A Ray of Hope for Mantas: Marine biologist makes case for protecting ‘devilfish’
This is the final installment in a four-part series on exhibits, public programs, lectures and scientific research in connection with “Shark Summer” at the Birch Aquarium at Scripps. The spotlight this time is on research and conservation efforts of manta rays, cousins of the shark.
By Lynne FriedmannOn his first day of college, Josh Stewart made a momentous decision. “I dropped a scheduled course and enrolled in a scuba diving class instead,” he said.
Stewart was an undergraduate at Indiana University, a land-locked school without a marine biology program.
But what Indiana University does have is one of the oldest academic diving programs in the country where Stewart found unparalleled opportunities for hands-on science experience accompanying underwater archaeologists studying shipwrecks in international waters. During a research excursion to the Dominican Republic, Stewart first glimpsed a giant manta ray — a disc-shaped fish more than 20 feet wide — gliding toward him like a slow-motion, underwater bat. This and subsequent encounters with the majestic creatures set the course for Stewart’s graduate studies.
Sometimes called “devilfish,” manta rays lack the barb of the stringray and are harmless to humans.
“They are filter feeders of zooplankton and copepods,” said Stewart.
Now a Ph.D. student at the Scripps Institution of Oceanography, Stewart said he is interested in manta ray population structure, habitat use, and its geographic range throughout the Pacific and Indian Oceans. His research methods include satellite telemetry as well as genetic analysis of tissue samples collected from animals in oceanic “hotspots” where large numbers of manta rays aggregate.
Questions for study“What we want to know is: Are these distinct populations, or is there one global, meta-population,” Stewart said.
The answer is critical to developing conservation recommendations and informing policy decisions to protect the species. Classified as vulnerable to extinction in the wild, manta rays are not captured for their meat (deemed of poor quality) but for their gill rakers: tight-knit, filament structures that mantas use to filter plankton from the water column. Some Asian cultures believe that dried gill rakers have medicinal properties.
“But this is not traditional Asian medicine like the use of rhino horn or tiger parts,” said Stewart. “The market for gill rakers has arisen in just the past decade.”
This new and growing market is possibly due to a decline in shark populations harvested for their fins, according to Stewart. Regardless of the motivation, the exploitation of manta rays is unsustainable as a fishery with several factors impacting a depleted population’s ability to recovery. These include the species’ long life, late sexual maturity (5 to 10 years of age), and a low reproductive rate of one pup, on average, every two to three years.
Based on Stewart’s field observations over several years, he hypothesizes that manta rays belong to regional subpopulations throughout the Indo-Pacific. This is good news since smaller, regional subpopulations would be easier to manage and protect than a global meta-population.
Swimming with mantas
“Swimming with mantas is at the top of everybody’s dive list,” said Stewart.
In May, a first-of-its kind study (
https://bit. ly/11cJgnl) was released that estimates the direct economic impact of manta-ray- watching tourism worldwide at $140 million annually. This far exceeds the estimated $5 million fisheries would derive if they harvested mantas for their gill rakers and depleted the stock in the process.
The report also highlights recent international treaties and laws that owe their adoption to an increased level of public awareness of manta rays as species of international conservation concern. Among the measures now in place is a binding treaty, adopted in March, by the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) that requires 178 nations and territories to demonstrate that any exports of manta rays or their parts have been obtained from legal and sustainable sources.
“These measures passed due to public support to protect mantas,” said Stewart. “It is unusual that public opinion mattered (in these deliberations).”
The Manta TrustTwo of the three authors of the study are members of The Manta Trust (
mantatrust.org), a non-profit organization, established in 2011, dedicated to worldwide conservation of manta rays and their habitat through research, awareness, and education to the general public and community stakeholders.
Stewart is a founding member of The Manta Trust and remains involved as an associate director. In August, he represented The Manta Trust during a special lecture titled “Manta Rays: Majestic and Threatened Icons,” at the Birch Aquarium at Scripps.
Stewart’s commitment to public outreach and education extends to film and television. An award-winning videographer, Stewart spent a year as a Rolex Scholar (only one is chosen from North America annually) traveling the world, working with leading filmmakers, biologists and conservationists on the most pressing marine conservation issues of our day.
Stewart has filmed for National Geographic and Animal Planet, as well as producing a number of independent documentaries and video shorts.
These are all examples of Stewart as a trailblazer, starting with becoming the first person to graduate from Indiana University with a self-directed degree in marine biology.
On the Web■ The Manta Trust: