Restoring endangered Staghorn and Elkhorn corals on Bonaire

For 10 years, Reef Renewal Foundation Bonaire has been restoring the critically endangered Staghorn and Elkhorn corals on Bonaire. Through coral gardening techniques, the Acropora palmata and Acropora cervicornis are grown in our coral nurseries and are outplanted on the reef. Our restoration sites show that our methods have been successful, as our nursery-grown corals have become sexually mature and are now reproducing naturally, thus creating a new generation of corals, and strengthening the biodiversity on the reef. But why do we work with these Acropora species? And how do we choose restoration sites? Read on to learn more!

What you should know about our favorite coral species: Staghorn and Elkhorn corals.

Staghorn (Acropora cervicornis) and Elkhorn (Acropora palmata) are stony coral species that play an important role in the coral reef ecosystem. These fascinating corals help to provide unique and complex habitats for thousands of fish species due to the distinctive structures they grow into. They get their names from their physical appearance resembling that of stag and elk antlers.

Staghorn – Acropora cervicornis

Depth range: 1 to 15m depth
Growth range: up to 1.5m high and 10m wide
Location: Can be found growing in the shallow waters of tropical reefs with low wave exposure

Elkhorn – Acropora palmata

Depth range: 0 to 5m depth
Growth range: up to 2m high and 13m wide
Location: Can be found growing on the shallowest parts of the reef with high wave action. Can handle low tide air exposure.

What are the biggest threats to Staghorn and Elkhorn corals?

Corals are currently facing several threats, which are increasing in scale, number, and intensity. The list of challenges the corals are facing is long and includes diseases, poor water quality, loss of sea urchins, overfishing, and the introduction of invasive species. On top of that, climate change is a major problem for coral reefs, as the rapidly changing climate creates thermal stress for corals and alters the ocean chemistry leading to more ocean acidification. All these factors play into creating stress for the coral species. On Bonaire, the loss of the long-spined sea urchins, poor water quality, diseases, and coral bleaching have played a big part in the decline of our shallow reefs.

Long spined sea urchins (Diadema antillarum) play an important role on the reef, and for Staghorn and Elkhorn corals. Due to a waterborne pathogen, the population of sea urchins in the whole Caribbean got decimated, which resulted in the growth of macroalgae, creating unfavorable conditions for corals to thrive and reproduce.

Poor water quality can lead to challenging conditions, which can stress the corals. The overall water quality is affected by water runoff, septic tank leaks, boats emptying wastewater tanks into the ocean, and erosion (Van de Water et al., 2021; Slijkerman et al., 2014). If you have visited Bonaire, you may have noticed that the island is also the home to thousands of free-ranging goats which eat rooted plants. This grazing pressure impacts the natural vegetation and makes runoff into the ocean much easier. All these elements can result in harmful outcomes to the corals such as excess sediment, chemical pollution, and higher levels of nitrogen and phosphate. Bonaire has been taking action to tackle these issues such as eradicating the goats, paving dirt roads, managing a water treatment plant, and replanting vegetation.

White Band disease has had a detrimental impact on Staghorn and Elkhorn corals in the past. The decline in abundance has created a change in the three-dimensional structure of the reef itself which has become much lower and has flatter structures. This lower structural complexity has provoked a decrease in coastal protection and habitat for many marine organisms such as fish, seahorses, crabs, and more (Meesters et al., 2015).

Due to the shallow depths that these Acropora species inhabit, they are also more susceptible to bleaching events due to a higher number of stressors. These can range from rising temperatures affecting shallower depths to human interaction from swimmers, pollution runoff, boat traffic, and more.

Photo by Casper Douma

Why does RRFB restore Staghorn and Elkhorn corals?

With all the threats that Acropora corals are facing, there has never been a more crucial time to focus on the restoration of these species. Both Staghorn and Elkhorn corals occupy the IUCN red list as critically endangered. We work to restore Staghorn and Elkhorn due to their important role in the reef ecosystem, providing nursery grounds, shelter, and food for thousands of marine animals, fast growth rate, and their critically endangered status. These Acropora species used to be two of the most abundant species of corals on the shallow reefs of Bonaire but were negatively impacted by the outbreak of White Band disease in the early 80s. Since then, successive bleaching events have resulted in almost all Acropora cervicornis (Staghorn coral) populations being destroyed and seriously impacted Acropora palmata (Elkhorn coral) (Meesters et al., 2015).

Both species tend to grow at a remarkably fast pace compared to other coral species and can quickly regenerate from damage which makes them exceptional candidates for restoration programs (Meesters et al., 2015). With Elkhorn coral growing at rates of 5-10 cm a year and Staghorn growing at 10-20 cm per year, there is a higher success rate and more corals fragmented. By kickstarting these two species’ natural recovery and reproduction cycle, we hope to restore a healthy population of these corals which will lead to more biodiversity and thriving coral reefs.

Outplanted Elkhorn Coral
Outplanted Elkhorn Coral - Ⓒ Paul Selvaggio
Staghorn coral nursery tree
Staghorn growing in a nursery - Ⓒ Paul Selvaggio

Staghorn & Elkhorn facts

  • Staghorn corals get their food from plankton but also hunt for food by extending their stinging tentacles to catch prey.
  • With 180 discovered species worldwide, Acropora is one of the most common coral genera and grows the fastest with only 2 species living in Caribbean waters.
  • Staghorn and Elkhorn corals can reproduce both sexually and asexually through fragmentation and spawning.
  • Staghorn and Elkhorn corals are hermaphrodites, which means they release both eggs and sperm.
  • Staghorn and Elkhorn become sexually mature when they are 3-5 years old.
Outplanted staghorn thickets
Thriving outplanted Staghorn corals.
Elkhorn coral nursery tree
RRFB Staff populating a coral nursery tree. Ⓒ Lorenzo Mittiga

How do we restore a healthy coral reef with Staghorn and Elkhorn corals?

We use two different methods of action in coral reef restoration practices: coral gardening and larval propagation. Coral gardening is one of the most common techniques used where asexual reproduction occurs by hanging coral fragments in nursery trees. In these coral nurseries, the fragments of Elkhorn and Staghorn corals are better protected from diseases and algae. With the help of routine maintenance from our team, dive shops, and volunteers, we are able to grow more than 15.000 coral fragments at any given time. Eventually, once these coral fragments grow, they are outplanted on the reefs where they will develop into healthy and thriving coral thickets.

Larval propagation is a relatively new method used in coral restoration projects which consists of assisting coral sexual reproduction through: the collection of gametes during coral spawning, assisted fertilization, settlement in nurseries, and finally outplanting them on the reefs. This coral restoration method used alongside fragment propagation on Bonaire and in other parts of the world shows promising results in increasing genetic diversity and coral reef resilience.
Through a partnership with SECORE International, we have been using this new and innovative reef restoration technique successfully for a few years.

Thanks to the success of our restoration sites, some Staghorn and Elkhorn have now reached sexual maturity and can be part of the larval propagation project. Indeed, by collecting Acropora gametes during their spawning event, the output of thousands of coral juveniles genetically unique is now possible, strengthening the local populations of these two species. Combining these two techniques, we can achieve a greater outcome of restored and healthy coral reefs.

Both newly settled larvae and fragments of corals from the nursery trees are outplanted back to the reef. With both techniques, the wedging of either seeding units or fragments in an appropriate location is key for their survival and growth. The criteria used to choose a suitable outplanting site are very similar for both methods. We look at several factors, including the pre-existing coral cover, the abundance of the targeted species at the site, water quality, species preferred conditions, bottom composition, wave exposure, and surrounding human impact (Van Duyl, 1985).

Efforts are made to restore Bonaire reefs in a sustainable and balanced way, by preserving the genetic diversity of the fragmented corals as well as aiming to significantly increase the density of the restoration targeted species. RRFB has achieved impressive results, outplanting more than 41.000 corals back to the reef, a number that is growing every year. With the addition of larval propagation to our methods, our ever-growing nursery capacity, and thriving restoration sites, we are hopeful to give the Bonaire reefs a fighting chance.

References

Aronson, R., Bruckner, A., Moore, J., Precht, B. & E. Weil (2008, 1 januari). Acropora cervicornis. IUCN Red List of Threatened Species. Geraadpleegd op 29 juni 2022, van https://www.iucnredlist.org/species/133381/3716457
Aronson, R., Bruckner, A., Moore, J., Precht, B. & E. Weil (2008, 1 januari). Acropora palmata. IUCN Red List of Threatened Species. Geraadpleegd op 29 juni 2022, van https://www.iucnredlist.org/species/133006/3536699
Van de Water, M., Crielaard, J., Pieternella, A., de Wolf, W, Breedveld, M. (2021, juni). Marine Waste Water Survey Bonaire 2021
Burke, L., Maidens, J. 2004. Reefs at risk in the Caribbean [online]. Washington, DC.: World Res Inst. Available from: http://pdf.wri.org/reefs_caribbean_full.pdf. Accessed April 2010.
Jones, G. P., McCormick, M. I., Srinivasan, M., & Eagle, J. V. (2004). Coral decline threatens fish biodiversity in marine reserves. Proceedings of the National Academy of Sciences, 101(21), 8251–8253. https://doi.org/10.1073/pnas.0401277101
Lohr, K., Bejarano, S., Lirman, D., Schopmeyer, S., & Manfrino, C. (2015b). Optimizing the productivity of a coral nursery focused on staghorn coral Acropora cervicornis. Endangered Species Research, 27(3), 243–250. https://doi.org/10.3354/esr00667
National Park Service (2015, 16 july). Elkhorn coral. Elkhorn Coral - Dry Tortugas National Park (U.S. National Park Service). Geraadpleegd op 29 juni 2022, van https://www.nps.gov/drto/learn/nature/elkhorncoral.htm#:~:text=Colonies%20of%20elkhorn%20coral%20are,about%2010%20to%2012%20years.
Slijkerman, D. M., León, R. D., & Vries, P. D. (2014). A baseline water quality assessment of the coastal reefs of Bonaire, Southern Caribbean. Marine Pollution Bulletin, 86(1–2), 523–529. https://doi.org/10.1016/j.marpolbul.2014.06.054
Van Duyl, F. C. (1985). Atlas of the Living Reefs of Curacao and Bonaire (Netherlands Antilles). Foundation for Scientific Research in Surinam and the Netherlands Antilles, 117.

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