Can we solve the riddle of halos of coral reefs?

Coral reefs around the world are threatened by a variety of impacts on humans. Fishing is one of the most pressing threats to reefs because it affects most reef systems and fundamentally alters food chains. At the same time, observing coral reefs, especially in remote and hard-to-reach places such as the Northwest Hawaiian Islands, remains notoriously difficult and expensive. But a researcher from the University of Hawaii (UH) in Mānoa and her collaborators may have discovered a mysterious natural phenomenon that can help us observe the health of coral reefs from space.

The coral reef areas are often surrounded by very large bare sand halos of several hundred to thousands of square meters. Beyond these halos are lush meadows of seagrasses or algae. Two recently published studies and a third feature film directed by Elizabeth Madin, assistant research professor at the Hawaiian Institute of Marine Biology (HIMB) of the UH at the School of Oceanic and Terrestrial Science and Technology from Mānoa, have brought to light these enigmatic features are visible from the space.

Scientists have observed halos in the reefs for decades and have explained their presence as the result of fish and invertebrates, which usually hide in an area of ​​coral and venture to eat seaweed and seaweed. herbaria that cover the surrounding seabed. However, it has long been feared that predators' fear of keeping these small animals close to safety explains why the open area is circular. Madin's recent work reveals that there is more to the story, and that these features may also be useful for observing aspects of reef ecosystem health from space .

In one of Madin's new studies, published in the Acts of the Royal Society B, his team of scientists has discovered that marine closed areas, where fishing is prohibited, dramatically change these patterns of vegetation at the scale of the seascape in coral reef ecosystems, thus influencing the presence of the famous pattern of "halo". This means that marine reserves could have even greater impacts on coral reef seascapes than before.

The team hypothesized that if "the fear of small fish being eaten was the cause of the formation of halos, the number of predators should be related to the presence of these bare spots and their size" Madin explained in a recently published article. in New scientist. "With fewer predators, we would expect the grazing fish to be less fearful and move further away from the reef, resulting in wider halos."

But to the great surprise of Madin and his team, who used freely available satellite imagery, they saw no difference in the size of the halos inside or outside marine reserves that were closed to fishing. . However, they found that halos are significantly more likely to occur in uncaptured marine reserves, demonstrating the new effects of marine reserves on the landscape.

In the second study, published in boundaryMadin et al. Found that a more complex set of species interactions than previously assumed probably influenced these halos. By combining remote high-definition underwater video camera traps and traditional ecological studies of the coral reefs of the Great Barrier Reef in Australia, Madin observed that, in addition to plant protection fish known for their role in the formation of coral reefs, Halo, fish eating invertebrates digging in the sand in search of prey disrupted the algae and made them larger. Another piece of the puzzle had been revealed.

Overall, Madin's work shows that the presence of halos can serve as an indicator of the health aspects of the reef ecosystem, as it is suspected that halos are the result Indirect healthy populations of predators and herbivores. Madin's ongoing studies on halos have shown that they can appear and disappear over time and change considerably in size, a phenomenon suggesting that environmental factors also influence halos.

"We urgently need more effective ways to control these reefs more economically and faster," Madin said. "Our work combines freely available satellite imagery, as well as traditional field experiences and observations, to unravel the mystery of what the global" halo "models can tell us about how reef ecosystems can transform the space and / or time due to fishing or marine reserves, thus paving the way for the development of a new technological solution to the challenge of monitoring large areas of reefs coral reef and allow the management of healthy ecosystems and sustainable fisheries. "