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Can we solve the riddle of coral reef halos?



  Can we solve the puzzle around the halo of coral reefs?
Halos in the Red Sea. Credit: CNES / Airbus; DigitalGlobe.

Coral reefs around the world are threatened by various human influences. Fishing is one of the most urgent reef threats, as it occurs in most reef systems and fundamentally alters food webs. Observing coral reefs, especially in remote, hard-to-reach places like the Northwest Hawaiian Islands (NWHI), remains notoriously difficult and expensive. A University of Hawai'i (UH) in Mānoa and her co-workers may have found a mysterious natural phenomenon that can help us observe the health of coral reefs from outer space.

Patches of coral reef are often surrounded by huge "halos" of bare sand that make up hundreds to thousands of square feet. Behind these halos are lush meadows of seaweed or algae. Two recent studies and a third feature story directed by Elizabeth Madin, assistant professor at the Hawai & # 39; i Institute of Marine Biology (HIMB) at the Mānoa School of Ocean and Earth Science and Technology's UH, shed light on these enigmatic Properties are visible from outer space.

For decades, scientists have been observing reef halos, explaining their presence as a result of fish and invertebrates that usually hide in a coral field and eat algae and seagrass that cover the surrounding ocean floor. The fear that predators keep these smaller animals close to safety has long been considered the reason why the cleared areas are circular. Madin's recent work reveals that history contains more, and moreover, these features can be helpful in observing aspects of ecosystem health from reefs in outer space.

In a new study by Madin, published in the Proceedings of the Royal Society B her team of scientists determined that marine protected areas without traffic, in which fishing is prohibited, these vegetation patterns in the marine landscape in coral reef ecosystems dramatically shape and influence the appearance of the prominent halo pattern. This means that marine protected areas may have even greater impact on the coral reef's seascapes than previously known.

  Can we solve the riddle of coral reef halos?
Heron Island (dark green oval in the upper left corner) and lagoon, Australia. Picture credits: DigitalGlobe.

The team hypothesized that if "the formation of halos is driven by the fear of the small fish before eating fish, the number of predators is related to whether these bare spots appear and how large they are" Madin article recently published in New Scientist . "With fewer predators, one would expect the grazing fish to be less anxious and venture farther from the reef, resulting in wider halos."

But Madin and her team did not see much of a surprise when they used free space satellite imagery of the halos inside versus the non-fishing reserves. However, they found that the halos occur significantly more frequently in marine protected areas without carryover, demonstrating novel landscape effects of marine protected areas.

In the second study, published in Frontiers Madin's colleagues found that a more complex set of species interactions than previously thought likely affect these halos. Using a combination of high-resolution video cameras for remote cameras and traditional coral reef environmental studies in the Australian Great Barrier Reef, Madin observed that in addition to the herbivorous fish known to play a role in halo formation, invertebrate eaters also play the role of digging in the sand for prey, the algae destroyed the edges of the halos and made them bigger. Another part of the puzzle had been revealed.

In summary, Madin's work shows that the presence of halos can serve as an indicator of aspects of the health of the reef ecosystem, as halos are thought to be an indirect result of a healthy predator and herbivore population. Madin's ongoing studies on halos have shown that over time they can appear and disappear and change significantly in size, suggesting that environmental factors also affect halos.

"We urgently need more cost and time efficient monitoring methods for such reefs," Madin said. "Our work combines free-floating satellite images with traditional field-based experiments and observations to begin to unravel the mystery of what global patterns of" halos "around coral reefs can explain how reef ecosystems can change space and / or time Fisheries or marine protected areas, which will pave the way for the development of a novel, technology-based solution for monitoring large areas of the coral reef, and will enable the management of healthy reef ecosystems and sustainable fisheries. "


Ocean currents bring good news for reef fish


Further information:
Marine Protected Areas Shape Seascapes on Scales Visible from Space, Royal Society B rspb.royalsocietypublishing.or … .1098 / rspb.2019.0053

Provided by
University of Hawaii in Manoa




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Can we solve the puzzle of coral reef halos? (2019, April 24)
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