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Scientists Check Coral Reef Health From Above

Bijal P. Trivedi
for National Geographic Today
October 10, 2001
 
Scientists have found a way to rapidly assess the health of the world's
coral reefs without getting wet. The technique can distinguish living
from dead coral by detecting spectral differences in the light that the
reefs reflect.

Flying as low as 3,000 feet (1,000 meters), the
scientists use a device called a spectrographic imager to measure the
corals' color characteristics. Every coral has a distinct spectral
"signature," and any changes in this may indicate a change in the health
of the reef.




"We measure the color—how much blue light, green light, yellow light, et cetera, is reflected by each coral—and compare it to the known signature," said Peter Mumby of the University of Newcastle upon Tyne, United Kingdom, and author of a report on the technique in a recent issue of the journal Nature.

Mumby's study began in November 1998, when he set out to determine the effects of El Niño, which had harmed reefs around the world. The study focused on two reefs in the lagoon of Rangiroa Atoll in French Polynesia.

The scientists flew up and down the reef taking pictures of every square meter. They then took to the water, mapping every type of coral—dead and alive—within several study plots.

When the aerial and sea-based data were compared, the results were both good and bad. "The aerial data," Mumby said, "was accurate to within 3 percent of what was really there," which suggests that the aerial approach could be a useful method of reef surveillance. The bad news was what the data showed: Rangiroa Atoll suffered tremendous damage during El Niño in 1997-98.

Major Losses

The survey showed that about 25 percent of the colonies of slow-growing corals known as Porites—some of which may be hundreds of years old—were dead. "Porites corals are considered the oak trees of the coral reefs, and are usually quite resistant to environmental stresses," said Mumby. "To see 25 percent destroyed was very disturbing."

The warm ocean waters created by El Niño also destroyed nearly all of the faster growing branched Pocillopora corals in the lagoon. Today, most of these corals have recovered, according to Mumby.

The advantage of remote sensing is that it enables scientists to assess an entire reef quickly. Images from reefs extending 92,500 square meters (110,630 square yards)—composed of several thousand research plots—were acquired in only an hour. In comparison, it took the underwater team three days to survey ten such plots.

One of the most surprising findings in the study, Mumby said, was the significant impact that local weather patterns have on the health of coral. "Local weather can play a huge role in reef health, either enhancing or protecting it from environmental stresses," he said.

When the seawater covering the coral is already quite warm, he explained, additional energy from the sun becomes poisonous and causes bleaching.

The team found that during the same period in which many of Rangiroa Atoll's corals died, the reefs in Tahiti, slightly southwest of the atoll, were unaffected. When the scientists examined the weather records for Tahiti, they found that the summer of 1998 was one of the cloudiest in recent history. "It seems that the clouds acted like a big sun hat and shielded the coral from the damaging sunlight," said Mumby.

Mumby's team is working with other researchers to design a satellite sensor for monitoring the health of coral.

Atlas of Coral Reefs

In another project important to coral preservation efforts, researchers at the United Nations Environment Programme-World Conservation Monitoring Centre (UNEP-WCMC) issued a new World Atlas of Coral Reefs in September. The contents are startling because they show that the world's remaining coral is considerably less than was thought to exist. Previous estimates of the total were from two to ten times greater than the current size.

All the worlds' remaining coral reefs cover about 284,300 square kilometers (109,768 square miles)—an area half the size of France. This is less than one-tenth of a percent of the Earth's oceans, with the reefs scattered along the coasts of more than 101 countries and territories.

"The atlas is a huge compilation of other people's work," said Mark Spalding, a marine ecologist for UNEP-WCMC and the primary author of the atlas. He and the other contributors spent eight years consolidating information on the location, health, and biological diversity of reefs from numerous sources, ranging from century-old maps to the most recent reef surveys and satellite data. "Some of the best maps of reefs in the Pacific and Indian Oceans were produced by Captain Cook in the 18th century," Spalding noted.

Preserving the health of the reefs documented in the atlas will require immediate attention. Experts say that as much as 60 to 75 percent of the world's coral reefs are in peril, threatened by blast fishing, pollution, disease, and ocean warming.

Almost 33 percent of the world's reefs are off the islands of the Philippines, Indonesia, Malaysia, and Papua New Guinea. These reefs are some of the most diverse, with as many as 600 species of coral, and are also the most threatened. Blast fishing, particularly common in Southeast Asia, "turns reefs into rubble" said Spalding. Shock waves from the explosives kill fish and tear apart the reefs.

But the situation is not all grim. Almost 25 percent of the world's reefs are in areas controlled by wealthy countries, including Australia, France, Britain, and the United States. These nations "have an obligation to preserve their reefs and support reef management in poorer nations," said Spalding.

He is also encouraged by successful reef restoration and management projects that illustrate the considerable social and economic benefits that reef preservation can bring to local people.

On Apo Island in the Philippines, for example, the destruction of reefs by blast fishing, along with overfishing, had destroyed the livelihood of fishermen and left many villagers starving. Residents of the island responded by setting aside about ten percent of the reef as a sanctuary, where fishing was banned.

"Within two years the reef was teeming with life and fish had come back," said Spalding. "And the catch in neighboring areas went through the roof. The spillover was enormous—a real win-win situation."

This research was funded by Prince Khaled bin Sultan bin Abdulaziz, president of the Khaled bin Sultan Living Oceans Foundation, in the United States. Jean Jaubert, director of the European Oceanographic Center, in Monaco, was the leader of the 1998 expedition on the vessel Golden Shadow to French Polynesia,during which the airborne remote sensing work of Peter J. Mumby and others was completed.
 

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