A marine sanctuary in the Gulf of Mexico off Texas is teeming with baby manta rays, a rare discovery that scientists have now described in a new study published in Marine Biology.
Giant oceanic manta rays (Mobula birostris), which can grow to reach a wingspan of up to 7 meters (23 feet) as adults and feed mostly on tiny plankton, are a spectacular sight to behold in subtropical and tropical waters around the world. Baby mantas, however, are rarely seen in the open ocean.
So when Joshua Stewart, a doctoral student at the Scripps Institution of Oceanography at the University of California, San Diego, saw several baby and juvenile mantas at the Flower Garden Banks National Marine Sanctuary off the Texas coast, he was surprised. Until then, Stewart had encountered hundreds of adult mantas in the wild, but not that many juveniles at one location.
After researchers who dove regularly at the marine sanctuary told Steward that baby and juvenile mantas were a common sight there, Stewart’s team combed through 25 years of dive data from the sanctuary, including photographs of manta rays. They found that around 95 percent of the mantas in the sanctuary measured an average 2.25 meters (7.38 feet) in wingspan, making them much smaller than known sizes of adult male mantas.
The researchers also found that the juveniles tended use the sanctuary area for extended periods, and were more commonly encountered there than in other areas. These findings confirm that the sanctuary is a nursery ground for giant manta rays, the researchers say — the first of its kind to be described formally.
“The juvenile life stage for oceanic mantas has been a bit of a black box for us, since we’re so rarely able to observe them,” Stewart said in a statement. “Identifying this area as a nursery highlights its importance for conservation and management, but it also gives us the opportunity to focus on the juveniles and learn about them. This discovery is a major advancement in our understanding of the species and the importance of different habitats throughout their lives.”
Why so many baby mantas seem to be aggregating at the Flower Garden Banks National Marine Sanctuary is not yet clear. But the researchers speculate that the shallow reef habitat of the sanctuary could provide the juvenile mantas protection from predators in the open waters, while also giving them a place to recover from deeper foraging dives.
“This is exciting news for the manta rays in the northwestern Gulf of Mexico,” said George P. Schmahl, superintendent of Flower Garden Banks National Marine Sanctuary. “The understanding that the mantas are utilizing the Flower Garden Banks National Marine Sanctuary, and possibly other reefs and banks in the region, as a nursery has increased the value of this habitat for their existence.”
Stewart, J. D., Nuttall, M., Hickerson, E. L., & Johnston, M. A. (2018). Important juvenile manta ray habitat at Flower Garden Banks National Marine Sanctuary in the northwestern Gulf of Mexico. Marine Biology, 165(7), 111.
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