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]
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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 called zooplankton, 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."
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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.
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Original article on Live Science.