Spawn, grow, sow: how coral seeding could boost recovery

16 February 2022

We know coral populations are declining globally. We know climate-related disturbances are driving this loss, and we know that the window of opportunity for reefs to recover and adapt is closing. 

We also know corals can replenish. 

Once a year, when the water is warm and the moon is full, corals on the Great Barrier Reef become ripe with eggs and sperm. Over a few nights, these corals burst in what is known as the ‘mass coral spawning event.’

Billions upon billions of coral sperm and eggs, bundled together, are released into the water in a highly synchronised event to maximise fertilisation. The bundles float to the surface of the ocean, where they mix and fertilise, grow into larvae and then, after a few days or weeks return to the reef to settle and grow. 

The new generation of corals begins.

close up of a small coral polyp sits on the edge of a round surface
A small coral polyp (Platygyra daedalea), just a few months old and a few millimetres across begins a new generation of colonies on a coral plug.

But with only a small fraction of the next generation reaching one year of age, scientists are exploring new ways to boost survival in this fragile stage of life to improve the rate of reef recovery. 

One method is ‘coral seeding’ – and so far, it’s exceeding expectations.

What is coral seeding? 

Coral seeding is a restoration technique designed to help speed the return of coral cover to reefs that are disturbed or damaged. 

At AIMS, it all begins in the National Sea Simulator (SeaSim), the world’s most sophisticated aquarium complex where corals spawn in synchrony with the Reef event.

As spawning approaches late in the year, scientists collect wild corals from the Great Barrier Reef – full of eggs and sperm – and take them back to SeaSim. Here, scientists stay close to the corals over several nights, patiently waiting for the spawning to occur so they can scoop up the corals’ egg and sperm bundles.

In the lab, the fertilised eggs are reared in the nursery until they reach the larval stage when they are settled on small plugs with coralline algae, creating a perfect place for the young coral to grow. Within 5-10 days of spawning, the larvae turn into single polyps: tiny baby corals.  

When the coral is healthy and established, scientists attach the coral plugs to small devices that house and protect the young coral. The baby corals are then returned to their home reef.

First things first – knowing your device and where they can help 

On the Great Barrier Reef, AIMS leads coral seeding research & development, contributing to a growing body of international study on the technique. A key component of AIMS’ research is the coral seeding device – the engineered home for the young corals, which take them from the lab to the reef. As coral seeding is relatively new, several designs are being developed, tested, and improved.

Seeding devices out of water - just three of the many designs being trialed to transport young corals from the lab to the reef. These examples are made of ceramic. Image: K. Green, M. Roman

AIMS' lead aquarist and sub program lead for the Reef Restoration and Adaptation Program Andrea Severati heads the research & development of the devices and has spent the last two years refining various aspects of the design. 

“Many factors need to be considered in the design to select the right materials and shapes. The design elements need to work together to address specific causes of mortality, like the retention of the device on the reef and the protection of the juvenile coral from grazers and predators.

"Other design criteria such as cost, size, ease of deployment, and suitability for automation are also important. We are even investigating the performance of different materials for manufacturing the seeding devices, all selected with environmental acceptability, cost and ease of manufacturing as key requirements," he said.

A second-generation seeding device deployed at Heron Island on the southern Great Barrier Reef, holding newly settled corals. This device is fixed to the reef in an experiment testing different the effects of water flow. Photo: C Randall.

Another key factor guiding the use of coral seeding as an intervention is knowing where it is required. 

AIMS benthic ecologist Dr Carly Randall said reefs with major limits on their recovery are most suitable for coral seeding. 

“We need to identify why the reef isn't recovering. If it's because it isn't receiving a natural supply of larvae, or if the corals aren't surviving after they have attached to the reef, despite historically thriving there, then it's a good candidate for this type of intervention,” Dr Randall said.

“Then we need to determine which device designs and which corals are best to deploy on the reef.” 

Seeding success! 

Dr Randall, who, along with Postdoctoral Fellow Dr Cathie Page led AIMS’ first coral seeding experiment in the Keppel Islands on Woppaburra sea country, said the survivorship of the corals’ first deployment have exceeded expectations. 

“In early 2021, we deployed about 300 devices onto reefs that were impacted by bleaching.  

“After ten months, we were amazed to see more than 90 percent of our devices had at least one live coral recruit,” she said.

These small corals are growing well on a seeding device on Woppaburra sea country in the Keppel Islands on the southern Great Barrier Reef. Photo: C. Randall

While the first coral seeding trial was successful, there are still a lot of questions scientists need to answer before coral seeding can be applied on severely degraded reefs at a large scale.

Coral seeding devices: why bother? 

Like many offspring in the wild, the first year of life can be tough. For corals, there are many drivers that place the young at risk, including grazing fishes, competition with other animals and plants living on the reef, and overgrowth by macroalgae, especially at degraded sites.  

By better understanding these drivers, scientists can engineer coral seeding devices that house and protect the young coral as it grows.

Scientists monitoring coral seeding trials on Woppaburra sea country around the Keppel Islands. Photo: C. Page

AIMS and James Cook University PhD student Taylor Whitman is investigating different styles of devices which protect the coral from intense feeding habits of large herbivorous fish.  

“Grazing fishes, like parrotfish, munch on reefs all day. This is hugely beneficial in keeping reefs clean and healthy, but for young corals it often means accidentally being eaten whole.  

“We’re trying to figure out what device design can protect the small and fragile corals from grazing fishes, until the coral becomes big enough to handle these feeding habits of fish,” Ms Whitman said.

Different coral seeding designs are being trialed to protect young corals from the grazing jaws of herbivorous fish, with caged devices acting as an experimental control. Photo: T. Whitman

The science team is also examining how waves and currents impact the growth of baby corals, looking at whether survival is higher at sheltered sites with little water movement, or at sites with strong currents and high water movement.  

Postdoctoral fellow Dr Saskia Jurriaans is testing this in the laboratory and the field.  

“The goal is to better understand which reef environments offer coral babies the best chance of survival, to maximise future restoration efforts,” Dr Jurriaans said. 

“We are also trying to work out what techniques are best in environments with low natural survival. It maybe that to boost coral numbers in these areas, the number of seeded corals may need to go up to compensate.”

Dr Saskia Jurriaans fertilising coral eggs in preparation for large scale trials testing the influence of water movement on coral seeding. Photo: K. Green

What’s different with the next coral seeding trials? 

With millions of young corals now being cared for in the National Sea Simulator, the science teams are preparing to bring these ‘coral seeds’ out onto the Reef in early 2022. 

For the Keppel Islands, the team are trialling seeding devices on three highly degraded reefs, mostly made of coral rubble, to mimic the most severe conditions where restoration is needed.   

Dr Cathie Page said the science team is also testing freely deployed devices. 

“We need to scale up our method so we can deploy these devices freely, rather than physically attaching them to reefs,” she said.  

“This is important because restoration techniques are needed that can be implemented on a large scale across many reefs. It has to be efficient and cost effective.” 

Trials for the fish grazing devices and the impact of different water flow are taking place on the northern, central, and southern regions of the Great Barrier Reef in early 2022.

Healthy coral colonies growing on a fixed coral seeding device. The next step – testing devices which are not secured to the reef and testing on reefs which are heavily degraded. Photo: C. Randall

This coral seeding research is led by the Australian Institute of Marine Science, funded through two programs: the Reef Restoration and Adaption Program, funded through the Australian Government’s Reef Trust Partnership with the Great Barrier Reef Foundation, and the Australian Coral Reef Resilience Initiative jointly funded by AIMS and BHP.