Rings of bare sand surround dozens of individual coral reefs in the Red Sea.
Halos of bare sand surround dozens of individual coral reefs in the Red Sea. Credit: CNES/Airbus; DigitalGlobe

Coral reefs are the rain forests of the ocean, home to staggering levels of biodiversity. And they are struggling, threatened by overfishing, warming oceans, and habitat loss. Monitoring the often remote ecosystems is logistically difficult.

A new study is exploring the possibility of tracking reef health from the air using the size of coral reef halos: circles of grazed algae that surround reefs small and large. But untangling the ecology of these halos is proving more complicated than anyone expected.

“If you look at Google Earth satellite imagery, you can see these grazing halos surrounding coral reefs all over the world.”

“If you look at Google Earth satellite imagery, you can see these grazing halos surrounding coral reefs all over the world,” says Elizabeth Madin, a marine ecologist at the University of Hawai‘i at Mānoa and lead author of two new studies on coral reef halos, published in the Proceedings of the Royal Society B and Frontiers in Ecology and Evolution.

Grazing halos are areas of bare sand surrounding coral reefs that are created by herbivorous fish eating algae and sea grass. In some places, halos stretch over 100 meters across, whereas in others, they are less than 10 meters in diameter, with the size of the halo thought to represent the farthest distance that grazing fish and invertebrates venture out from the relative safety of the reef.

“Everybody has assumed that the size of the halos was determined by predator-prey relationships,” Madin says.

In protected marine reserves with higher populations of large predators, smaller halos would make sense because the risk of predation is much higher closer to the reef than in areas with fewer predators.

“That was our working hypothesis,” she says, “but it turns out that halo size is not as straightforward as we thought.”

Satellites Teach Researchers to “Ask Questions That We Don’t Know the Answers to Yet”

Madin and her colleagues used satellite imagery to compare halo size surrounding reefs inside and outside protected no-fishing marine reserves. “For whatever reason, we found no correlation between the size of the halos and the boundaries of the reserves. So we have a mystery: What causes halos to be different sizes in different places? We’re still scratching our heads.”

The team is now trying to isolate the factors that may govern halo size, including fish and algae species, water temperature, and latitude. “Once we understand those mechanisms a little better, we’ll be in a better position to decode what halos are telling us about how reef ecosystems are functioning. Not only is this a fascinating ecological phenomenon, it holds a lot of potential for reef conservation management using remote sensing methods.”

The new studies represent the next generation of ocean management, says Doug McCauley, a marine ecologist at the University of California, Santa Barbara, who was not involved in the new study.

“I’m really excited to see this kind of cross-fertilization between marine biologists and satellite technology. As the stakes in the oceans keep going up in terms of biodiversity, fish populations, food supplies, and industry, we need to keep upping the ante with how we monitor the health of the ocean. Satellites help us see the bigger picture and teach us to ask questions that we don’t know the answers to yet.”

—Mary Caperton Morton (@theblondecoyote), Science Writer


Morton, M. C. (2019), Mysterious coral reef halos can be seen from space, Eos, 100, https://doi.org/10.1029/2019EO122611. Published on 03 May 2019.

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