In five years diving to observe manta rays, Joshua Stewart had only encountered adults – until 2016 at the Flower Garden Banks National Marine Sanctuary in the Gulf of Mexico. That’s where he spotted several juveniles, a rare sight in nature, due to the declining numbers of these gentle giants and their wide global distribution.
His sighting of the juvenile mantas turned out to be a big clue to the ecological role of the area. After reviewing 25 years of dive logs and photos, Stewart, a doctoral candidate at the Scripps Institution of Oceanography, realized about 95 percent of manta rays found in Flower Gardens Bank were likely young ones. In a recent paper, he suggested the area is the world’s first confirmed manta ray nursery.
That such a discovery was only recently made underscores how little is still known about manta rays even as they face growing conservation threats. Far off the coasts of Asia, they are caught and slaughtered en masse, their gill rakers, a feeding apparatus designed to filter plankton from seawater, extracted and sold as an ingredient for Chinese traditional medicine. Their bodies, which can grow up to 8.8m (29ft) wide, are easily entangled in fishing gear. In early 2018, the United States placed giant rays on the endangered species list.
As populations decline, scientists are working to better track their movements in hopes of designing more effective conservation plans. They also are trying to learn more about their behavior in the wild from the relatively few giant mantas in captivity.
“Tag data can show us what habitats mantas are using, which helps us define important regions for protection,” said Stewart, who is also associate director of Manta Trust, a conservation organization.
Giant manta rays, the world’s largest ocean rays and a species distinct from somewhat smaller (but still massive) reef rays, inhabit a wide belt of tropical and subtropical ocean waters across the equator, off every continent except Antarctica. They often congregate far offshore, out of sight of humans – and so very little is known about their life cycle, especially during their juvenile stage where they appear to live mostly in secluded nursery habitats. This stage is also crucial to the health of the overall population, Steward said, because mantas have few offspring and take years to reach sexual maturity.
As a next step in his work, Stewart is planning to track Flower Garden Banks’ juvenile mantas using GPS-enabled satellite tags. His goal is to understand how they use the nursery and where they go after they leave, which could give scientists insight into where else to look for other nursery habitats around the world.
Other scientists are looking to mantas released from captivity to learn more about their behaviors. At Florida’s Mote Marine Laboratory, researchers have worked with the hotel Atlantis Paradise Island resort in the Bahamas, one of just two facilities in the Western Hemisphere to house the giant manta rays. Recently, the hotel announced the release of the program’s 13th manta, “Leyley” and the sixth to be tagged by scientists.
Robert Hueter, senior scientist at Mote, led a medical evaluation of Leyley before he inserted two tiny satellite tags just beneath the skin on her back that would later transmit data on the manta’s movements, according to Mote Marine Laboratory. One tag gathers data on water depth and temperature, as well as the manta ray’s location, and sends data when the animal breaks the water’s surface. The other tag collects data until a set date when the tag detaches and floats to the surface. So far, Leyley’s location tag has not set off any pings, but when they do, her movements will be viewed online.
Georgia Aquarium is the only other facility in the Western Hemisphere to house captive giant mantas; however, it does not currently tag and release them. But Harry Webb, its project coordinator of research and conservation, works with the Florida Fish and Wildlife Commission and Marine Megafauna Foundation to tag and track wild mantas.
“We’ve studied the seasonal migration of mantas along the northeast Florida coast for over eight years and have found that almost every spring there’s a huge migration of mantas along the near-shore area off of the Atlantic coast,” said Webb. “What we want to uncover is a bit about their natural history. We want to observe them and gain insight into why they’re here with such regularity and what makes this area so special.”
Webb said the Georgia Aquarium, which keeps the animals for educational and observational research purposes, is working to overcome common challenges to keeping mantas in captivity: Making sure these enormous animals get enough food and training them to safely receive routine veterinary checkups and care so that they can thrive outside of their wild habitats. In captivity, scientists take blood samples and perform DNA tests to learn more about manta ray biology and explore the potential for captive breeding.
Andrea Marshall, co-founder and principal scientist at the Marine Megafauna Foundation, which works with the Georgia Aquarium and other facilities that keep captive mantas, said she does not think manta rays – highly nomadic, intelligent and social animals – can thrive in captivity.
Yet, she said that tagging could be extremely useful in helping scientists not only understand the lives of wild manta rays, but also how well captive mantas assimilate with their wild counterparts upon release. She said the goal of any kind of captive manta ray research should be to protect manta rays in nature.
“We collaborate with many aquariums around the world, some of which do have manta rays in captivity,” said Marshall. “[We] try get as much science as we can from the animals that happen to be in captivity and apply these findings to protecting them in the wild.”