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UH study shows reef halos could be key to remote checkup of coral health worldwide

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What if there was a way to inexpensively survey coral reefs worldwide remotely and understand how they’re doing? Thanks to researchers at the University of Hawai‘i, a tool to do just that might be within reach.

Coral reef halos, also known as grazing halos or sand halos, are bands of bare, sandy seafloor that surround coral patch reefs. These features can be seen clearly from satellites and could provide a window into global reef health, according to a published study by scientists at the UH-Mānoa School of Ocean and Earth Science and Technology.

Grazing halos are worldwide. Compilation: Madin, et. al, 2022. Images: Google Earth Pro.

“Once we’ve been able to decode the clues that halos are giving us about the health of coral reef ecosystems, specifically in terms of the health of populations of plant- and fish-eating reef fishes, we plan to develop a tool that will allow scientists, reef managers, conservation practitioners and others to remotely and cheaply survey coral reefs and understand how they’re doing,” Elizabeth Madin, an associate research professor at the School of Ocean and Earth Science and Technology’s Hawai‘i Institute of Marine Biology who led the study, said in a press release. “This approach will never fully replace underwater reef monitoring, but would provide a first-cut idea of how reefs are faring over much larger spatial and temporal scales than we can possibly achieve with traditional underwater surveys.”

Scientists have observed reef halos for decades, mostly in the tropics, and explained their presence as the result of fish and invertebrates, which typically hide in a patch of coral, venturing out to eat algae and seagrass that cover the surrounding seabed. However, the fear of predators keeps these smaller animals close to the safety of the reef — and focused on eating the marine plants nearby.


In the UH-Mānoa study, Madin and her team analyzed high-resolution satellite imagery and historical aerial imagery from the 1960s that was captured from around the world. They also documented the previously undescribed presence of halos outside the tropics surrounding seagrass “reefs,” and revealed the timescales for which coral reef halos change, merge and persist.

“We found that halos, a fascinating phenomenon occurring on coral reefs worldwide, are much more common around the world than we would have expected,” Madin said in the press release. “We also see that they are quite dynamic. Halos can change in size over relatively short timescales, on the order of months, despite persisting for at least half a century, which is as long as we could go back in time with aerial imagery.”

Reef halos have been associated with marine reserves designed to protect predator and herbivore species from being overfished. Previous research indicated that halo presence is a potential indicator of predator — and possibly herbivore — recovery from fishing and that the size of a halo is likely indicative of herbivore recovery.


Madin and her research team recently finished developing an artificial intelligence algorithm that will cut the time needed to find and measure halos from large volumes of satellite imagery from days to minutes. They are also exploring how predator populations around the world affect the presence and size of halos. They are testing the mechanisms behind halo development and changes in size as well.

“All of this information will help us use halos as the basis for what we hope will be a valuable, freely-accessible, globally-relevant reef health assessment tool,” Madin said in the press release.

Halos surround coral reef in Florida Keys. From Google Earth Pro.

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