Australasian Science: Australia's authority on science since 1938

Setting the Record Straight on Coral Bleaching

By Russell Reichelt

The mass bleaching on the Great Barrier Reef this summer is the most damaging recorded.

Over the past 12 months, corals in most tropical regions across the world have experienced the most severe mass coral bleaching ever recorded. By June this year, 22% of coral died on the 3000 reefs of the Great Barrier Reef, with most of the mortality occurring in the far north above Cooktown.

Coral bleaching occurs in hard and soft corals, and is triggered by prolonged heat stress that causes corals to expel the colourful, symbiotic organisms that live inside their tissue. These organisms, called zooxanthellae, normally mask the underlying white skeleton. Giant clams and sea anemones also have these symbionts, and consequently also turn white when under heat stress.

Under the presence of a strong El Niño, sea temperatures from February to May over much of the Reef were a degree or more over their long-term monthly averages, exceeding all previous temperature records. The El Niño came on top of an underlying trend of ocean warming caused by human-induced climate change – since 1871 the average temperature on the Reef has risen by 0.67°C.

The physiology of corals is adapted to their local region, and they thrive in waters close to their thermal tolerance limits, which can vary by 5–6°C across the full geographic range of tropical corals.

The mass bleaching on the Great Barrier Reef this summer is the most damaging ever recorded. To date, surveys by the Great Barrier Reef Marine Park Authority (GBRMPA) and the Australian Institute of Marine Science found that 85% of the die-off has occurred between the tip of Cape York and just north of Lizard Island, 250 km north of Cairns. The impact on reefs between Port Douglas and Cairns varies considerably from no mortality to high levels, while the 1300 km stretch below Cairns has escaped significant rates of mortality.

Surveys by James Cook University provide a similar synoptic picture. Preliminary results show the average coral loss within each management area is:

  • 50% in the Far Northern Management Area;
  • 16% in the Cairns–Cooktown Management Area (surveys around Lizard Island were conducted in March, but more recent reports indicate that mortality levels are likely to be higher in this management area);
  • 3% in the Townsville–Whitsunday Management Area; and
  • 0% in the Mackay–Capricorn Management Area.

Reef waters are now cooling, and many reefs will recover as ocean temperatures come down. However, mortality figures are also likely to increase north of Cairns, as there remain substantial levels of highly stressed corals that are completely bleached but not yet dead.

With initial in-water surveys now complete, GBRMPA is turning its attention to measures to help the Reef recover and to protect the more than three-quarters of coral that survived the bleaching event to date. The priority across all governments, industries and the community must be to take actions that will build the resilience of the Reef in the face of continuing ocean warming.

Along with supporting global initiatives to reduce greenhouse gas emissions, actions can be taken on and near the Reef. A high priority is to decrease the run-off of fertilisers, sediments and pesticides into waterways connected to the Reef. Reducing nutrient loads, in particular, is critical as corals are adapted to a low-nutrient environment, and waters over-enriched with nutrients can lower the thermal tolerance of inshore coral reefs.

A program to continue culling the coral-consuming crown-of-thorns starfish will also act as a key insurance policy. Every adult coral protected from this native predator is a coral that can potentially produce larvae to repopulate other reefs downstream.

The existing zoning plan across the Great Barrier Reef Marine Park also plays a critical role in maintaining the resilience of the ecosystem by providing protective measures against the effects of overfishing and potential shipping risks.

Education and compliance with the rules associated with these protective measures is also a high priority.

Dr Russell Reichelt FTSE is the Chairman of the Great Barrier Reef Marine Park Authority. He is a board member of the Great Barrier Reef Foundation and the Australian Maritime Safety Authority. He has also served as CEO of the Australian Institute of Marine Science.