Rays Don't Stray: Giant Mantas Stick Close to Home
Rays Don't Stray: Giant Mantas Stick Close to Home
Scientists have found that oceanic manta rays (Manta birostris) usually live their lives in distinct, local subpopulations, changing how conservationists approach protecting the mysterious fish species.
It turns out that humans aren’t the only species weary of long commutes: A new study reveals that manta rays living in the open ocean prefer staying close to home, rather than migrating over long distances.
The findings, based on years of tracking data, tissue samples, and genetic tests, are the latest to overturn long-held ideas about how the giant, mysterious fish eke out a living—and how they should be protected from overfishing.
Bigger than their reef-dwelling relatives, oceanic manta rays (Manta birostris) grow up to 23 feet (7 meters) wide and weigh up to 4,440 pounds (2 metric tons). They filter their food out of the water, snacking on plankton, fish eggs, krill, and occasionally small fish.
“We sort of assumed that they were behaving the same way that other large pelagic species do,” says lead author Josh Stewart, a Ph.D. student at the Scripps Institution of Oceanography in San Diego. (Stewart's work was supported in part by the National Geographic Society/Waitt Grants Program.)
But when Stewart and his colleagues used satellite tags to track 18 manta rays at sites in Mexico and Indonesia for up to six months at a time, they found that manta rays were far from long-distance travelers.
On the contrary, they seemed to prefer a short commute.
The tracking data, published Monday in Biological Conservation, indicated that 95 percent of the time, the manta rays at each site stayed in patches of ocean as small as 140 miles (220 kilometers) across and rarely if ever journeyed outside of them.
In Mexico, for instance, mantas tagged near the Revillagigedo Islands—some 373 miles (600 kilometers) off the country’s Pacific coast—never ventured to the coast, and vice versa.
And when the researchers analyzed tiny muscle samples taken from each tagged manta, they found that the mantas in each location had their own genetic and dietary quirks—shooting down the idea that they regularly traveled and mixed with other populations.
“The results are surprising, especially when considering the mantas off Mexico [and] that the overlap between populations isn't greater,” says Lydie Couturier, a manta expert at France’s Institut Universitaire Européen de la Mer who wasn’t involved with the study.
Couturier and Stewart say that the findings have major conservation implications for the species, which is currently listed as vulnerable on the IUCN Red List. Manta rays are frequently caught as bycatch and are hunted for their gill plates, a popular ingredient in traditional Chinese medicine.
“If you had a fishery that was drawing from the entire population of Indo-Pacific mantas, then [killing] 10 [to] a hundred mantas a year wouldn’t be a huge number, necessarily,” says Stewart. “But if there are these very local, isolated subpopulations, then you’re talking about removing half of the population in a year.”
Paradoxically, that vulnerability may also bring benefits, by intensifying pressure on regional and local governments to conserve mantas on their own.
Currently, oceanic mantas are protected mainly by two international agreements: CITIES, which forbids the international trade of wild manta-based products, and the Convention on Migratory Species, which provides a framework for international agreements on manta conservation.
Both treaties have had their successes, says Stewart, but they are difficult to enforce because of the large numbers of countries involved. Local and regional agreements, however, have fewer stakeholders—speeding up the adoption of conservation protections.
“If we can see that there are discrete subpopulations within Mexican waters, that would enable Mexico as one country to protect breeding, sustainable pockets of these animals themselves,” says Guy Stevens, the founder and chief executive of the conservation nonprofit Manta Trust, which supported the research. “That doesn’t require any international agreement. They can get on with it [and] protect these animals.”
This kind of local-first strategy already has borne fruit. In 2013, conservation groups, including the Manta Trust, worked with the local government of Raja Ampat, an archipelago in northeast Indonesia,to create Indonesia’s first shark and manta ray sanctuary.
But more work remains, especially to figure out how manta rays survive by staying in one place while their similarly filter-feeding peers opt for wider travels.
Another Stewart-authored study, published in May in Zoology, suggests that manta rays instead travel vertically, swimming into deeper waters periodically to make their diets more varied. That hunch won’t be confirmed until researchers have video of the mantas’ feeding behaviors—a project National Geographic is currently supporting with its Crittercam initiative.
Study Finds Manta Rays Are Local Commuters; Not Long-Distance Travelers
Scripps-led study has important implications for the threatened species’ conservation
Oceanic manta rays–often thought to take epic migrations–might actually be homebodies, according to a new study. A Scripps Institution of Oceanography at the University of California San Diego-led research team studied satellite-tracked manta rays to shed light on the lives of these mysterious ocean giants.
Manta rays (Manta birostris) spend much of their lives swimming in remote open-ocean environments, such as on seamounts and offshore islands, in search of tiny free-floating plankton, their main source of food. They can live for over 40 years and reach a wingspan of up to seven meters (23 feet).
The findings, published in the journal Biological Conservation, have important implications for the conservation of the threatened species.
To better understand their travels, the researchers tagged and collected muscle tissue samples from the rays at four different sites in the Indo-Pacific separated by 600-13,000 kilometers (373-8,078 miles), to see if the local aggregations of mantas were in fact a network of highly connected subpopulations.
Using the tagging information, which included up to six months of data on their movements, along with genetic and stable isotope analyses on the collected tissues, the researchers found that manta rays remained close to their tagged location, and are very likely distinct subpopulations with very limited connectivity between regions.
“These animals are showing a remarkable degree of residency behavior compared to the migrations we were expecting,” said Scripps Oceanography PhD candidate Joshua Stewart, a researcher in the Scripps Gulf of California Marine Program and the study’s lead author. “While mantas do make the occasional long-distance movement, it appears that the norm is to stay put. This means that any one population of mantas is highly susceptible to fisheries and other human impacts, but that local populations are also more easily protected.”
Populations of manta and closely related mobula rays are in decline worldwide due to targeted fishing mainly for their gill plates, which are used in traditional Chinese medicine, and from accidental bycatch in other fisheries.
Scientists had previously assumed manta rays to be long-distance travelers, similar to other large marine vertebrates such as sharks, tunas, and whales, largely based upon their size and pelagic habitat preference.
“We found that these patterns of residency remain true on multi-year and generational time scales, with both genetic and isotopic separation between populations,” said Stewart, also a researcher at the UK-based nonprofit The Manta Trust.
According to the authors, this study demonstrates that oceanic manta rays can be effectively protected by local and regional management strategies, which are often not considered viable for highly migratory species.
“The research we’ve conducted has shown that perhaps the most effective management strategies for oceanic manta rays will come from the local and national level,” said study co-author Calvin Beale of the Misool Manta Project.
The population of manta rays studied in Indonesia appears to reside exclusively in Indonesian waters, where there is a complete moratorium on the landing of manta rays, and local marine protected areas that cover a substantial portion of the population’s range.
“If more countries follow suit and protect their local manta populations, the outlook for the species may improve from the current downward trajectory,” said Beale.
In a separate study recently published in the journal Zoology, Stewart and his team analyzed the diving behaviors of six satellite-tagged oceanic manta rays at the Revillagigedo Archipelago in Mexico. They found seasonal shifts in diving behavior, likely the result of changes in the location and availability of their main prey source–zooplankton.
“This additional study helps explain why the mantas may remain resident, unlike most other large marine animals,” said Stewart. “Rather than move horizontally over long distances to track specific prey items, it seems that oceanic mantas are quite flexible in their foraging behavior, perhaps allowing them stay put rather than migrate.”
Stewart and colleagues at National Geographic Crittercam are conducting a follow-up study to affix cameras to the animals to directly observe their feeding behaviors.
The New England Aquarium’s Marine Conservation Action Fund, PADI Foundation, Save Our Seas Foundation, Misool Baseftin, Carl F. Bucherer, Conservation International, SEA Aquarium Singapore, The Punta Mita Foundation, National Geographic Society/Waitt Grants Program, and private donors provided funding for the research study. Stewart was supported by a National Science Foundation Graduate Research Fellowship, a Switzer Environmental Fellowship, and a Nancy Foster Scholarship through the NOAA Office of National Marine Sanctuaries.