A dramatic rise in the surface temperature of Indonesian waters has resulted in a large-scale bleaching event that has devastated local coral populations. Following a report of a bleaching incident in May, WCS-Indonesia dispatched a “Rapid Response Unit” of marine biologists to investigate. Their initial survey revealed that over 60 percent of corals have bleached. The incident took place in the province of Aceh, on the northern tip of the island of Sumatra.

Coral “bleaching,” or whitening, occurs when algae living within coral tissues are expelled. The condition results from stress triggered by environmental factors such as sea surface temperature fluctuations. Some bleached corals may recover over time, while others die.

Subsequent monitoring conducted by marine ecologists from WCS, James Cook University in Australia, and Syiah Kuala University in Indonesia were completed in early August. The rate and extent of the coral mortality exceeds that of most other bleachings on record. The scientists found that 80 percent of some species have died since the initial assessment and more colonies are expected to die within the next few months.

Sea surface temperatures in the Andaman Sea—an area that includes the coasts of Myanmar, Thailand, the Andaman and Nicobar Islands, and northwestern Indonesia—have been on the rise. According to the National Oceanic and Atmospheric Administration’s Coral Hotspots website, temperatures in the region peaked in late May, when the temperature reached 34 degrees Celsius. This represents a dramatic 4-degree rise over the long-term averages for the area.

“It’s a disappointing development, particularly in light of the fact that these same corals proved resilient to other disruptions to this ecosystem, including the Indian Ocean Tsunami of 2004,” said WCS-Indonesia Marine Program Director Dr. Stuart Campbell.

WCS and JCU have been working in the region since March 2005. Surveys conducted in the wake of the Indian Ocean tsunami in 2004 revealed that the many reefs of Aceh were largely unaffected by this massive disturbance. Indeed, reefs severely damaged by poor land use and destructive fishing prior to the tsunami had recovered dramatically in the intervening years due to improved management by local governments and communities. But the recent bleaching and mortality will have a profound effect on reef fisheries.

Of particular concern is the scale of the sea surface temperature anomaly, which the NOAA website indicates has affected the entire Andaman Sea and beyond. Similar mass bleaching events in 2010 have now been recorded in Sri Lanka, Thailand, Malaysia and many parts of Indonesia.

“This is a tragedy not only for some of the world’s most biodiverse coral reefs, but also for people in the region, many of whom are extremely impoverished and depend on these reefs for their food and livelihoods,” said WCS-Marine Program Director Dr. Caleb McClennen. “It is another unfortunate reminder that international efforts to curb the causes and effects of climate change must be made if these sensitive ecosystems and the vulnerable human communities around the world that depend on them are to adapt and endure.”