Why do we need coral reef restoration?
Today's coral reefs are threatened worldwide (ICRI, NOAA, Reefbase). This is nothing new, but the extent to which corals die around the globe is unsettling. The impact of the ongoing climate change, directly affecting coral reefs by an already measurable rise of sea water temperature, was manifested by the third global bleaching event in 2015/16. This event caused a mass die-off of corals (NOAA declares third ever global coral bleaching event, El Niño prolongs longest global coral bleaching event).
The recovery of degraded coral reefs can be supported by a well-conceived restoration strategy. However, it will only be successful, if it addresses local causes of reef degradation. Most importantly, we need to reduce our Carbon-dioxide emissions to limit ocean warming (ISRS consensus statement: Climate Change Threatens the Survival of Coral Reefs).
Coral restoration may include to grow asexually or sexually derived corals in land-based or ocean nurseries for later restoration, to directly transplant coral colonies or fragments from intact areas (often to-be construction sites) to degraded reefs, and to transplant corals to substrate stabilization structures after ship groundings or dynamite fishing, respectively.
After some time at a nursery, when corals have reached a certain size, they are usually out-planted in the restoration area. Restoration methods using asexually derived corals are somewhat invasive to the donor reef or to individual donor corals. They do not promote genetic diversity, since fragments are genetically identical to the donor colony―they are clones of their parents. Yet, fragmentation used in restoration efforts may be a good outreach tool to involve local communities. It can be a useful addition to sexual techniques, for example for coral species that naturally reproduce asexually and sexually (staghorn corals, some Acropora spp.).
Using sexual coral reproduction for reef restoration ('sexual coral restoration') requires expertise and initial research to understand reproduction of a specific species. However, it can be applied basically for all coral species, keeps natural populations unharmed and contributes to genetic diversity. The latter is most important as modern coral reefs changed their face fundamentally within the last decades and certain new genotypes may cope better with these conditions than their already struggling parents.
Today, coral restoration is still cost and time intensive and limited in scale. Each coral has to be handled manually and transplanted to the reef by hand. It may not be possible to match the scale at which reefs decline worldwide, nevertheless, to restore reefs at a meaningful scale requires new approaches (restoration techniques urgently need to be upscaled). In particular by reducing cost and labor likewise.
SECORE has developed coral settlement substrates that self-attach to the reef (SECORE Seeding Units, larval settlement). This new concept will enable us to seed large numbers of coral recruits settled on SECORE Seeding Units without the need to transplant each coral by hand. Our studies show that, depending on the conditions at the restoration site, short nursery periods can be sufficient before seeding coral recruits on the reef (Chamberland et al. 2015). To implement our findings at larger scales, we are currently developing new methods to handle large numbers of gametes, larvae and coral recruits with low tech-approaches.
In 2015, we started larger scale pilot projects using our new approaches for sexual coral restoration in Mexico and in Curacao.
A Caribbean example
Once, the elkhorn coral (Acropora palmata) and the staghorn coral (A. cervicornis) dominated shallow reef habitats throughout the Caribbean. Over the past few decades, their populations have dramatically decreased and today, more than 90% of these main reef builders have disappeared. Although some local populations still reproduce, no significant recruitment has been observed throughout the Caribbean (Status and trends of Caribbean coral reefs: 1970-2012, IUCN).
SECORE started to investigate sexual reproduction of elkhorn corals in 2006. A few years later, we expanded our research to include reef restoration. This comprised the development of suitable settlement substrates, the elaboration of ideal conditions for larval culture and settlement, and the best timing to out-plant the recruits. Since 2010, several generations of recruits have been transferred to the reefs in Curacao. In 2015, our first recruits spawned together with the natural population on the reef. This is the first time that offspring of an endangered coral species reached sexual maturity in the wild (laboratory bred corals reproduce in the wild).
We should keep in mind that the reefs we may be able to rehabilitate, will not become the reefs they were before. Coral restoration efforts may aim at rebuilding ecological functioning of coral reefs, such as giving a home to diverse animals and plant and maintaining biochemical cycles, and sustaining economic services, such as coastal protection, fisheries, and tourism. Endangered key species such as the elkhorn coral that have difficulties to reproduce and to deal with a changing environment may be rehabilitated as well.
It is always easier to protect what you have, rather than try to rebuild it up from scratch once it is gone!
We are confident that a well-conceived restoration strategy, combing larger scale restoration efforts itself with education, outreach and conservation, and strongly involving partners at projects on-site, is giving coral reefs a fair chance for the future.