Philippines · Global Manta Ray Program

Philippines Manta and Devil Rays

Four species of manta and devil rays landed together in the Bohol Sea fed almost entirely on krill, and reproduce so slowly that no level of fishing appeared sustainable.

Led by LAMAVE, with MMF

1,509rays examined to study their growth and reproduction

91%of stomachs contained the same species of krill

86.6 yearsfor the slowest-growing population to double

80%decline in oceanic manta ray sightings at two sites

Four species of manta and devil rays were once landed together at Jagna, on the island of Bohol. Working with LAMAVE (Large Marine Vertebrates Research Institute Philippines), we studied their diet and reproductive rates, and helped map the distribution of manta rays across the country.

The Bohol Sea is the main fishing ground for manta and devil rays in the Philippines, and Bunga Mar, in the town of Jagna, was its largest landing port. Fishers set drift gillnets at night, up to 60 km (37 miles) offshore, mostly over water more than 1,000 m (3,300 ft) deep. Between 2012 and 2016, LAMAVE researchers led by Joshua Rambahiniarison, together with MMF’s Dr. Chris Rohner, collected samples from rays landed at Bunga Mar, with the consent of the Local Government Unit of Jagna, the fishers and buyers, and with the regional offices of the Bureau of Fisheries and Aquatic Resources and the Department of Environment and Natural Resources. The targeted fishery was banned in April 2017, following the listing of manta and devil rays under CITES, the Convention on International Trade in Endangered Species of Wild Fauna and Flora.

What the project has done

Identified the diet of four species of manta and devil rays from the stomach contents of rays landed at Jagna, in a study led by Dr. Chris Rohner and published in Royal Society Open Science in 2017.
Co-authored a DNA study of the rays’ diet (Ecology and Evolution, 2019) and a stable-isotope study, which reads diet from chemical signatures in body tissue, of their longer-term diet (Marine Ecology Progress Series, 2017).
Co-authored a study of the growth and reproduction of four species in the Bohol Sea, from 1,509 rays, published in Frontiers in Marine Science in 2018.
Co-authored a national study of manta ray sightings across the Philippines from 2004 to 2020, published in the Journal of Fish Biology in 2022.

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Research findings

Four species in one catch

The rays landed at Jagna were oceanic manta rays (Mobula birostris), sicklefin devil rays (M. tarapacana), spinetail devil rays (M. mobular), and bentfin devil rays (M. thurstoni). Between February 2015 and May 2016, the team examined 1,509 of the more than 4,000 rays landed over two fishing seasons: 817 bentfin devil rays, 450 spinetail devil rays, 173 oceanic manta rays, and 69 sicklefin devil rays. (Photo: Duncan Murrell.)

Three devil rays swim together in blue water

A diet of krill

A single species of krill, Euphausia diomedeae, was present in 81 of 89 stomachs and accounted for 93% of identifiable prey. Every spinetail and bentfin devil ray stomach contained krill; oceanic manta rays had also eaten lanternfishes and copepods, and sicklefin devil rays squid and fish. The rays fed at night near the surface, on krill that spends the day at depths of 140 to 280 m (460 to 920 ft), and the November to May fishing season coincides with a seasonal upwelling in the Bohol Sea. One oceanic manta ray’s stomach held an estimated 631,167 kcal of krill.

The four species differed little in diet, and stable isotopes from 192 Philippine muscle samples confirmed that their diets also overlap over the long term. Because the rays share feeding grounds that overlap with fishing, identifying those grounds could help guide where fishing takes place.

Joshua Rambahiniarison of LAMAVE examines samples under a microscope

Evidence from DNA

DNA analysis of 101 stomachs identified 44 prey taxa, or distinct groups of organisms, including a surprising variety of bony fishes. Krill, however, accounted for 95% of the 1,335,305 prey DNA sequences, confirming that it dominates their diet.

Growth and reproduction

Females of all four species mature at a larger size than males and become pregnant later still. Females matured at a disc width (the span from wingtip to wingtip) of 163.6 cm (64.4 in) in bentfin devil rays, 217.8 cm (85.7 in) in spinetail devil rays, and 448 cm (14.7 ft) in oceanic manta rays. The study also recorded the largest known bentfin devil ray, at 197 cm (78 in), and showed that the Bohol Sea is a pupping and mating ground for at least three of the species. Estimated population growth rates were just 0.008 to 0.044 per year (allowing for the age at first pregnancy), so populations would take 15.8 to 86.6 years to double. The authors concluded that “the sustainability of any level of harvesting and fishery exploitation seems unrealistic and strongly discouraged”. (Photo: Sally Snow, LAMAVE.)

Dr. Chris Rohner and Joshua Rambahiniarison look at data on a laptop

Manta rays across the Philippines

A national study, with MMF’s Dr. Simon Pierce, Dr. Chris Rohner, and Anna Flam among its co-authors, compiled 2,659 manta ray sightings from 2004 to 2020. Photo-identification recorded 392 reef manta rays (Mobula alfredi) and 107 oceanic manta rays at 22 sites, with 89% of the individuals concentrated at four sites in Masbate and Palawan. Sightings of oceanic manta rays fell by 80% at Daanbantayan and San Jacinto. At Tubbataha and Cagayancillo, 309 sightings were of 32 individual reef manta rays.

A dark-colored manta ray swims past a diver at Tubbataha Reefs, Philippines

Research recommendations

Continue monitoring rays caught as bycatch in tuna fisheries and at other landing sites, and regulate non-selective gillnets.
Because rays have continued to be caught as bycatch since the ban, protect the cleaning stations they visit, reduce bycatch, and develop a national recovery plan.