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Guest post by Proyecto Manta: Manta Rays are not long-commuters

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Guest Post by Proyecto Manta.

mantaOceanic manta rays are one of the most iconic animals in the ocean. They can grow up to 7 meters across, live over 50 years, and are incredibly intelligent and curious. Because of these traits and their harmless nature, mantas are sought out by divers and snorkelers around the world. Bahía de Banderas is the second most popular destinations to dive and snorkel with oceanic manta rays, providing frequent and predictable encounters that make tourism operations surrounding the species possible.

Proyecto Manta is a multidisciplinary study which will examine the population structure of oceanic manta rays, enabling the assessment of fisheries impacts on manta populations and will identify habitats and regions that can be targeted for spatial protection to prevent severe population decline.

Until recently, scientists had long thought that oceanic manta rays migrated great distances across ocean basins to follow shifts in the distribution of their food, similar to the movements of other pelagic -open ocean- filter feeders such a baleen whales and whale shark do.

But a new study reveals that these big fish have a much smaller range. Manta rays prefer staying close to home, rather than migrating over long distances.

In order to find out this, researches tagged out (using tagging technology with a recovery rate of 80 to 90 percent) 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.

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

In the very first batch they collected, Stewart and his colleagues noticed 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 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 were, in fact, different individuals in every location.

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.

Why Mantas don’t migrate?

Mantas were known to feed primarily on tiny marine organisms called zooplankton filtering them from seawater with specialized gill plates but when the researchers analyzed tiny muscle samples, 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 tissue analysis revealed that mantas diets are broader than scientists had expected.

Stewart made a suggestion to this fact: greater flexibility in their diet.

The tags also record the mantas were some months close to the surface and some months, much deeper, this lead to believe they were searching for available food. “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.” Stewart said.

Conservation of the specie

Manta rays are currently listed as a vulnerable, they are frequently caught as bycatch and are hunted for their gill plates, a popular ingredient in traditional Chinese medicine.

It is true mantas by stay put are highly susceptible to fisheries and other human impacts, but it’s also more easily to protect. Recognizing mantas are local is easier to manage conservations programs by local and regional people.

“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.” Said Stewart

“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.

More work remains

In a separate study, 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. It suggests that manta rays instead travel vertically, swimming into deeper waters periodically to make their diets more varied. It will be confirmed until researchers have video of the mantas’ feeding behaviors (Junio 20 journal Zoology)

The study could help to explain why mantas remain resident. 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.

For more information about Proyecto Manta and their extraordinary effort on the protection and conservation of this species please visit their website.

Learning more about Manta Rays

Manta rays (Manta birostris) can live for over 40 years and reach a wingspan of up to seven meters (23 feet) wide and weigh up to 4,440 pounds (2 metric tons). They 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 filter their food out of the water, snacking on plankton, fish eggs, krill, and occasionally small fish.