Marine biologist Josh Stewart was floating underwater, looking up at the manta ray that materialized out of the blue above him, when he did a double take: The animal was a juvenile, only a few feet across—nowhere near as big as a mature giant manta ray, which can be as wide across as a giraffe is tall.
Stewart, a graduate student at the Scripps Institution of Oceanography, had studied mantas for years, diving with them all over the world, but in all his time in the oceans he had only seen one or two other young mantas.
When he got back to the surface, he shared his excitement with the researchers who regularly dove and surveyed marine life at this reef in the Flower Garden Banks National Marine Sanctuary, off Texas and Louisiana.
But they were unfazed—they saw juveniles all the time. Even so, Stewart knew it wasn't normal to see so many baby rays. When he dug through 25 years of carefully collected dive data from the site, his hunch was confirmed: This was a nursery ground for giant manta rays. (Read why manta rays prefer “staycations.”)
In a study published today in Marine Biology, Stewart and his colleagues describe a nursery brimming with young mantas, from newborns through adolescents—a safe space for the young rays to grow and develop.
“It's really important for us to know where these nursery sites are,” says Andrea Marshall, a ray expert at the Mozambique-based Marine Megafauana Foundation and a National Geographic Explorer. “Anywhere that has tiny mantas is really important for us to learn about, so we can target our protection strategies.”
Manta Chill Zone
Giant oceanic mantas take their time to grow up. Females, which can live to 40 years old, don't generally have their first babies until they're eight to 10 years old.
So every teenage ray loitering around the Flower Garden Banks nursery zone is valuable—a product of time, effort, and energy from its mother.
They’re also valuable for the continuation of their species, which is listed as threatened under the U.S. Endangered Species Act. (Learn how to put a camera on a manta ray.)
Total population estimates are few and far between—especially for the mysterious giant manta ray. But sightings of closely related reef mantas have dropped by 90 percent over the past decade in many parts of the world.
Fishing is the primary threat to mantas, which are sought after for their gill plates, the soft, fine sieves through which they filter their food from seawater.
Mantas are also “incredibly talented at getting caught in fishing gear,” explains Stewart, so the graceful creatures often end up as bycatch, their wide wings tangled in purse seines or trawl nets.
Researchers don’t yet know why, exactly, the Flower Garden Banks nursery are so attractive to young mantas, but they want to understand what makes the vibrant, shallow offshore reef habitat such a good place to grow up.
“These are the areas that hold the animals in their most delicate life phase,” says Giuseppe Nortarbartolo di Scioria, a marine conservation ecologist at the Tethys Institute, based in Italy. He hopes that, by understanding this nursery site, researchers can figure out how to find and protect other important nurseries around the world.
Stweart agrees. “The more we understand why these areas are important,” he says, “the better we can tailor [conservation strategies] to them—” and help ensure the young rays living in these nurseries survive long enough to have their own pups.