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Indonesian researchers study how to help rays released as bycatch survive

  • Researchers in Indonesia are studying the survival rate of manta rays and devil rays released after being caught unintentionally by fishers.
  • The study, which has so far tagged five of the animals with satellite trackers, aims to come up with best practices to boost the survival of these threatened rays.
  • Populations of mantas and devils rays, from the genus Mobula, have been hit by the global trade of their parts, particularly their gills, for traditional medicine and food.

JAKARTA — Researchers in Indonesia are working to identify the best practices for safely releasing threatened manta rays and devil rays that are caught alive unintentionally by fishers.

Populations of these large rays from the genus Mobula have been devastated in recent years by targeted and bycatch fisheries in Indonesia amid growing local demand for their meat and international demand for parts like gills for traditional Chinese medicine. The significant population decline prompted the government in 2014 to ban the intentional capture and trade of mantas, but not of devil rays.

“The question now is that when a ray is caught alive and the conservation effort is to release it back, what is actually the survival rate when it’s released back into the ocean?” Sila Kartika Sari, a marine researcher with the conservation collective Mobula Project Indonesia, told Mongabay recently.

A Japanese devil ray (Mobula japanica) on the auction floor at Tanjung Luar Port in Lombok, Indonesia. Tourists here often mistakenly identify the species as a reef manta ray (Mobula alfredi). Image by Melati Kaye.

The group has generally been studying Mobula in Indonesian waters since 2015. Last November, Sila’s team began tagging rays that were caught alive and released under certain methods to learn how they would fare over the next 60 days.

“This is the first study of its kind in Indonesia,” she said.

The trackers they attach to the rays record a range of variables, though SIla noted that factors such as the type of net they’re initially caught in to the kind of handling they experience prior to their release also influence their prospects of survival.

While the released rays are tracked over the course of 60 days, Sila and her team consider it a successful survival if a ray stays alive a month after its release.

The researchers also interview local fishers to understand the socioeconomic aspects of Mobula ray fisheries. Among the fishers who catch these rays, the rays account for less than 3% of their total catch by volume — but almost half of their income, dwarfing what the fishers can earn from other commercially valuable species such as snappers, groupers and mackerel tuna. Mobula rays have another economic impact on fishers: the process of releasing them often destroys the fishing net, Sila said.

“So we’re also looking at the fisheries aspect of this: what is the best policy from the government to encourage fishers to do this practice of releasing rays caught alive in their net,” she said.

Sila added the study has so far tagged five rays out of the 10 that the researchers hoped to study from two sites. Those sites, the waters off East Java province and the island of Flores, are some of the hotspots for Mobula ray fisheries in Indonesia. Sila said she had hoped to include other hotspots, such as the waters off Lombok Island and off Makassar on the island of Sulawesi, for the ongoing research, but those plans were curtailed by the COVID-19 pandemic and not enough people to do the tricky sampling.

All mantas and most devil rays are listed as threatened on the IUCN Red List, and their international trade is subject to strict regulation under CITES to ensure the survival of their wild populations.

A reef manta ray in Flores, Indonesia. Image by Herry Jeremias/Mongabay Indonesia.

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