Research raises possibility that Flower Garden Banks National Marine Sanctuary is part of manta ray nursery

By Elizabeth Weinberg

June 2018

Oceanic manta rays are graceful, almost magical creatures: these relatives of sharks swim through the water as if in flight, circling in search of tiny zooplankton. They can grow to be enormous, with some reaching 22 feet from fin-tip to fin-tip. Relatively little is known about them – including where they spend their early years. But Dr. Nancy Foster Scholar Joshua Stewart has an answer to the question of where some young mantas go: Flower Garden Banks National Marine Sanctuary.

underside of a manta ray
A manta ray swims in Flower Garden Banks National Marine Sanctuary. Unique markings on the undersides of manta rays – like fingerprints – help researchers identify individuals. Photo: G.P. Schmahl/NOAA

In a paper published in Marine Biology, Stewart and sanctuary researchers Marissa Nuttall, Emma Hickerson, and Dr. Michelle Johnston suggest that Flower Garden Banks National Marine Sanctuary and the area surrounding it may represent the first documented nursery habitat for oceanic manta rays (Manta birostris) and a potential new species of manta (Manta cf. birostris).

Manta rays are frequent visitors to the sanctuary, which protects three coral cap regions in the northwestern Gulf of Mexico, 75 to 115 miles off the coast of Texas and Louisiana. But the mantas seen there tend to be smaller than average, with a 14-foot wingspan at most, with many measuring less than six feet. Stewart was originally interested in whether these mantas were part of the same population often sighted in the Yucatan, off of Florida, and elsewhere in the Gulf of Mexico. The very first manta he saw in the sanctuary was a small juvenile male.

“I was incredibly surprised because I had only seen a juvenile manta once before, and they’re very rare in most field studies,” he says. “When we got up from the dive I was telling the team how excited I was to have seen a juvenile, which is when they told me that’s nothing new for the Flower Garden Banks!”

a diver photographs a manta ray
A researcher photographs a juvenile manta ray. Photo: G.P. Schmahl/NOAA

Manta ray central

Monitoring is a linchpin of efforts to protect the resources within Flower Garden Banks National Marine Sanctuary. By making regular trips out to the sanctuary and recording observations, sanctuary researchers establish a baseline of information that can help them detect changes in the environment. These data can also help researchers like Stewart better understand population dynamics.

For over 28 years, Flower Garden Banks National Marine Sanctuary scientists and research partners have recorded data on manta rays as part of regular monitoring expeditions. Sanctuary research specialist Marissa Nuttall has maintained a database of images of many of the mantas seen by research divers, logging the mantas’ sex, maturity status, and size, and noting when individual mantas have been seen multiple times. Based on those images and observations, the sanctuary research team realized that 95 percent of the manta rays spotted in the sanctuary are smaller than mature oceanic manta rays, and many have been spotted multiple times over the course of several years. Those observations and other data collected by sanctuary staff indicate that the sanctuary fulfills several criteria for elasmobranch (ray, skate, and shark) nursery habitats, suggesting that Flower Garden Banks National Marine Sanctuary and its surrounding areas are places young mantas regularly visit as they are growing up.

“This is exciting news for the manta rays in the northwestern Gulf of Mexico,” says Emma Hickerson, Flower Garden Banks National Marine Sanctuary research coordinator and a co-author on the paper. “Nowhere else in the world has a manta ray nursery area been recognized, which heightens our understanding of just how important the sanctuary is for these iconic species.”

A juvenile manta ray swims above coral cover in Flower Garden Banks National Marine Sanctuary. Its claspers identify it as a young male; scars also show evidence of past shark bites. Video courtesy of Joshua Stewart

What makes a nursery so special?

Oceanic manta rays were recently designated as threatened under the Endangered Species Act. However, much is still unknown about these rays, including where their critical habitats exist in U.S. waters, making it difficult to determine how best to protect them. Knowing the location of a nursery could help.

Though little is known about the life cycle of oceanic manta rays, their close cousins, reef manta rays (Manta alfredi), take years to mature. Males mature between ages three and six, while females take longer, maturing between ages eight and 10. It is likely that oceanic manta rays mature at similar times. Because of this late maturity, the juvenile stage is likely important for oceanic manta ray population growth.

With that in mind, it’s essential that juvenile manta rays can find safe habitat in which to grow and reach reproductive age – like Flower Garden Banks National Marine Sanctuary. Manta rays typically dive deep into cold, dark waters to forage on zooplankton, and then bask at the surface to warm themselves. But spending time at the ocean surface exposes them to predation by sharks and other predators. The relatively shallow reef habitats of the sanctuary may give them the access they need to warmer waters to rest in, while also providing opportunities to evade predators.

Plus, Flower Garden Banks National Marine Sanctuary may help juveniles avoid another threat. In the past decade, manta rays and closely-related mobula rays have been heavily impacted by global fisheries, which seek to fulfill demand for gill plates in Asian markets. Manta rays are also sometimes caught as bycatch. In sanctuary waters, though, juvenile manta rays are relatively safe from both these threats. “The sanctuary already protects incredible coral reef habitats,” says Stewart. Identifying that the sanctuary may be part of a nursery area for mantas emphasizes why it's so important to protect it.

An expansion of Flower Garden Banks National Marine Sanctuary could also be good news for mantas. “The sanctuary is actively pursuing an action to expand the area protected within the sanctuary, by adding additional reefs and banks located in the northwestern Gulf of Mexico,” explains sanctuary superintendent G.P. Schmahl. “Some of these areas are also known to provide habitat for the manta rays.”

Juvenile manta rays are often seen by researchers visiting the sanctuary. Video: Emma Hickerson/NOAA

Space for manta rays – and scientists – to grow

Stewart has something in common with manta rays: Flower Garden Banks National Marine Sanctuary has also been a place for him to grow, in this case as a scientist. Stewart is a recipient of the Dr. Nancy Foster Scholarship, which supports graduate students in oceanography, marine biology, and maritime archaeology who conduct research related to national marine sanctuaries. For Stewart, the scholarship has enabled him to get out to this offshore sanctuary and collaborate with Flower Garden Banks researchers to better understand manta populations. “But beyond the mantas,” he says, “it’s also given me the opportunity to embed with the sanctuary team so that I can learn about their research and conservation priorities, and make sure that the work I’m doing at the sanctuary is most relevant to their objectives.”

His work isn’t done yet: “Now we’re especially interested to find out how this population of juveniles relates to the adults seen elsewhere in the Gulf and Caribbean.” For the next few years, that will be the researchers' focus: when the mantas leave the sanctuary of Flower Garden Banks, where do they go?

Editor's note: Some changes have been made to this article to reflect clarifications that Flower Garden Banks National Marine Sanctuary may be part of a wider nursery area. In the paper that supported this article, "Important juvenile manta ray habitat at Flower Garden Banks National Marine Sanctuary in the northwest Gulf of Mexico," the authors state that "Temporal patterns of use and the prevalence of juveniles suggests that this region may serve as nursery habitat for M. birostris and M. cf. birostris." The research team "suggest that FGBNMS and the surrounding region may represent the first documented nursery habitat for M. birostris and M. cf. birostris."

Elizabeth Weinberg is the social media coordinator and editor/writer for NOAA’s Office of National Marine Sanctuaries.