Editor’s Note, June 26, 2019: A previous version of this article incorrectly stated that the Flower Garden Banks National Marine Sanctuary manta ray nursery was first described in a paper by Josh Stewart et al. of the Scripps Institution of Oceanography, when, in fact it was first described by Jeffrey Childs in his master thesis for Texas A&M titled ‘The occurrence, habitat use, and behavior of sharks and rays associating with topographic highs in the Northwestern Gulf of Mexico.’ The story has been edited to correct that fact.
You should consider yourself lucky if you ever come across the giant manta ray (Manta birostris), the crazy wing-shaped creature that can measure up to 29 feet across. Even more fortunate? Finding a juvenile manta ray, which are downright rare in most parts of the ocean. But that’s exactly what marine biologist Josh Stewart found while diving in Flower Garden Banks National Marine Sanctuary off the coast of Texas and Louisiana, which led him to the manta ray nursery, reports Alejandra Borunda at National Geographic.
Stewart, a graduate student at the Scripps Institution of Oceanography who has studied mantas around the world, had only seen one or two juveniles in his studies. So he was thrilled when, in 2016, he came across several juveniles while diving in the sanctuary in 2016. Local researchers told him they saw young rays all the time, which led Stewart to investigate.
According to a press release, he and his fellow researchers combed through 25 years of dive logs and photo records from Flower Garden Banks. What the data showed was that a remarkable 95 percent of the manta rays viewed in the Marine Sanctuary were juveniles, averaging just 7.38 feet across (compared to an average adult wingspan of around 22 feet). The research appears in the journal Marine Biology.
“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," says Stewart, who also serves as executive director of the Manta Trust, in the release. “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.”
Mantas worldwide are listed as vulnerable, which is the ranking just below endangered, by the IUCN Red List of Threatened Species. The animals typically found out in the open ocean in tropical and subtropical waters, occasionally congregating in sites far from the coast where they gorge on deep-water zooplankton. That has made tracing them back to their nursery grounds, usually along the coast or in reefs, difficult.
Flower Garden Banks may be attractive to the juvenile mantas because it's an area where the sea floor slopes into deep water. “We think they may be feeding on specific types of zooplankton there, then migrating up toward the surface, where we saw them,” Stewart tells Gary Robbins at The San Diego Union-Tribune. “They might be hanging around the banks because it could be a little safer than open water."
Borunda reports that protecting a manta ray nursery site is critical for their survival. While population numbers for giant manta rays are hard to come by, a related species, the reef manta, which until a decade ago was seen as the same species, has decreased by 90 percent in the last decade. (Other recent research shows that there may even be a third species of manta ray along the Yucatan Peninsula.) Their gill plates are used in Chinese medicine, which has led to a sharp increase in illegal fishing of rays. The animals are slow growing and can live up to 40 years, and don’t reach breeding age until around 8, meaning recovering the population will take time.
All this makes protecting nurseries even more important. Luckily, Flower Garden Banks is currently looking at expansion and hopes to add additional reefs and banks in the Gulf of Mexico to its 56 square miles, which were first protected in 1992.