Tags: Gill Raker,  Ray Threats 

Threats to Rays

Targeted Fisheries

Historically, subsistence fishing for manta and mobula rays occurred in isolated locations with simple gear, restricting the distance and time fishers could travel to hunt. In recent years, however, fishers have begun targeting manta and mobula rays with modern fishing gear while expanding fishing range and season. The emerging market for dried gill rakers is the primary driver of mobulid fisheries. However, shark population declines also have boosted mobulid fisheries: the rays provide a cheap substitute for shark cartilage used in nutritional supplements1.

Bycatch

Thousands of manta and mobula rays are caught “incidentally” as bycatch in industrial and artisanal fisheries throughout the Atlantic, Pacific and Indian Oceans and the Mediterranean Sea2. Purse seines, gillnets and longlines, all commonly used in tuna fisheries, are most frequently responsible for manta and mobula bycatch. Unfortunately, the intensive “dolphin safe tuna” conservation campaigns that were intended to reduce dolphin bycatch, increased the bycatch of manta and mobula rays, sharks and many other marine animals3. Because mobulid bycatch data is rarely recorded and when recorded is not classified by species4, the impact of incidental fishing on manta and mobula ray populations remains largely underestimated and unknown5.

Habitat Destruction

Coral reef degradation could negatively impact manta and mobula rays by disrupting feeding aggregations, cleaning station behavior, or disrupting reproductive behavior6.

Climate Change

Most mobulid rays depend on plankton as their primary food source. As changing sea temperatures disrupt the phytoplankton’s natural ecological cycles, manta and mobula rays may struggle to find adequate food supplies7.

Marine Debris

Many manta and mobula rays die from marine debris, phantom nets, plastics and pollution from vessels. Fishing line entanglement, and resulting amputation or damage to cephalic fins, can also impair the rays’ ability to feed8. Ingestion of plastic debris has been extensively documented to result in a range of health problems, injuries and death in several marine species, and may also pose a significant threat to manta and mobula rays9.

Boat Strikes and Entanglement

Manta and mobula rays often fall victim to boat strikes as they pass through regions of heavy maritime traffic10. Manta rays can also become entangled in mooring and boat anchor lines. When these lines get caught around the cephalic fins and head, they trap the manta ray and cause it to drown11.

Unregulated tourism

As aggregation sites become tourist attractions, unregulated interactions (i.e. a high number of boats in the vicinity and a large volume of people in the water close to or touching the rays) may cause undue stress12.

Captivity

Although few mobulids live in captivity, the unmonitored removal of these species from the wild for the public aquarium trade may negatively impact small and geographically isolated populations13.

Natural Predation

Natural predation, primarily from sharks and also killer whales, is not considered to be a leading threat to mobulid rays. Non-fatal injuries from shark bites have been observed on both manta and mobula rays in several parts of the world, and the impacts of these injuries on long term survival and reproduction are not known14.

References & Resources

1 - Hilton 2011, Setiasih 2011.

2 - Molony 2005, Perez and Wahlrich 2005, White et al. 2006, Zeeberg et al. 2006, Pianet et al. 2010

3 - Lack and Sant 2006

4 - Lack and Sant 2009, Camhi et al. 2009

5 - G. Stevens, pers. comm.

6 - Deakos 2010b

7 - Chin and Kyne 2007

8 - Deakos 2010a

9 - Couturier et al. in press

10 - Notarbartolo di Sciara 2005, Deakos et al. 2011

11 - Deakos et al. 2011, A. Marshall pers. comm.

12 - Deakos et al. 2011

13 - Deakos et al. 2011

14 - Couturier et al. in press

Camhi, M.D., Valenti, S.V., Fordham, S.V., Fowler, S.L. and Gibson, C. 2009. The Conservation Status of Pelagic Sharks and Rays: Report of the IUCN Shark Specialist Group Pelagic Shark Red List Workshop. Newbury, UK: IUCN Species Survival Commission Shark Specialist Group, x +78 pp.

Chin, A., Kyne, P.M. 2007. Vulnerability of chondrichthyan fishes of the Great Barrier Reef to climate change. In: Climate Change and the Great Barrier Reef: A Vulnerability Assessment, Johnson, J.E., and Marshall, P.A. (eds). Great Barrier Reef Marine Park Authority and Australian Greenhouse Office, Townsville, Australia. P 393-425.

Deakos, M. H. 2010b. Paired-laser photogrammetry as a simple and accurate system for measuring the body size of free-ranging manta rays Manta alfredi. Aquatic Biology, 10: 1-10.

Deakos, M., Baker, J., and Bejder, L. 2011. Characteristics of a manta ray (Manta alfredi) population off Maui, Hawaii, and implications for management. Marine Ecology Progress Series, 429: 245-260.

Deakos, M.H. 2010a. Ecology and social behavior of a resident manta ray (Manta alfredi) population off Maui, Hawai’i. PhD thesis, University of Hawai’i, Manoa, Hawai’i.

Hilton, P. 2011. East Asia Market Investigation. Manta Ray of Hope, 49pp.

Lack, M and Sant, G. 2009. Trends in global shark catch and recent developments in management. TRAFFIC International, 33 pp.

Lack, M. and Sant, G. 2006. Confronting Shark Conservation Head on! TRAFFIC International. 35 pp.

Molony, B. 2005. Estimates of the mortality of non-target species with an initial focus on seabirds, turtles and sharks. 1st Meeting of the Scientific Committee of the Western and Central Pacific Fisheries Commission, 84 pp.

Notarbartolo di Sciara, G. 2005. Giant devilray or devil rays Mobula mobular (Bonnaterre, 1788). In: Sharks, Rays and Chimaeras: The Status of Chondrichthyan Fishes. Fowler, S.L., Cavanagh, R.D., Camhi, M., Burgess, G.H., Caillet, G.M., Fordham, S.V., Simpendorfer, C.A. and Musick, J.A. (eds). Gland, Switzerland and Cambridge, UK: IUCN ⁄ SSC Shark Specialist Group, pp. 356–357.

Perez, J.A.A. and Wahrlich, R. 2005. A bycatch assessment of the gillnet monkfish Lophius gastrophysus fishery off southern Brazil. Fisheries Research, 72: 81-95.

Pianet, R., Chavance, P., Murua, H., Delgado de Molina, A. 2010. Quantitative estimates of the by-catches of the main species of the purse seine fleet in the Indian Ocean, 2003-2008. Indian Ocean Tuna Commission, WPEB-21.

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

White, W.T., Clark, T.B., Smith, W.D. & Bizzarro, J.J. 2006. Mobula japanica. In: IUCN 2011. IUCN Red List of Threatened Species. Version 2011.2. <www.iucnredlist.org>

Zeeberg, J. Corten, A. and de Graaf, E. 2006. Bycatch and release of pelagic megafauna in industrial trawler fisheries off Northwest Africa. Fisheries Research, 78: 186-195.