This article illustrates two midwater nursery technique achievements by means of cultivating thousands of coral fragments, which showed resilience from the 2016 bleaching event.

Coral fragment suspended in a rope nursery. Image Credit: Maren Toor

The Maldives is an island nation that relies on healthy coral reef ecosystems. Comprised of 1,190 islands and with an average elevation of 1.6 meters, this low-lying nation hosts the seventh largest reef ecosystem in the world [1], [2]. Roughly 3,500 square kilometers of coral reef support more than 1,000 species of fish, nearly 200 species of stony coral, and numerous migratory megafauna [1]. Coral reefs and associated systems have a critical economic and ecological importance for the Maldives through tourism, fisheries, and coastal protection [3]. Additionally, these valuable ecosystems are some of the most vulnerable. Mass bleaching events, coral mining, pollution, and physical damage have caused coral cover to decline, leaving this island nation at risk to sea level rise and losses in livelihood [4], [5], [6].

Coral restoration is a widely practiced field in the Maldives [7], [8], [9]. Both local and resort islands employ strategies to preserve remaining coral cover and restore the abundance and diversity of corals that previously surrounded islands [7], [9]. Different methods have been used depending on island and reef topography with varying success [7], [8]. Common methods include coral domes/frames, coral nurseries, and Biorock [9], [10], [11]. While all of these methods help to restore small areas of reef, restoration scientists have said that only large-scale nurseries, combined with transplantation efforts, will help to restore reefs on a big enough scale to cope with the current levels of degradation [12]. This method of using large-scale nurseries has been recently implemented at the resort Velaa Private Island in the northern Maldives and has been successful after the first year.

The aim of this restoration project is to increase the abundance and diversity of coral colonies on the reef to promote a more self-sustaining ecosystem and increase overall reef biodiversity. The emphasis is on holistic restoration by using a variety of coral genotypes and morphologies to maintain the structural integrity and genetic diversity of the reef. This project will consist of two phases, a nursery phase to cultivate coral fragments for one to two years and a transplantation phase where the corals will be taken from the nurseries after cultivation and cemented back to the nearby reef. The project is fully funded by the resort at which it is taking place, as part of their environmental commitment and social responsibility.

The nursery phase consists of eight mid-water floating nurseries set up around Fushivelavaru Island in Noonu Atoll, Maldives. Nursery methods are those of Frias-Torres, Montoya–Maya, and Shah [13] and Frias-Torres et al. [14] from Nature Seychelles. As of May 2017, there are four nurseries set up, three rope nurseries and one net nursery, with the fifth nursery underway. Nurseries are anchored in sandy substrate at 30 meters, approximately 100 meters away from the reef edge, and suspended at 8 meters in the water column. This method gives the fragments optimal conditions to grow, including minimal interaction with corallivorous fish and invertebrates, good water flow, nutrient rich waters, and a relatively controlled environment that can be cleaned of algae and other overgrowing organisms. Loose fragments and small pieces from donor colonies are collected to stock all nurseries. In the future, nursery grown corals will also be re-fragmented to fill future nurseries. Rope nurseries hold 2,500-3,500 fragments from branching species that remain in the nursery for up to one year. Net nurseries hold 500-1,500 fragments of massive, encrusting, and sub-massive species that remain in the nursery for up to two years. This will yield a total of 12,000-15,000 fragments viable for transplantation after one to two years in the nurseries.

Nurseries are surveyed every three months. Health and bleaching of each fragment are monitored based on methods adapted from Nature Seychelles (see [13], [14] and [15]), while ecological volume is measured based on methods of Rinkevich and Loya [16] and Shafir et al. [17]. Temperature and light intensity are recorded at the nurseries as well as the nearby reefs. Nurseries are periodically cleaned to reduce abundance of biofouling organisms.

Pocillopora fragments hanging in a rope nursery with resident spadefish (Platax teira) below. Image Credit: Maren Toor

Successes

Overall, the nursery phase has shown success in cultivating a large number of coral colonies. Breakdown of survival rate per nursery can be seen in Table 1, below.

Table 1. List of each nursery, nursery type, number of species in each nursery, and the survival rate as of May 2017. *Note: Nursery 4 is not fully stocked with coral yet so survival rate cannot be determined at this time.

Nursery 1, in place for nearly a year, has the highest survival rate, likely due to species composition. It holds two Acropora species and three Pocillopora species; however, over 95 percent of the fragments are in the genus Pocillopora. This genus has been one of the most resilient on the reefs throughout the Maldives and has shown similar success in the nurseries. The average survival rate of Pocillopora fragments across all nurseries is 87.39 percent. This genus has shown the largest increase in size, growing, on average, 300-400 percent of initial size after one year, irrespective of species. Similar growth patterns have been seen in other nurseries; however, because the other rope nurseries were set up more recently, there are insufficient data to determine accurate growth rates at this time.

After 11 months in the nursery, more than 2,000 corals from the first nursery are viable for transplantation. The survival rates and growth rates are indicative that this nursery method provides favorable conditions for fragments to get a head-start in regards to growth and can be cultured in large numbers. This allows for a high abundance of colonies to be transplanted as well as used to fill future nurseries with a single colony providing around 30 fragments.  

In addition to providing a large number of healthy colonies for transplantation, the nurseries act as a refuge for resilient coral species. Those in the nurseries are ones that survived the mass bleaching event caused by the El Niño Southern Oscillation in 2016 and showed minimal to no bleaching during the hottest months of 2017. This includes Pocillopora, Acropora, and Porites species. Those that did show signs of bleaching in 2017 readily recovered within a few weeks, even before water temperatures dropped.

Pocillopora verrucosa fragment when first placed in the nursery in July 2016 (left) and again in May 2017 (right). Image Credit: Maren Toor

Setbacks

Throughout the first year there were some minor issues but only three significant setbacks. They include a high mortality rate in Acropora species, a lower than expected survival rate of the first net nursery, and an outbreak of disease in multiple nurseries.

Acropora has displayed a much lower survival rate as compared to other species in the nurseries with an average survival rate of 15.45 percent. The exact cause of this is unknown and likely due to various factors. Moving forward, methods will be adjusted, and successful nursery colonies will be re-fragmented to fill future nurseries. The net nursery has also shown a lower than expected survival rate of 41.1 percent after six months. Certain genera, including Physogyra, Platygyra, and Diploastrea, have shown positive growth, growing over the cable ties connecting them to the netting. Methods have been altered so that future nurseries provide more stability for fragments and hopefully increase the survival rate. Disease was also observed in the rope nurseries with a type of white syndrome affecting multiple Pocillopora species during the hottest part of the year (March 2017). Fragments showing signs of disease were carefully removed from the nurseries after consulting disease experts, and spread of disease appeared to drastically slow once water temperature and light intensity dropped.

Phase II – Transplantation

The transplant phase has begun in early June 2017. Because this phase will involve transplanting thousands of corals, a pilot study is being conducted prior to the official start of transplanting. This study aims to look at three things: optimal depth range for transplants, optimal volume of colonies for transplanting, and optimal colony density. It will utilize 1 meter² plots in which the corals will be cemented to the reef. The pilot study will run for three months before the full transplantation of the corals. Multiple surveys will be carried out on fish assemblages, benthic composition, and changes in coral cover within plots both before and during transplantation.

Coral colonies cemented back onto the nearby reef. Image Credit: Maren Toor

Conclusions

This project is still in the early stages with half of the nurseries built and transplantation just starting. However, one year after the start of the project, the nurseries have cultivated thousands of healthy coral colonies. The nurseries serve as refuges for small fragments to grow by removing certain pressures from the reef and providing favorable conditions. While there have been setbacks over the year, the scale of the project allows for these to happen as they naturally would on the reef. The survival rates so far, while not as successful for the net nursery, are positive indicators that this method is viable for this area. The rope nurseries hold thousands of healthy fragments, and while many of them, for nursery 1, are in the genus Pocillopora, current and future nurseries aim to hold a more diverse selection of species. Based on issues faced in the past year, small adjustments to the nurseries will be made as well.

While debate remains about the usefulness and success of active restoration work, the success of this nursery method in the Maldives is encouraging. The transplant phase will reveal more about the viability of this method to restore reefs on a scale large enough to counter current levels of degradation. This project will be ongoing in the coming years with the hope that research being conducted here can help restore this reef as well as shape the future of coral restoration elsewhere.

Maren Toor is the coral biologist at Velaa Private Island Maldives. Any questions regarding the project can be directed to her at marine.biology@velaaisland.com