As alarm bells ring worldwide over damage to the planet’s coral reef habitats, an ACIAR team working in the Philippines has achieved the first mass reseeding of coral colonies to help restore damaged reefs.
The project developed techniques to raise coral larvae either in aquaculture facilities or directly in the sea, with the successful restoration trials arousing interest among reef management agencies around the world, including Australia.
The ACIAR project is led by Professor Peter Harrison, the director of the Marine Ecology Centre at Southern Cross University in northern NSW. Professor Harrison made seminal discoveries about the reproductive biology of corals during PhD studies on the Great Barrier Reef in the 1980s. Now, Professor Harrison says the reasons to attempt coral restoration efforts are overwhelming.
'Corals are everything to a reef—they are foundation species that provide the reef’s three-dimensional structure,' he says. 'Those structures then serve as nurseries for fish and habitats for hundreds of thousands of reef organisms that simply disappear if the coral colonies are killed.'
Corals are faring badly worldwide, but the problem is especially acute in South-East Asia where about half of all coral reefs have been destroyed. Besides pollution and an increase in environmental stresses (such as rising ocean temperatures), these reefs are also being damaged by blast fishing. This involves detonating cheap explosives over coral reefs to stun or kill fish, which are then scooped up as they float to the surface. Professor Harrison considers the practice a sign of poverty and desperation, an easy way to harvest the last of dwindling fish stocks to feed impoverished families. The blasts, however, are so damaging that they convert complex underwater coral forests into rubble. Paradoxically, in the search for alternative and more profitable income streams, coral reefs and their conservation and restoration stand to play pivotal roles in the Philippines and around the world.
Corals are unusual organisms. They are animals, yet they host photosynthesising, single-cell algae and they also secrete a calcium carbonate exoskeleton, which is responsible for the rock-like appearance of corals.
The animal part of a coral colony is made up of genetically identical polyps that can reproduce asexually by simply splitting into two. Polyps, however, can generate new genetic identities— the grist for the mill of adaptation and evolution— by releasing eggs and sperm bundles that undergo sexual reproduction.
In the 1980s, Professor Harrison was involved in the discovery of something unusual about coral sexual biology. To maximise opportunities for novel genetic recombinations, many corals spawn en masse, releasing vast quantities of egg and sperm bundles that form slicks on the surface of calm waters.
Once fertilised, the eggs grow into millimetrelong, maggot-like larvae over several days. If the larvae can find a suitable spot to settle on the reef, they metamorphose into juvenile polyps, grow tentacles and start feeding. After six weeks, the polyps are big enough to start dividing and the colony-producing process gets under way.
'Colonies can be many decades old and grow into areas that are over 10 metres wide,' Professor Harrison says. 'But they all start as tiny larvae, with only a minuscule percentage surviving to form large coral structures.'
How to foster baby coral
The Philippines lies within the Coral Triangle that includes 35% of the world’s coral reefs and more than 75% of all coral species—a degree of biodiversity greater than the Great Barrier Reef. The Philippines’ reefs are worth more than US$2 billion annually from fishing, tourism and the storm protection services they provide. Despite their environmental, social and economic value, about 98% of the Philippines’ reefs are classified as threatened.
ACIAR’s reef restoration work is being undertaken in the reefs of northern Luzon. The method involves raising millions of coral larvae and then assisting their settlement into damaged areas using underwater enclosures or tents made of fine mesh. To obtain coral larvae, two techniques have been developed.
The first involves temporarily relocating mature coral colonies to a laboratory tank with flowthrough seawater at the aquaculture facility of the Bolinao Marine Laboratory. The controlled environment makes it possible to experimentally maximise fertilisation rates (currently at close to 100% efficiency) and larval survival rates.
The team is also developing protocols for the wild capture of egg-sperm bundles during mass spawning events. The capture involves using protective mesh enclosures on the reef’s surface before larvae are transferred to the underwater tents for five to seven days for settlement. The mesh tents are then removed and the settlement site monitored.
'To scale up this kind of restoration work for larger reef areas, it will be essential to have reliable methods to rear larvae at sea,' Professor Harrison says. 'Our first successful spawn capture took place in 2016 and each time we do it we refine our ability to do the rearing work at sea.'
Restoration gets under way
In April 2016, colonies of Acropora tenuis branching coral derived from artificially reared larvae became sexually active and spawned for the first time, just three years after the ACIAR team’s first coral restoration attempt. A second trial with a slower-growing branching coral species is due to spawn in 2017.
The successful restoration trials have since spawned a larger ACIAR project that so far has raised 4.5 million larvae, resulting in about 250 new baby corals that in mid-2017 were the size of 20-cent pieces, with their growth rate due to accelerate.
'With the new project, the aim is to get as many new coral colonies on the reef as possible,' Professor Harrison says. 'By using sexually produced larvae for the restoration, we bring into the new colonies the extra genetic diversity needed to ensure survival of the fittest larvae and the best-adapted genotypes.'
As coral restoration efforts gain pace, new habitats for many other species are being created, including the fish that local communities rely on for food and income. The success of ongoing restoration work, however, hinges critically on the Filipino technical partners at the University of the Philippines—who will eventually take over the reseeding work—and the engagement of local communities who rely on or visit the reefs.
To aid with capacity-building, the ACIAR team works closely with its in-country technical partners. They also provide numerous opportunities for Filipino students to undertake postgraduate research work on the coral project. Among them is Mr Dexter dela Cruz, who has proven integral to the project’s success.
Professor Peter Harrison (middle) with his research team in the Philippines.
The team also works closely with local communities and municipal authorities to foster mutually beneficial stewardship roles for ongoing preservation and restoration efforts.
Helping to build community engagement is Professor Jeffrey Bennett, an environmental economist at the Australian National University. His socio-economic surveys have created an understanding of the reef’s value to local communities and the experience of visitors to the region, even comparing responses to the restored and damaged reefs. This information has informed the development of training workshops and education materials for communities, visitors and local schools that explain the importance of corals to both the health of the environment and of people.
'Where we work, the municipal mayor was elected on a platform of stopping blast fishing,' Professor Harrison says. 'As a consequence, detonations are becoming rare occurrences. These days, local communities respond to the sound of blasting by jumping in their boats and chasing away the blast fishers from the reef, so the reefs have a chance to recover.'
Among the many reefs around the world that stand to benefit from coral restoration technology is Australia’s Great Barrier Reef. Prior to recent mass bleaching events (which are due to more frequent spells of abnormally high seawater temperatures), the Great Barrier Reef had already lost half of its corals in the previous 30 years. Public concern is running high over the reef’s health.
With word spreading about the success of restoration efforts in the Philippines, new policies may be developed to support reef restoration in other reef regions, including the World Heritagelisted Great Barrier Reef.
Given that the Great Barrier Reef is worth about $6 billion to the Australian economy each year, the combination of conservation and restoration work has important cultural, ecological and economic implications. In addition, technical manuals are under development to help guide restoration efforts worldwide and scientific papers are being published based on the data produced at the Philippines restoration sites.
'Once we know how best to scale-up the rearing of millions of larvae at sea, we want to test the feasibility of selecting for heat tolerance within the reseeding population so that restored coral colonies can better withstand bleaching events,' Professor Harrison adds.
'The potential even exists to accelerate the adaption process to warmer ocean temperatures that reefs are experiencing through selective breeding for heat tolerance of the coral polyps, of the photosynthesising single-cell algae, or both. As such, restored coral is a source of genetic diversity that could help us future-proof reef systems against climate change.'