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Can we solve the riddle of the coral reef halo?

Coral reefs worldwide are threatened by a variety of human impacts. Fishing is among the most pressing threats to reefs, because it occurs on most reef systems and fundamentally alters food webs. Meanwhile, observing coral reefs, particularly remote, hard-to-access locations such as the Northwestern Hawaiian Islands, remains notoriously difficult and expensive. But a National Science Foundation-funded researcher may have found a mysterious natural phenomenon that can help observe coral reef health from space. Patches of coral reef are often surrounded by very large "halos" of bare sand that span hundreds to thousands of square meters. Beyond these halos lie lush meadows of seagrass or algae. Scientists have observed reef halos for decades and explained their presence as the result of fish and invertebrates, who typically hide in a patch of coral, venturing out to eat algae and seagrass that cover the surrounding seabed. But the fear of predators keeping these smaller animals close to safety has long been thought to explain why the cleared area is circular. The researchers have revealed there is more to the story -- these features may be useful in observing aspects of reef ecosystem health from space.

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