Quirks & Quarks

Holy coral reefs? They've got a 'halo' that could show if they're healthy

Reef patches are surrounded by rings of bright sand that's been cleared by reef fish

Reef patches are surrounded by rings of bright sand that's been cleared by reef fish

Coral reef halos in the Red Sea seen from satellite images. (CNES/Airbus; DigitalGlobe)
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Halos of sand surrounding patches of coral reefs that can be seen in satellite images, may help researchers monitor the health of coral reef ecosystems.

Coral reefs are  threatened by human activities such as climate change, pollution and overfishing.

Scientists and conservationists are interested in monitoring the health of these ecosystems, but current methods are time consuming and expensive.

Elizabeth Madin, an assistant research professor at the Hawai'i Institute of Marine Biology at the University of Hawaii at Manoa, has been studying coral reef halos for almost a decade. She thinks these halos could provide an inexpensive and more efficient alternative to monitoring coral reef ecosystem health.

What are coral reef halos?

Reef halos are created by the interaction between small herbivorous fish and larger predators.

Small fish like to take shelter in the coral patches as protection from larger predatory fish. As a result they tend not to venture to far from this safe haven. So they tend to feed on the algae and sea-plants in the immediate vicinity of their reef patch. 

A fish swims through the coral on Australia's Great Barrier Reef. (WILLIAM WEST/AFP/Getty Images)

"It's almost like someone took an underwater lawnmower and went around the reef and you get this pretty clear bare patch of sand," explained Madin.

Reef halos can be found around isolated patches of small reefs in shallow lagoons near large reefs. They can be anywhere from one metre to 100 metres in diameter and are found all over the world.

Monitoring coral reef health with halos

The presence of reef halos is can be a sign that the ecosystem is healthy, said Madin. It means there are sufficient predators and prey living in and around the coral reef, cultivating the relationship that results in these halos.

Madin and her colleagues found that reef halos were more likely to be found inside marine reserves, where fishing is banned.

Coral reed halos in Heron Island lagoon within Australia’s Great Barrier Reef. (DigitalGlobe)

Humans generally catch larger predatory fish, so in areas where these predators are protected from fishing, like along the Great Barrier Reef, halos are more common. However, they also found that there was no difference in the size of reef halos inside or outside the marine reserves, which was contrary to their expectations.

"What we're trying to figure out is more detail about what exactly we can say about the health of the food web if we see halos or if we don't see halos or if they change in size over time," said Madin. "We're trying to decode the halos."

Her ultimate goal is to develop an online tool for conservationists to monitor hundreds of coral reefs at once by just checking their halos from satellite imagery.

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