The Rowley Shoals are a chain of three coral atolls 300km off Broome, Western Australia. High sea temperatures cause coral, which can kill coral. Rowley Shoals showed the first signs of mass coral bleaching earlier this year. A follow-up survey found a surprising recovery.
Tom Holmes, the marine monitoring coordinator at the WA Department of Biodiversity, Conservation and Attractions, said it appeared the coral had pulled off an "amazing" return towards health over the past six months. The survey was a follow-up to an April survey that found as much as 60 per cent of corals on some Rowley Shoals reefs had bleached in April 2020 after the most widespread marine heatwave since reliable satellite monitoring began in 1993.
"So when a coral bleaches, it's actually just a sign of initial stress," Dr Holmes said. "Tiny little microscopic algae that live in the coral are expelled by the corals themselves, so the actual coral animal underneath is still alive." Corals rely on these microscopic algae as a food source and cannot survive for long without them. "If that stress continues for a long time and those corals remain white, then it can lead to mortality," Dr Holmes said.
Last summer bush fires raged across Australia's east coast, and an unprecedented marine heatwave enveloped the west coast, reaching from South Australia all the way up to the Kimberley in northern Western Australia. "It was actually the most significant bleaching event we've ever recorded out there," Dr Holmes said.
"It's one of the only places in WA, and only one of the places in Australia, where we've had long-term stability in coral cover and to date it's been largely free of wide scale bleaching events."
"We had this sudden temperature peak, which is when the bleaching occurred," Dr Holmes said. "But then the water temperatures dropped down fairly quickly." This likely would have reduced the stress on the coral and enabled it to recover.
Only 10 per cent of the coral at the Rowley Shoals has been killed, rather than the feared 60 per cent.
