Inside the fight to save the Great Barrier Reef from climate change

“I RECOMMEND getting inside the net. It’s very good for you,” jokes marine ecologist Peter Harrison. “It’s good for your skin, it’s good for your clothes.”

The net in question is a giant, slimy thing, with a fine mesh at its base that contains a precious cargo: coral larvae that have been incubating in the ocean for five days. Some white sun shirts have already fallen casualty to the net, getting coated in a greenish algal stain on contact.

It is early December and we are on Wistari Reef, which forms part of the southern end of the Great Barrier Reef off the east coast of Australia. I’m with 17 others on three research boats, on a field trip to reseed reefs with coral larvae in the hope that they will eventually grow into new coral.

Harrison, who is leading the expedition, is the founding director of the Marine Ecology Research Centre at Southern Cross University in New South Wales. He was one of a group of researchers who first discovered the mass spawning of corals on the Great Barrier Reef in the early 1980s, and has spent the intervening four decades researching coral reproduction and restoration.

Coral spawning occurs once a year. On the outer reefs off the east coast of Australia, the action begins a few nights after a spring full moon in late November or early December. Various coral species release sperm and eggs en masse, in trillions of small balls that rise to the surface and open, resulting in fertilised larvae known as planulae.

These larvae form slicks of vivid pink or orange on the ocean surface, and drift for days or weeks until eventually attaching themselves to hard surfaces underwater to form new colonies. As they mature and become ready to settle, they fade to a greyish hue.

Harrison estimates that only one in a million coral larvae will become an adult coral: some die naturally, some are eaten by plankton or fish, and others are carried by currents into waters that are too cold for the coral to grow. This is why the researchers have come to Wistari. By increasing the number of larvae that settle on reef systems, they are hoping to speed up the reef regeneration process – particularly crucial given that the Great Barrier Reef has suffered three mass bleaching events due to warm ocean temperatures in the past five years.

The team has been based on Heron Island, a coral cay where there is a permanent research station, since the end of November. When spawning occurred in the first week of December, the researchers collected millions of larvae from some of the slicks.

The task on the boats today is to collect larvae from three floating nursery pools in Wistari Reef, where they have been maturing, and seed them onto damaged sections of reef that no longer have live corals. The net of each pool hangs from a square pontoon that is roughly 3 by 3 metres in size.

“Coral species release sperm and eggs en masse, in trillions of small balls that rise to the surface and open”

Researchers gather around the perimeter of each pontoon and lift each net out of the water in sync, concentrating the larvae at the bottom, while Harrison hoses the sides to wash down any larvae caught on the fine mesh. The process is laborious: each net contains about 80 kilograms of water, which takes time to drain out.

All the larvae are collected by 9.30 am, and decanted into tubs in preparation for seeding. Concentrated, the larvae float in a cloudy brown suspension. “It looks like miso soup,” someone remarks. The smell is less appetising: like the fishy saltiness of seafood, but more pungent.

We have been on the boats since 7 am, and will spend at least another 5 hours out on the water. Wistari Reef has a central lagoon edged by a wall of coral. At low tide, the tips of the coral jut out of the water and it becomes impossible for boats to pass. The water level will only be high enough for us to return to Heron Island in the mid-afternoon. But low tide, at 10.40 am today, is perfect for coral seeding – there are fewer currents that will disperse the larvae away from the targets.

We weave the boats around the corals for more than an hour, searching for suitable sites: the researchers are after flat reefs that aren’t too shallow, which might limit the future growth of corals. Four sites are chosen. At each, 30 small, square tiles made from crushed coral skeletons are laid down. These will be used to measure the rate at which coral larvae have settled.

We take a quick break for lunch, but the food is all aboard one boat. This poses little problem: our sandwiches are delivered by Floatyboat, a small, remote-controlled robotic boat developed by Matthew Dunbabin at the Queensland University of Technology and his colleagues.

Floatyboat serves a greater purpose than just aquatic deliveries: at two of the chosen release sites, the team uses it to disperse larvae from two thin pipes that dangle beneath it. Snorkelling behind as it gets to work, I watch the jets of larvae shooting out. At two other sites, larvae are manually dispersed via a larger pipe.

The team won’t be able to see the results of its handiwork for several years. The larvae have been captured from corals that have survived recent mass bleaching events, so the idea is that their offspring may also be more heat tolerant. The researchers hope to upscale the restoration across more sites in the future, using robots to disperse larvae more efficiently.

But climate change is an existential threat. The ideal temperature for larvae along the Great Barrier Reef is between 26°C and 28°C. At warmer temperatures – the same that result in adult coral bleaching – the larvae won’t settle.

The coral restoration programme is one of a range of tools to help boost the resilience of the reef, says Anna Marsden, managing director of the Great Barrier Reef Foundation, a charity dedicated to protecting the reef.

The research under way is a race against time. “We’ve got 30 years to solve this, otherwise we will not have coral reefs on this planet,” says Marsden.