Reef snapshot details widespread coral bleaching on the Great Barrier Reef

The 2023−24 summer has seen substantial climate driven impacts across the Great Barrier Reef, with widespread coral bleaching, two cyclones and several severe flood events.

The Reef Snapshot 2023−24, released today by the Reef Authority, Australian Institute of Marine Science and the CSIRO, confirms the cumulative impacts experienced across the Reef this summer have been higher than previous summers. 

This also includes outbreaks of the coral-eating crown-of-thorns starfish.  

The Reef Snapshot applies the Coral Reef Bleaching Framework developed by AIMS, James Cook University and the Reef Authority that describes bleaching events in a clear and consistent way.

Monitoring coral condition is still ongoing as the Reef’s fifth widespread bleaching event since 2016 continues to unfold. 

Aerial surveys were conducted over 1000 reefs spanning the Great Barrier Reef Marine Park and Torres Strait. Coral bleaching was observed on 73 per cent of surveys reefs within the Marine Park and 6 per cent in the Torres Strait. 

Reefs in the far north of the Marine Park and in the Torres Strait recorded lower levels of coral bleaching.

The Snapshot shows the highest levels of coral bleaching were found across the southern region and parts of the central and northern regions, where in some areas, corals were exposed to record levels of heat stress. Parts of the central and northern regions also experienced the highest levels of coral bleaching. 

The Great Barrier Reef is but one of many coral reef systems across 53 countries that have experienced widespread coral bleaching in the last 12 months, prompting the USA’s National Oceanic and Atmospheric Administration (NOAA) to announce a fourth global bleaching event. 

The Reef Authority’s Chief Scientist Dr Roger Beeden said monitoring and responding to the conditions on the Marine Park over summer is vital. 

“Climate change is the greatest threat to the Great Barrier Reef, and coral reefs globally,” said Dr Beeden.  

“The Great Barrier Reef is an incredible ecosystem, and while it has shown its resilience time and time again, this summer has been particularly challenging.” 

“The management work we do 365-days a year is all focused on protecting the resilience of the Reef.  Work to control coral predators such as crown-of-thorns starfish, compliance with the Marine Park rules to protect the Reef’s precious biodiversity and heritage values, coupled with conservation actions undertaken with partners such as the tourism industry, are all vital to support Reef recovery following periods of stress,” said Dr Beeden.

The Reef Authority is continuing to work with the Australian Institute of Marine Science, the Reef Joint Field Management Program, crown-of-thorns starfish control Program, Tourism Operators, and researchers on further in-water surveys. 

In-water surveys complement aerial surveys by providing data on fine-scale impacts of heat stress, such as the severity of coral colony responses, the prevalence of bleaching in different habitats and depths, and any coral mortality at the time.

Tourism operators as always have helped with heavy lifting, conducting 15,450 reef health surveys and taken, 65,000 images at 272 high value tourism sites and submitting this data into the Eye on the Reef system. 

The data from these surveys give a greater overview of the severity of bleaching among different coral types, habitats, and depths over the coming months.

Dr David Wachenfeld, Research Program Director at Australian Institute of Marine Science said mortality from the bleaching event has already been observed by AIMS research divers in each region of the Reef.   

“AIMS long-term monitoring of the condition of reefs over the next 12 months will help us understand the ultimate balance between loss and survival of corals from bleaching, cyclones, floods and starfish over the last few months.

“While these results are still to come, the extent of the heat stress, and the result of the aerial surveys indicate this is one of the most extensive bleaching events the Reef has experienced in AIMS’ nearly 40 years of monitoring.

“The Great Barrier Reef has seen increases in coral cover to high levels in recent years, indicating it is still a resilient system. But this resilience has its limits.”

CSIRO Executive Director Environment, Energy and Resources Dr Peter Mayfield said research partnerships were critical to tackling the range of pressures on the Reef. 

“Modelling, monitoring and innovation has a critical role to play in better understanding the impacts of climate change on the Reef, not just now but into the future. This includes looking at how we can scale up ecosystem repair and adaptation, both on land and in coastal and marine areas.“

View a copy of the Reef Snapshot 2023-24 and stay up-to-date with the latest Reef health information.

Feature image: aerial survey of a coral reef showing signs of bleaching on 23 February 2024 | Neal Cantin