The baby rays? Gliding at the offshore nursery
Juvenile mantas are found where sea floor enters deeper water.
SAN DIEGO — UC San Diego has discovered the world’s first known nursery for manta rays, an idyllic spot in the Gulf of Mexico where the “gentle giants” approach divers along colorful reefs that seem drawn from the imagination of Walt Disney.
An exploration team led by Josh Stewart, who is finishing his doctorate in marine biology at the university’s Scripps Institution of Oceanography, made the discovery and confirmed its importance over the last few years.
Stewart was aided by colleagues from National Oceanic and Atmospheric Administration who manage the site, which is part of the Flower Garden Banks National Marine Sanctuaries, about 70 miles south of Galveston, Texas.
Finding adult manta rays is easy; divers see them in many locations. But in a mystery that has long perplexed scientists, juveniles rarely swim openly in defined places during the four to five years it takes for them to become adults.
Stewart has been trying to solve the mystery for years, and had some initial success in 2016 when he saw a juvenile manta ray (Mobula birostris). He later saw more juvenile mantas and, with help from the NOAA researchers, determined that he’d happened upon a nursery. The discovery was published this week in the journal Marine Biology.
The juveniles were largely found in an area where the sea floor slopes into deeper water.
“We think they may be feeding on specific types of zooplankton there, then migrating up toward the surface, where we saw them,” Stewart said. “They might be hanging around the banks because it could be a little safer than open water.
“We’ve seen them so rarely [in the past] that we know very little about these juveniles. We don’t know how far they move, or exactly what they feed on, or all of the habitats these access.”
But Stewart can say with certainty that simply hanging in the water, waiting for manta rays to arrive, is a remarkable experience.
“You’re out there, doing your work, and you look up and there’s the distinctive shape of a manta,” Stewart said. “At first, you can barely see it. But then it comes into view and my heart skips a beat. [The adults] are gigantic. They can have a 21-foot wingspan. And they’re super-friendly. They’ll get really close to you.
“That makes them a lot different from other large marine species, who keep their distance. Mantas are curious. They’re interested in why divers are blowing bubbles. They’ll hang out. I can’t think of another large animal that will do that with you.”