Manta rays, abundant around Raja Ampat, eastern Indonesia, were listed last year as "threatened" under the International Convention on the Conservation of Migratory Species in 2011, NBC News reported.
Scientists say mantas are being caught as bycatch, getting caught in industrial fishing nets targeting different types of tuna and, increasingly, for their gill rakers -- which allow them to filter food from water, and are used in traditional Chinese medicine.
A report called Manta Ray of Hope found an estimated 3,400 manta rays and 94,000 mobulas, which are related to the manta ray, are caught each year, but the numbers reflect only reported catches.
"Unreported and subsistence fisheries will mean true landings are much higher," the report said.
Scientists in China are working to have manta rays protected by the government.
"In the last two years, we have conducted evaluations of the manta ray and submitted a recommendation to the government to list it as a protected species," said Professor Wang Yanmin from the Chinese Shandong University's Marine College.
Feng Yongfeng, founder of Green Beagle, a group that promotes environmental protection, said, "There is no regulation for protecting the manta ray so sales of mantas are not illegal."
"They're such an iconic species, beloved by divers," said Andrea Marshall, director of the Marine Megafauna Foundation. "They're just amazing."