larvae have matured.
Now, a similar project being conducted by SECORE International, on the southern Caribbean island of Curaçao, is also reporting success with its reef repopulation trials by collecting larvae from Favia fragum – ‘golf ball’ coral – in which fertilization is internal, meaning ‘a few less steps for the researchers,’ according to Dr Margaret Miller, Research Director at SECORE
‘In the current proof-of-concept study,’ Dr Miller told DIVE, ‘we used larvae from a species of coral that has internal fertilization and broods the larvae within the mother colony. These larvae are well-developed and already competent to settle when released from the mother.’
Once the larvae were collected, SECORE’s scientists transferred them to sheltered tanks containing specially designed concrete substrates where the larvae were able to settle, without risk of predation.
The ‘seeding units’ are concrete tetrapods, each with four arms with equal angles between each – rather the large tetrapods that are used as artificial breakwaters. Grooves cut into the surface help to create ‘micro-habitats’, giving the young larvae a better chance of early survival.
'This is crucial, as our results show that the early post-settlement life stage is the bottleneck for the survival of young corals,' says Dr Dirk Petersen, project lead and Executive Director of SECORE International.
Once the larvae had settled on the tetrapods, the seeding units were transferred to a ‘flow-through’ aquarium where the larvae could begin to grow in more natural conditions, before being ‘sowed’ back onto the main reef outside Curaçao Sea Aquarium.
‘The specific shape of the tetrapod substrates allowed us to simply wedge the seeding units into natural crevices of the reef,’ says Valérie Chamberland, who is heading up the project on Curaçao. ‘Most seeding units were stable within few weeks, either secured in crevices or naturally cemented on the reef’s framework,’
Because of the ease of which the seeding units can be distributed on the reef, SECORE’s project allows for a much greater amount of coral to be re-introduced than if they were manually attached to the reef.
With the report suggesting that transplanting 10,000 individual corals manually might take a few thousand person-hours of work, ‘sowing the same number of corals could be achieved in less than 50 person-hours, a time saving of over 90 per cent,’ says Dr Miller. ‘Additionally, material costs could be reduced up to one third, representing a substantial advance for future restoration work.’
The potential for much greater coral coverage, coupled with the reduction in time and expense, creates a new level of potential for reef regeneration. ‘If we want restoration to play a more meaningful role in coral reef conservation, we need to think in new directions,’ says Dr Petersen. ‘Our sowing approach is an important step towards reaching this goal since it will allow the handling of large numbers of corals in a very short amount of time at significantly lower costs.’
Another distinct advantage of both the Australian and the American-led projects is that they are working with sexually propagated corals, which maintains genetic diversity, as opposed to coral transplantation, which simply clones an existing colony. Different genetic combinations arise through sexual reproduction, which may equip coral offspring to better cope with changing conditions.
‘This is of vital importance for any coral species in the face of climate change. In this way, we may get corals that, for example, are more resilient to rising water temperatures,’ says Dr Petersen.
In the twelve months following the sowing at Curaçao Sea Aquarium, SECORE’s scientists closely monitored the attachment of the substrates to the reef and the survival rates among the new colonies.
‘We settled between 20-30 larvae on each substrate to ideally have one coral established per seeding unit,’ says Valérie Chamberland. ‘After one year, more than half of the units were recovered and still harboured at least one coral, meeting the target required to yield a successful restoration outcome.’
Although the initial trials were by-and-large successful, certain problems were noted after some of the substrates became overgrown with algae. ‘Their light-exposed upper surfaces became rapidly overgrown by competing algal turfs once put on the reef,’ says Chamberland. ‘This likely contributed to a relatively high mortality of coral settlers during the first three months on the reef.’
Different materials such as glass or glazed ceramics could deter the formation of turf algae, something that SECORE’s scientists are currently working on.
After the successful research pilot, testing now needs to begin on a much larger scale. Sowing 50,000 to 100,000 substrates in a single location during the spawning season is a huge logistical challenge, which SECORE plans to address together with the California Academy of Sciences, The Nature Conservancy, and other partners, all part of the recently launched Global Coral Restoration Project.
‘We are routinely able to produce one million larvae from a single spawning event,’ says Dr Petersen. ’Until now, the number of substrates was limited as they were produced manually using moulds. We are currently working together with our partners to implement industrial manufacture of the tetrapod substrates in large numbers at significantly lower costs.’
SECORE and their partners are currently working to refine each step of the sowing approach, including different shapes of seeding units to cover a wide range of reef habitats and a wider range of species.
One day, the report concludes, it may be possible to sow corals from a boat, or with underwater drones.