Tags: Ray Biology 

The Impact of Overfishing on Rays

Manta birostris

Directed fisheries in Indonesia, Sri Lanka and India account for the largest share of recorded M. birostris mortality (~ 90%), with annual landings of over 3,000 animals. Global landings are reported as ~ 3,400 M. birostris, but actual landings are likely much higher:

• Directed and organized harpoon fisheries on both coasts of India are reported to land large numbers of M. birostris but are not represented in fisheries data.

• Additional ports in Indonesia, other than those documented in this report, have been observed to land M. birostris regularly but also are not represented in fisheries data.

• Potentially large fisheries for M. birostris in Africa have been reported, but again little to no landings data is available.

• Manta and mobula rays are frequently mentioned as bycatch in industrial and artisanal fisheries, especially in purse-seine and gillnet fisheries for tuna, however, these landings are not recorded separately by species creating further gaps in landings data.

M. birostris are large and tend to feed close the surface, making them extremely vulnerable to opportunistic hunting by coastal fishermen. A quick Internet search reveals images of captured manta rays from many countries, but no data on these fisheries are available.

Large declines have been reported following increases in directed fishing for M. birostris:

Indonesia - Large declines in number and size of manta ray catches reported over the past decade1.

Sea of Cortez, Mexico - Disappearance following intense fisheries in the 1980’s2.

Sri Lanka - Fishermen reported declines in manta ray catches over the past five years as targeted fishing pressure has increased3

India - Manta catches have declined in several regions, including Kerala, along the Chennai and Tuticorin coasts and Mumbai, despite increased fishing effort4.

Philippines - Fishermen reported a 50% decline in manta ray landings from the 1960s to 1990s, following directed fisheries there5.

Thailand - Dive operators in the Similan Islands have witnessed increased fishing for manta rays, even in Thai national marine parks, and have reported steep declines in manta ray sightings6.

While there are no worldwide population estimates for M. birostris, one can put the estimated landings figure into context based on the maximum number of M. birostris individuals recorded in some of the largest known aggregation sites: 1) Mexico’s Revillagigedos Islands (>350 animals); 2) Ecuador Isla de la Plata (~300 animals); and 3) Southern Mozambique (~600 animals population estimate).

The implications are serious: each year fisheries are extracting 6-12 times the number of mantas documented in these sites, which are the largest known aggregations of this species.

In addition, manta ray researchers from Western Australia report that sightings of M. birostris have dropped precipitously over the past ten years. Tagging data from whale sharks tagged in Western Australia reveals migration routes that frequently pass directly through known Indonesian manta ray fisheries, where whale sharks are also harpooned with regularity7. The seasonal correlation between the M. birostris and whale sharks, their migration through confirmed Indonesian fisheries areas, and the dramatic decline in both species in Western Australia over the past decade, suggest that the Indonesian fisheries may be having a significant impact on M. birostris populations8.

The targeting of juvenile M. birostris in a potential manta ray ‘nursery’ ground close to shore in southern Sri Lanka, possibly the first M. birostris nursery reported anywhere in the world, poses yet another serious conservation concern. 

Manta alfredi

Prior to the recent re-evaluation and splitting of the Manta genus9, all manta rays were identified as M. birostris. It is often not possible, therefore, to determine if published landings referring to M. birostris might actually be M. alfredi or a mix of the two species in some cases. What is confirmed is that no landings of M. alfredi were observed in investigations of fisheries in Lamakera and Lombok in Indonesia10 or in Sri Lanka11.

In Mozambique it is estimated that 20 to 50 M. alfredi are taken by subsistence fishermen annually along a ~ 100 km area12. An ongoing observational study on manta abundance in Southern Mozambique also reports an 80% decline in M. alfredi over the last 9 years.

Local fishermen are known to opportunistically target animals belonging to small M. alfredi populations around islands throughout the western and central Pacific. Because of their isolation and low numbers, these local populations of M. alfredi are extremely vulnerable to any fishing pressure.

Mobula rays

Mobula rays are subject to even greater global fisheries pressure, yet we know even less about the state of their populations. The Sri Lankan and Indian fisheries combined land more than 79,000 mobulas per year, with Sri Lanka accounting for more than 50% of recorded global landings of over 94,000 animals. In the Sri Lanka fishery, the most frequently landed species are M. japanica (~ 87%), followed by M. tarapacana (~ 12%), and M. thurstoni, (~ 1%).

Factors including illegal, underreported, and unrecorded fisheries suggest that the total number of mobula rays landed in global fisheries is likely to be significantly greater than the ~94,000 accounted for in the aggregate fisheries data. For example, there are numerous anecdotal reports of large numbers of mobulas landed in parts of Mexico, despite laws prohibiting their harvest and no available landings data13.

In Sri Lanka and Indonesia (Lombok and Lamakera), fishermen and traders reported declines in catches of mobula rays over recent years as targeted fishing pressure has increased.

Mobulid catches have declined in several regions of India, including Kerala, along the Chennai and Tuticorin coasts and Mumbai, despite increased fishing effort14.

Swordfish fisheries in the Aegean and Levantine Seas also report bycatch of Mobula mobular, a species endemic to that region15 and classified by the IUCN as “Endangered.”

References & Resources

1 - Setiasih 2011, IUCN Red List 2011

2 - Homma et al. ’99, Notarbartolo di Sciara 1995, White et al. 2006.

3 - Fernando and Stevens 2011.

4 - Couturier et al. in press.

5 - Alava et al. 2002.

6 - R. Parker pers. comm., R. Dion pers. comm.

7 - Pet-Soede 2002, Setiasih 2011.

8 - F. McGregor pers. comm.

9 - Marshall et al. 2009,

10 - Setiasih 2011

11 - Fernando and Stevens 2011.

12 - Marshall et al. 2011.

13 - M. McGettigan, SeaWatch pers. comm.

14 - Couturier et al. in press

15 - Akyol and Ceyhan 2011

Akyol, O. and Ceyhan, T. 2011. The Turkish swordfish fishery. Collective Volume of Scientific Papers ICCAT, 66(4): 1471-1479.

Alava, E.R.Z., Dolumbaló, E.R., Yaptinchay, A.A., and Trono, R.B. 2002. Fishery and trade of whale sharks and manta rays in the Bohol Sea, Philippines. In: Fowler, S.L., Reed, T.M., Dipper, F.A. (eds) Elasmobranch Biodiversity, Conservation and Management: Proceedings of the International Seminar and Workshop. Sabah, Malaysia, July 1997, pp 132–148

Fernando, D. and Stevens, G. 2011 A study of Sri Lanka’s manta and mobula ray fishery. The Manta Trust, 29 pp.

Homma, K., Maruyama, T., Itoh, T., Ishihara, H., and Uchida, S. 1999. Biology of the manta ray, Manta birostris Walbaum, in the Indo-Pacific. In: Seret, B. and Sire, J.Y. (eds) Indo-Pacific fish biology: Proc 5th Int Conf Indo-Pacific Fishes, Noumea, 1997. Ichthyological Society of France, Paris, p 209–216

Marshall, A. D. 2009. Biology and population ecology of Manta birostris in southern Mozambique. PhD Thesis, University of Queensland

Marshall, A.D., Dudgeon, C.L. and Bennett, M.B. 2011. Size and structure of a photographically identified population of manta rays Manta alfredi in southern Mozambique. Marine Biology, 158 (5): 1111-1124.

Notarbartolo di Sciara, G. 1995. What future for manta rays? Shark News, 5: 1.

Pet-Soede, L (Ed.). 2002. The Solor and Alor islands: Expedition Results, data collected during 2 reconnaissance trips: 9-12 September, 2001 and 7-19 May, 2002, Report from WWF Wallacea Bioregional Program and The Nature Conservancy Southeast Asia Center for Marine Protected Areas, Bali, Indonesia. 110 p.

Setiasih, N. 2011. Indonesia Fishery Investigation. Manta Ray of Hope, 15 pp.

White, W. T., Giles, J., Dharmadi, and Potter, I. C. 2006 b. Data on the bycatch fishery and reproductive biology of mobulid rays (Myliobatiformes) in Indonesia. Fisheries Research, 82(1-3), 65-73.

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