Frequently asked questions

Current as at 13 June 2016

What is coral bleaching?

Corals are animals which live in a symbiotic relationship with microscopic algae called zooxanthellae. The zooxanthellae, which live within the coral tissue, convert sunlight into food, providing corals with up to 90 per cent of their energy needs. Zooxanthellae also give corals much of their colour.

Bleaching occurs when stressful conditions, such as heat, cause this relationship to break down, resulting in the corals expelling their zooxanthellae. This leaves the coral tissue mostly transparent, revealing the coral's bright white skeleton.

This loss of their symbiotic algae means bleached corals are essentially starving.

Mass coral bleaching events have only ever occurred during unusually high sea temperatures.

Why are ocean temperatures unusually warm?

Great Barrier Reef waters have warmed by approximately 0.67 degree Celsius since 1871, with most of the warmest years occurring in the past two decades.

According to the Bureau of Meteorology, this year the Great Barrier Reef recorded its highest-ever sea surface temperatures for February, March, April and May since records began.

These record-breaking temperatures occurred because of the underlying ocean warming trend caused by climate change, the recent strong El Nino and local weather conditions.

How extensive is the coral bleaching and coral mortality?

The mass bleaching follows a distinct pattern whereby the severity declines from north to south.

The most severe bleaching continues to be in the northern half of the Great Barrier Reef Marine Park. Bleaching is less severe towards the southern part of the Reef.

Overall, 22 per cent of coral on the Great Barrier Reef has died from this current mass bleaching event — about 85 per cent of that die-off has occurred between the tip of Cape York and just north of Lizard Island, 250 kilometres north of Cairns.

The level of bleaching and mortality varies within different sections of the Marine Park.

This map provides a summary of bleaching and mortality levels in each of the four sectors.

This map provides average mortality levels on surveyed reefs.

Are all bleached corals dead?

Bleached white corals are not dead. These corals are under stress from unusually hot conditions.

If the heat stress does not persist for too long, the corals can recover and regain their colour. If the heat stress persists for a month or more, the stressed corals will eventually die and be covered in algae.

What does it mean if a coral is fluorescent?

Stressed corals may initially display a striking fluorescent hue of pink, yellow or blue — this is also a sign of severe stress and bleaching.

Most corals contain fluorescent proteins that help minimise damage from ultraviolet light. When there is excessive sunlight, the fluorescent proteins can absorb and dissipate some ultraviolet light. Most fluorescent proteins are invisible in daylight, meaning bleached corals may appear completely white rather than fluorescent.

A fluorescent coral can die from heat stress without becoming a totally bleached white coral.

What are the stages of coral bleaching?

It is a common misconception that corals progress through various stages of bleaching, between being healthy, totally bleached or dead.

Corals can skip stages, even going from healthy to dead during a bleaching event without displaying any other signs of bleaching if the heat and light stress is very acute.

Photos are available of various stages of bleaching.

What has changed in the past month?

Some reefs in the far northern management area of the Marine Park were surveyed again. Mortality was found to have increased substantially, particularly on inshore and midshelf reefs.

Sea temperatures have cooled over the past month, but are still well above average for this time of year.

The effect of continued higher than usual sea temperatures during autumn and winter on potential coral recovery or further bleaching and disease is unknown. The situation is being closely monitored to determine its effects.

What is the extent of coral mortality overall?

It is too early to tell, given the bleaching event is still unfolding.

While substantial mortality has been detected on some reefs between Port Douglas and the tip of Cape York, further in-water surveys will be needed to gauge the overall extent of coral die-off.

What is the prognosis for recovery?

At 348,000 square kilometres, the Great Barrier Reef is large and resilient with the ability to recover from major events — as demonstrated by recently released data from the Australian Institute of Marine Science’s Long-term Data Monitoring Program.

The data shows between 2012 and 2015, coral cover improved in the central and southern sectors through post-cyclone coral growth, but declined in the northern sector due to more recent cyclone activity.

Since 2012, coral cover on the Reef increased to almost 20 per cent from a low point of about 17 per cent.

The new results show coral in the southern sector increased from 15 per cent in 2012 to 27 per cent in 2015. This strong coral growth was the main contributor to an increase in the Reef-wide figure. 

Recovery from the current bleaching event will largely depend on how long ocean temperatures remain high locally.

If conditions return to normal, the tiny algae that give corals their food and colour can repopulate corals. However, if ocean temperatures are too high for too long, the corals will eventually starve and die.

Given the unprecedented scale and nature of the current bleaching event, it is too early to determine how long it will take for corals to recover from this period of extreme heat stress.

On the most resilient reefs and in ideal circumstances, bleached corals can regain their colour within a period of weeks to months once water temperatures return to normal.

However, corals experiencing chronic poor water quality and/or other stressors are unlikely to recover within these short timeframes and recovery will be impeded.

Even if a coral regains its colour, this does not necessarily mean it is in good health.

Research shows bleaching can deplete the corals' energy resource to the extent that corals do not reproduce for one or two years. Its weakened state means the coral is also more vulnerable to disease.

What does bleaching mean for visitors?

At 348,000 square kilometres, the Reef contains many places where visitors can experience and enjoy the marine environment. In the short to medium term, there will be minimal effect on fish and invertebrates and other diverse reef organisms.

There are also many experiences on offer throughout the Marine Park, from hiking on islands and kayaking through to swimming and boating.

We have general information about the Reef, best practices and experiences on offer.

For detailed advice about tourism and to plan your visit, please see Tourism Queensland's website.

How does this year’s coral bleaching compare to mass bleaching events on the Great Barrier Reef in 1998 and 2002?

Due to its extent and severity, the 2016 bleaching event is worse than the 1998 and 2002 mass bleaching events.

In 1998 and 2002, about 40 to 50 per cent of the Reef experienced some bleaching. Up to five per cent of reefs suffered significant coral mortality in each of these events, as sea temperatures came back down again in time for recovery to occur.

How many surveys were conducted?

Along with the Queensland Parks and Wildlife Service, we conducted 2641 reef health and impact surveys of 186 reefs across the Marine Park since the beginning of summer. Additional in-water surveys were completed by our science partners.

More than 900 reefs in the Marine Park and the Torres Strait were surveyed from the air by the ARC Centre of Excellence for Coral Reef Studies. Researchers found of 911 individual reefs only 7 per cent (68 reefs) were not bleached to some extent. Between 60 and 100 per cent of corals were severely bleached on 316 reefs, nearly all in the northern half of the Reef.

We also continue to receive additional observations from the public and tourism operators through our Eye on the Reef program.

How was bleaching measured?

Aerial surveys quickly provided information about the extent of coral bleaching (that is, how many reefs in an area exhibited any bleaching) and what proportion of the coral on an individual reef was bleached.

These results helped direct in-water survey efforts to determine how severe the bleaching was (including to what depth) and the rate of coral mortality. It also helped ground-truth aerial data.

In-water surveys measured the extent of coral bleaching based on the proportion of bleached coral on an individual reef. They also measured severity by assessing the most common appearance of bleached corals.

This included recording whether corals were bleached only on the upper surface, if corals were pale or fluorescent, or if corals were completely white. It also measured the proportion of corals that had already died from bleaching.

How will you use the data from in-water surveys?

We will use the survey information to compile a comprehensive assessment of the overall impact of the bleaching event,  the percentage of bleached reefs in the Marine Park, the percentage of bleached corals on each individual reef, how severe the bleaching was, and the percentage of bleaching-induced coral mortality.

By comparing information over time, we will also know how much bleaching is occurring as our climate changes, which species and locations are most affected and how much of an impact other stressors, such as crown-of-thorns starfish, pollution and disease, are having. 

Monitoring over time will also allow us to measure how quickly corals return to a healthy state. 

The resulting data will guide management policies and actions designed to protect and assist recovery of corals.

What resources were devoted to conducting the surveys?

The Great Barrier Reef Marine Park Authority and the Queensland Parks and Wildlife Service (QPWS) worked with scientists from the Australian Research Council Centre of Excellence for Coral Reef Studies and the Australian Institute of Marine Science (AIMS) to conduct the surveys. 

Resources committed to the task totalled approximately $3.5 million across these organisations.

The University of Queensland will also resurvey sites in the Far Northern Management Area as part of its Catlin Seaview Survey program.

What happens next?

Another round of surveys is scheduled to start in October to assess recovery rates and survivorship.