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They are the seed banks of the sea. Hubs of healthy coral in Australia鈥檚 Great Barrier Reef could help rebuild damaged areas by spreading their larvae via ocean currents. Protecting these areas could be key to the future of the ecosystem.

The Great Barrier Reef is threatened by repeated coral bleaching, outbreaks of coral-eating crown-of-thorns starfish, choking sludge from farms on land and cyclones. Surveys published in April showed that two-thirds of the reef is now badly degraded.

But there鈥檚 still hope, says at the University of Queensland in Australia. His team has identified 112 of 3800 coral reefs in the Great Barrier network that are relatively safe. These reefs could at least partially regenerate their neighbours.

The researchers looked for reefs that had the best chance of surviving future stressors, and which could conceivably seed new reefs if and when conditions improve. These refuges needed to be located in areas with low risks of overheating and starfish outbreaks, based on historical records. They also needed to be connected to other reefs by ocean currents, so they could spread their coral larvae and replenish struggling areas.

Picking winners

The refuge reefs were mainly located towards the聽south and away from the mainland.聽This area is flushed with cool water from an ocean current known as the South Caledonian Jet. Most of the coral here survived the heatwaves of 2016 and 2017, which devastated northern parts.

The ocean current also partially protects the southern offshore reefs from crown-of-thorns starfish, because it acts as a 鈥渉eadwind鈥� slowing them down.

In addition, the cool jet聽facilitates the spread of coral, because it pushes coral larvae from healthy offshore reefs to less robust ones near the coast. The researchers calculated that the 112 鈥渕other reefs鈥� could supply up to 47 per cent of the Great Barrier Reef with coral larvae after a single spawning. 鈥淲e call them 鈥榮uper-spreaders鈥�,鈥� says Mumby.

Coral larvae are known to travel long distances. In August, a satellite study revealed coral larvae being swept 280 kilometres across the Red Sea in two weeks.

Protecting the protectors

Mumby believes the 112 reefs identified by his team should be targeted for extra protection. 鈥淚magine you have a reef that鈥檚 just been damaged by a cyclone,鈥� he says. 鈥淭hen you can focus some of your management efforts on preserving intact reefs that you know are going to resupply the damaged one.鈥� Preserving the hub reefs might involve removing any crown-of-thorns starfish and tightening fishing restrictions in the area, he says.

It鈥檚 feasible that surviving corals could bring others back, says at the University of Queensland. 鈥淚t鈥檚 like a forest: if most of it dies but you still have a few trees left, they can spread seeds and multiply.鈥�

Nevertheless, even the most resilient reefs will die if we don鈥檛 rein in our greenhouse gas emissions, because the ocean will become too hot and acidic, says Marshall. 鈥淚t doesn鈥檛 matter how much conservation or re-seeding or genetic-engineering or other fancy things we do. If we don鈥檛 act on climate change now, there won鈥檛 be a reef in 50 years鈥� time.鈥�

Mumby agrees the threat is severe, but is more optimistic. 鈥淚 remember diving on the reefs around Tahiti after the huge bleaching event of 1998 and seeing these 8-metre-wide corals completely dead except for a few living bits the size of your finger,鈥� he says. 鈥淲ithin 10 years, those little fragments regrew right across the skeletons and the reefs bounced back. So I haven鈥檛 given up all hope for the Great Barrier Reef yet. Nature has surprised me too many times for that.鈥�