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Rays Don't Stray: Giant Mantas Stick Close to Home

Until recently, manta rays — which sail through tropical and temperate ocean waters, looking much like enormous kites — were thought to migrate great distances across ocean basins, as do many of the largest marine animals.

But a new study finds that these big fish have a much smaller range than scientists had thought.

Researchers investigated data gathered from tracking devices on the manta rays, as well as chemical and DNA analysis of the rays' muscle tissues. The scientists were surprised to find that these giants of the deep are not long-distance seasonal commuters at all. Rather, they spend their lives in much more localized areas, the researchers found. The discovery radically changes scientists' understanding of mantas' habits and carries dramatic implications for their conservation. [Watch 'Homebody' Manta Rays Get Tagged]

With a "wingspan" that can extend more than 23 feet (7 meters), mantas are the largest rays and one of the ocean's biggest fishes. But tracking even very large animals in the open ocean can be extremely difficult, and mantas have always been especially so, according to lead study author Josh Stewart, a graduate student at the Scripps Institution of Oceanography in San Diego.

"They live in hard-to-reach places — and in a lot of these places, it's challenging to find them consistently. So for a long time, no one was tagging them," Stewart told Live Science.

Stewart, who is also the associate director of the nonprofit conservation organization Manta Trust, explained that individual mantas can be identified by unique patterns of spots on their bellies; photos of mantas captured by researchers, dive tours and citizen scientists were used totrack mantas over time.

But sometimes, nearly two decades would elapse between sightings, Stewart said. And in some locations, researchers would see the mantas for a few weeks or months, but they wouldn't find any at all for the rest of the year. And because mantas are so big, it was thought that they were simply doing what large migratory ocean creatures such as whales,leatherback turtles and bluefin tuna do — following their food.

"If you look at every other big animal that lives in remote pelagic [open ocean] environments, they're making long, epic migrations," Stewart said. "So we thought the mantas were migratory, too. They're certainly big enough and capable enough."

The researchers set out to tag and sample manta-ray populations at four sites that were up to 8,000 miles (13,000 kilometers) apart, to find out how far the rays traveled.

Tagging technology has been used by oceanographers for more than two decades, but recent innovations have made devices much more robust and reliable, with a recovery rate of 80 to 90 percent, Stewart said.

The tags were programmed to detach after six months and then float to the ocean surface, where scientists could retrieve them.

In the very first batch they collected, Stewart and his colleagues noticed something unexpected: The tags popped off within about 62 miles (100 km) from where they were originally attached, and when the scientists mapped the mantas' movements over months, they found that the tags remained in largely the same area.

Stewart said their initial reaction was, "Well, that's interesting," though they needed to collect more data to be sure. But every tag they deployed after that returned the same results over a six-month period. And their genetic analysis confirmed that mantas in the different sample sites were not, in fact, the same individuals traveling from place to place, but rather established groups that staked out their ranges and stayed put. [Marine Marvels: Spectacular Photos of Sea Creatures]

So why don't mantas seasonally roam the oceans as other massive predators do? Greater flexibility in their diet might be the answer, Stewart suggested.

"The tags also record where in the water column they are," he said. "Some months, they were close to the surface, and some months, they were much deeper, which correlates to where we think different types of food may have been available."

Mantas were known to feed primarily on tiny marine organisms calledzooplankton, filtering them from seawater with specialized gill plates, but tissue analysis of the rays revealed that their diets are broader than scientists had expected.

"They can feed on everything from really tiny copepods that you can barely see to big shrimp, and even fishes," Stewart said. "We think they're able to shift what they're feeding on at different times of the year, which may allow them to stay put and not migrate."

Recognizing that mantas are local and affected by smaller groups of people could shift conservation efforts to local communities — which tend to be more effective, Stewart said.

On the other hand, he added, mantas that don't stray as far are more likely to be negatively affected by activities from local fisheries and poaching for the illegal wildlife trade.

"It's a double-edged sword," Stewart told Live Science. "It's good in terms of facilitating management. But it also means we have to act much more quickly, because these populations are more vulnerable due to their restricted ranges."

The findings were published online today (June 20) in the journal Biological Conservation.

Original article on Live Science.


Manta Rays Prefer Staycations Over Long Migrations 

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.

  • Scientists had long thought that oceanic manta rays migrated thousands of miles around the world to follow shifts in the distribution of their food, similar to the movements of other pelagic, or open-ocean, filter feeders such as baleen whales and whale sharks. Previous studies had even documented individual manta rays traveling hundreds of miles at a time.

“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.

Protecting the Gentle Giants

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.

http://news.nationalgeographic.com/2016/06/mantas-rays-migrations-conservation-oceans/

 

 


Study Finds Manta Rays Are Local Commuters; Not Long-Distance Travelers

Scripps-led study has important implications for the threatened species’ conservation

 Media Contact: Mario Aguilera  |  Phone: 858-534-3624  |  Email: This email address is being protected from spambots. You need JavaScript enabled to view it.

 

A giant manta ray off Revillagigedo Archipelago in Baja California, Mexico. Credit: Josh Stewart

 

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.