Life Science Environment Coral bleaching casts a dark cloud over Reef’s annual sex-a-thon

Coral bleaching casts a dark cloud over Reef’s annual sex-a-thon

Microscopic specks of new life fill the waters of the Great Barrier Reef as the annual mass spawning continues. Photo: AAP
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The world’s biggest sex event — the mass coral spawning on the Great Barrier Reef — is underway.

Three nights after the full moon and right on cue, corals right up and down the length of the reef have begun filling the ocean with eggs and sperm.

Researches said they were already confident that the spawning would happen this lunar cycle as the coral egg bundles had begun changing colour, according to reef expert Katie Chartrand from James Cook University’s TropWATER Centre.

“There’s already been reports of spawning up on the Ribbon [Reefs] — it started two nights ago,” Mrs Chartrand said.

“The plate corals up there, they were gravid. They had pink egg bundles ready to go.”

The ABC is live streaming the coral spawning event this weekend, in a two part series called Reef Live on Friday and Sunday nights on ABC TV and iView.

Though it’s different depending on the species, many coral egg bundles change colour in the lead up to spawning, often turning a pink or rich red.

Researchers can break open small branches or pieces of coral to check the colour of the eggs to get an idea of whether they’re getting ready to spawn.

Scientists are setting up coral creches for the Reef’s baby polyps. Photo: ABC

Further south on Heron Island, Peter Harrison from Southern Cross University had also found that the corals there were ready.

“Some of my team have sampled some of the corals at Heron [Island],” Professor Harrison told the ABC earlier this week.

“If we gently break some of the polyps open underwater you can see these ripe red [eggs].”

Professor Harrison along with Bette Willis from JCU were part of a team of scientists that first discovered the annual spawning on the reef in 1981.

Prior to that it was thought that coral brooded, or released fertilized embryos into the water.

Emeritus Professor Bette Willis said polyps can release 20 egg bundles or more, and there are trillions of polyps across the reef.

“There are something like 200 species of corals that will spawn during the week-long event,” she said.

‘Very severe bleaching’

But while this year’s mass spawning is a welcome sight for reef researchers and the public alike, there are fears that the severe bleaching that hit the reef last summer and autumn could limit the size of this season’s spawning, Mrs Chartrand said.

“It didn’t get a lot of media coverage because of COVID but, especially from Townsville south there was a very severe bleaching event,” she said.

“There’s a chance that the spawning event will be diminished because of that.”
This year’s bleaching was considered the second most severe to ever hit the reef, and followed similar bleaching events in 2016 and 2017.

Coral bleaching driven by human influences is taking a tragic toll. Photo: ABC

Corals begin producing their eggs about nine months before spawning, and sperm around five months before. Bleaching can therefore lower the fertility of corals in the following spawning season, according to Professor Harrison.

“One of the issues is we can’t [know] how many of the corals on the reef were bleached this year,” he said.

“Depending on how badly stressed they were, sometimes they don’t produce strong enough eggs and sperm to reproduce.”

In other cases, stressed coral may forego reproducing altogether, according to Mrs Chartrand.

“Instead, they put that energy into recovery.”

The conservation outlook of the reef was downgraded from “Significant Concern” to “Critical” under the IUCN World Heritage Outlook assessment cycle last week.

The outlook, which is published every three years by the IUCN — an advisory body on natural heritage properties to UNESCO — identified climate change, bleaching, catchment runoff, and coastal development as key drivers of the reef’s “deteriorating” status.

A study published earlier this year estimated that Great Barrier Reef corals have halved since 1995.Study co-author Terry Hughes said that there were two issues which would affect the size and success of this year’s mass-spawning.

“The impact of 2020 bleaching on the reproductive capacity of surviving corals that may be stressed, and the dwindling number of adult corals over

Katie Chartrand is the proud foster mum to 150 million wriggling water babies. Photo: James Cook

a longer period,” Professor Hughes from JCU said.

“Dead corals don’t breed.”

In a bid to help boost the reproductive success of corals on the reef, Mrs Chartrand has been planting coral larvae onto degraded and “source” reefs.

Source reefs are areas identified as providing significant amounts of coral spawn and larvae to other reefs during spawning.

“We trialled large-scale larval seeding using these inflatable nursery systems,” she said.

“We’ve had somewhere over 150 million baby corals which we reared last year and settled onto the reef.”

Despite trillions of eggs and sperm being released into the water during this mass-spawning event, as few as one in one million will form larvae that successfully settle and survive to reproductive age.

coral bleaching
Reef experts are hoping this year’s cooler La Nina will lower water temperatures and bring at least a temporary respite from bleaching. Photo: The Ocean Agency

Planting coral larvae helps to significantly increase the odds by taking much of the chance out of the equation.

The first year is when young corals are the most vulnerable to disturbances like bleaching, according to Mrs Chartrand.

“It’s especially hard for corals to make it through the first 12 months when they’re under pressure,” she said.

She’s going out in January to check on the recruits she “seeded” at the start of this year, and said she hopes the forecast for this summer may offer a window for some coral recovery.

“February and March is when we get the biggest build up of heat in the system,” she said.

“In favour of corals this year is the La Nina event. So there is a strong chance that they’re going to get a reprieve from [bleaching].”

Increased rainfall typically means cooler water temperatures and less coral stress.

New corals can take three years or more to reach sexual maturity, so it’s critical that we get a run of bleaching-free seasons to allow those corals to develop.

While Mrs Chartrand said she hopes her project can help add resilience to the reef, it’s no substitute for taking bigger measures like stopping anthropogenic climate change and sediment runoff.

“It’s not this in isolation. It’s together acting on things like emissions,” she said.

“[Reef seeding is] really about buying time, so at a large scale we can support the processes that are there for recovery.”