Grace, size, and mystery: Foster Scholar Joshua Stewart’s work in Flower Garden Banks National Marine Sanctuary supports manta conservation
By Yaamini Venkataraman
“I was struck by grace, size, and mystery.”
This is how Dr. Nancy Foster Scholar Joshua Stewart describes his first time seeing the subject of his dissertation work, a manta ray. Diving near a shipwreck in the Dominican Republic, he was caught off guard when a manta ray shot up towards the surface. Stewart turned his video camera to the creature, and it mocked him with a belly flash and swam off. Stewart checked his camera. It wasn’t recording, and no one believed his story. But that experience kicked off his interest in this playful gentle giant, and inspired his research in Flower Garden Banks National Marine Sanctuary.
“I was just obsessed instantly,” Stewart says. “They’re one of the most interesting animals to spend time with in the water. There aren’t many huge animals that you can just hang out with, whereas mantas are these huge, playful, curious beasts that are totally gentle.”
Manta rays are large — their average wingspan is 22 feet — but their size does not protect them from human impacts. Mantas and their relatives, mobulas, can easily get caught as bycatch in fishing gear like purse seins, gill nets, and longlines. The recent creation of targeted fisheries for mantas and mobulas also raises management concerns. But to protect them effectively, all countries along the species’ migration route have to agree on a management plan, which can be difficult. Currently, there is no concrete idea of where mantas go, who should manage them, and how.
That’s where Stewart and his work in Flower Garden Banks National Marine Sanctuary come in. As a Ph.D. student at Scripps Institution of Oceanography at the University of California, San Diego, he is working to understand the migratory behavior and population structure of these animals. Satellite tags allow him to see how far mantas move and where their home ranges are. He then uses stable isotope and genetic analyses to understand how different populations are connected to each other. By linking population structure with migration patterns, Stewart can identify important manta habitats and better inform management.
“If you have mantas swimming through a huge fishery region like Indonesia, will they swim over to Mexico or Hawai‘i? Are the foreign fisheries impacting the same populations?” Stewart asks. “One of the interesting things we found is that unlike the vast majority of other large marine vertebrates, mantas are not moving around that much. They have these isolated subpopulations that do not have extremely large home ranges or migratory routes. They tend to actually stay pretty close to home.” Based on Stewart’s findings, countries have the ability to protect their own subpopulation within their regional waters rather than relying only on international agreements to manage populations.
One important habitat Stewart is studying is Flower Garden Banks National Marine Sanctuary, approximately 115 miles due south of the Texas and Louisiana border in the Gulf of Mexico. The sanctuary is one of the only locations in the continental United States with resident manta populations. More importantly, it is the only place in the world where juvenile oceanic mantas have been regularly seen in the wild.
While fisheries sometimes catch juvenile mantas as bycatch, researchers know very little about nursery habitats worldwide. Like most sharks and rays, juvenile mantas are born live and are fully independent after birth. Seeing pregnant oceanic mantas is a rare occurrence, and newborns are even rarer. Reviewing data collected at the Flower Garden Banks over the past two decades and building on research conducted by Jeffrey N. Childs (2001), Stewart found that 95 percent of the mantas observed at the sanctuary are small individuals. He believes these are most likely juvenile mantas.
“Having a potential nursery habitat for this threatened species is a pretty important thing,” Stewart says. By confirming that Flower Garden Banks as an important manta habitat, he can then evaluate how that protected habitat helps manta conservation, and whether more areas should be considered to support protection of the species.
Stewart shares his findings as an associate director of The Manta Trust, an international collective of researchers, policy experts, photographers, and videographers seeking to improve manta and mobula conservation globally. Formed when the impact of growing manta fisheries was unclear, the group sought to fill knowledge gaps in manta research. Members work in several regions globally with either large wild populations or manta fisheries, making it easier for researchers to collaborate and address the most pressing conservation questions.
“The collaborative spirit helps drive everything forward,” Stewart says. “If we were all working independently, we would not have nearly the same impact.” The organization got mantas listed on the Convention on International Trade of Endangered Species and on the Convention on Migratory Species. This means mantas cannot be traded internationally, and countries that are a part of the convention have to implement and enforce laws that protect the animals.
For Stewart, spreading a love of mantas doesn’t end in the conservation world. Stewart tries to bridge the gap between science and the general public through photography. As an undergraduate, Stewart was an active underwater photographer and filmmaker, with some of his work featured on Animal Planet and National Geographic.
“Having beautiful images of either ecosystems or animals stops people in their tracks — that’s the hook,” Stewart says. “Once people are captivated by the subject, they are willing to read a little into the details.”
The mystery and magic of mantas may have sparked an interest in the animal itself, but it’s the collective efforts of the research and conservation community that motivate Stewart to pour energy into his research, nonprofit, and photography.
“We as humans have had an impact on something so ancient and epic,” Stewart says. “We have had a huge impact on [manta] populations, and now we are working to preserve and boost population recovery for these animals. I find that inspiring.”
Editor's note: Some changes have been made to this article to correct an oversight in mentioning Jeffrey N. Childs' earlier work: "Spatial and temporal resource use of the Flower Garden Banks by charismatic megafauna" (1996, pages 74-79); "Sharks and rays of Stetson and the Flower Garden Banks" (2000, pages 183-193); and "The occurrence, habitat use, and behavior of sharks and rays associating with topographic highs in the Northwestern Gulf of Mexico" (2001).
Yaamini Venkataraman is a volunteer social media intern at the NOAA Office of National Marine Sanctuaries and a graduate student at the University of Washington.