Several heatwaves in the Indian Ocean have killed more than two-thirds of corals in two years, a study has shown.

Research from the Zoological Society of London (ZSL) suggests some corals were more resilient to high temperatures, despite 70% of the hard corals in the ocean being lost between 2015 and 2017.

But, as these sorts of heatwaves become more frequent, the ability to recover will become “increasingly compromised”, the study says.

Seawater temperatures around the reefs in the Chagos Archipelago, part of the British Indian Ocean territory, were unusually high for eight weeks in 2015. Another heatwave hit the region before the corals could recover in 2016, this time lasting for four months.

The seafloor surveys before and after the heatwave saw the amount of healthy coral fall by 60% in 2015. Although scientists were unable to judge the impact of the 2016 heatwave across all the islands, data from the Peros Banhos atoll suggests that 70% of hard corals were lost due to the rise in temperature.

Coral bleaching affected the Archipelago in 1998. It took the reef ten years to recover, according to the study.
Image:
Coral bleaching affected the Archipelago in 1998 and it took the reef 10 years to recover

But, while the second heatwave was longer, fewer of the surviving corals were killed.

Lead author of the study, Dr Catherine Head, a marine biologist at ZSL’s Institute of Zoology, said: “We know it has taken about 10 years for these reefs to recover in the past.

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“But, with global temperatures rising, severe heatwaves are becoming a more regular occurrence, which will hinder the reef’s ability to bounce back.”

She said preliminary reports from April 2019 suggested further high sea temperatures had led to more coral bleaching in the British Indian Ocean Territory, though it is not known yet how serious it is.

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“It is encouraging that reefs may have some degree of natural resilience, though further research is needed to understand the mechanisms by which some corals are able to protect themselves,” said Dr Head.

“This may be our best hope to save these vital habitats from the catastrophic effects of climate change.”

Similar coral death and changes to the reef’s make-up were seen in the Chagos Archipelago after global coral bleaching in 1998. Recovery took 10 years, according to the study.

The relatively fast recovery of the coral suggests the reef is highly resilient. It has also benefited from a lack of disturbance from humans, a result of the controversial removal of the Chagossian people by the UK in 1971 to make way for a US military base.

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