Coral reef halos are bands of bare, sandy seafloor that surround coral patch reefs. They are also known as grazing halos or sand halos. According to a recently published study by researchers at the University of Hawai'i at Mnoa School of Ocean and Earth Science and Technology, these features, which are clearly visible in satellite imagery, may provide a window into reef health around the world (SOEST).
|Halos surround coral reef in Florida Keys. Credit: Google Earth Pro.|
For decades, scientists have observed reef halos, mostly in the tropics, and explained their presence as the result of fish and invertebrates hiding in a patch of coral and venturing out to eat algae and seagrass that cover the surrounding seabed. However, the fear of predators keeps these smaller animals close to the reef's safety—and focused on eating the nearby marine plants.
The team analyzed very high-resolution satellite imagery and historical aerial imagery from the 1960s from around the world in the recently published study led by Elizabeth Madin, associate research professor at SOEST's Hawai'i Institute of Marine Biology. They discovered previously unknown halos surrounding seagrass "reefs" outside of the tropics, as well as the timescales over which coral reef halos change, merge, and persist.
"We discovered that halos, a fascinating phenomenon that occurs on coral reefs worldwide, are much more common than we would have expected," Madin said. "They are also quite dynamic, as we can see. Halos can change size over relatively short timescales, on the order of months, despite persisting for at least a half-century, which is as far back as we can go with aerial imagery."
|Halos surrounding seagrass 'reefs' within algal beds near Playa des Codolar, Ibiza Island, off the coast of Spain. Credit: Google|
Decoding health indicators
Reef halos have been linked to marine reserves established to protect predator and herbivore species from overfishing. Previous research published in 2019 in Proceedings of the Royal Society B: Biological Sciences and Frontiers in Ecology and Evolution suggested that the presence of halo is a potential indicator of predator—and possibly herbivore—recovery from fishing, and that halo size is likely indicative of herbivore recovery.
"Once we've decoded the clues that halos are giving us about the health of coral reef ecosystems, particularly the health of populations of plant- and fish-eating reef fishes," Madin explained, "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."
"While this approach will never completely replace underwater reef monitoring, it will provide a first-cut idea of how reefs are faring over much larger spatial and temporal scales than traditional underwater surveys can possibly achieve."
|Grazing halos are visible worldwide. Credit: Compilation: Madin, et. al, 2022 via Google Earth Pro.|
Madin and his colleagues have also recently completed the development of an artificial intelligence algorithm that will reduce the time required to find and measure halos in large volumes of satellite imagery from days to minutes. They're also looking into how predator populations around the world influence the presence and size of halos. Finally, they are investigating the mechanisms underlying halo formation and size changes.
"All of this data will allow us to use halos as the foundation for what we hope will be a valuable, freely accessible, globally relevant reef health assessment tool," Madin said.
The new findings were published in The American Naturalist.