Manta birostris

Atlantic Manta

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Atlantic Manta

 

Description

The graceful manta ray is a stunning member of the devil ray family, if only for its sheer size. These large rays have a distinctive body shape with triangular pectoral 'wings' and paddle-like lobes extending in front of their mouths. They are generally dark on the upper surface, ranging from black to greyish-blue and brown, with pale undersides; individuals have a unique pattern of blotches and scars that can be used to identify them. The large, cavernous mouth is situated at the front of the body and contains 18 rows of teeth on the lower jaw.

Geography

Circumglobal, tropical to temperate: in the Northern Hemisphere, as far north as southern California and Rhode Island on the United States west and east coasts, Mutsu Bay, Aomori, Japan, the Sinai Peninsula, Egypt and the Azores Islands; in the Southern Hemisphere, as far south as Peru, Uruguay, South Africa and New Zealand. Insome locations, including Mozambique, it is sympatric with Manta alfredi.

Ecosystem

Manta birostris, unlike most other rays, are found near the surface of the ocean and to depths of 120 meters. Atlantic manta rays stay closer to shore in the warmer waters where food sources are more abundant, but occasionally can be found further from shore.

Range depth: 0 to 120 m.

Habitat Regions: tropical ; saltwater or marine

Aquatic Biomes: pelagic ; reef ; coastal

Conservation

Vulnerable.

 

Threats

Major Threats
The main threat to both Manta species is fishing, whether targeted or incidental. Manta rays are currently killed or captured by a variety of methods including harpooning, netting and trawling. These rays are easy to target because of their large size, slow swimming speed, aggregative behaviour, predictable habitat use, and lack of human avoidance.

Manta species have a high value in international trade markets. Their gill rakers are particularly sought after and are used in Asian medicinal products. This market has resulted in directed fisheries for manta rays which are currently targeting these rays in unsustainable numbers. Over 1,000 manta rays are caught per year in some areas (Alava et al. 2002, Dewar 2002, White et al. 2006, C. Anderson and G. Stevens pers. obs.). Artisanal fisheries also target both species for food and local products (Essumang 2010, Marshall et al. 2011).

Aside from directed fisheries, manta rays are also incidentally caught as bycatch in both large-scale fisheries and small netting programs such as shark control bather protection nets (Carlson and Lee 2000, Young 2001). In some populations, such as the ones identified at Isla de la Plata, Ecuador, Laje de Santos, Brazil, and the Similan Islands, Thailand, high percentages of all individuals encountered or identified have evidence of entanglement or are dragging lines or nets (A. Marshall unpubl. data 2011).

As a result of sustained pressure from fishing (both directed and bycatch) certain monitored subpopulations appear to have been rapidly depleted (e.g., Gulf of California, Mexico; Indonesia; and, Philippines (Anon 1997, Alava et al. 2002, White et al. 2006)). Targeting either species of Manta at critical habitats or aggregation sites, where individuals can be caught in large numbers in a short time frame, is a particular threat. Regional populations of both species appear to be small, and localized declines are unlikely to be mitigated by immigration. This situation is exacerbated by the conservative life history of these rays, which constrain their ability to recover from a depleted state.

Cryptic threats such as mooring line entanglement and boat strikes can also wound manta rays, decrease fitness or contribute to non-natural mortality (Deakos et al. 2011). Many other threats have been postulated and identified such as habitat degradation, climate change, pollution (e.g., from oil spills), ingestion of micro plastics and irresponsible tourism practices.

Known directed fisheries:

Reported World Catch
Manta and devil ray catches increased from 900 tonnes to over 3,300 tonnes between 2000–2007 (FAO 2009, Lack and Sant 2009).

Trade-driven Fisheries
Manta rays, predominately the Giant Manta Ray, are currently taken in fisheries that have transitioned from bycatch fisheries to directed fisheries, with the birth of a market for manta ray products in Asia.

Chondrichthyan landings from drift gillnets were examined at four different sites (a total of 263 sampling days) in Indonesia from 2001 until 2005. Mobulid rays including manta rays were commonly represented in the catch, which was estimated to be approximately 4,110 individuals annually, a biomass of approximately 544 tonnes of which manta rays (Giant Manta Ray) comprised 13.7% (White et al. 2006). Individual manta rays were worth up to $200 in the early 2000s. Dried filter plates were being exported to Hong Kong, Taiwan and Singapore (for up to US$30 per dry kilo) (White et al. 2006).

Manta rays (predominately the Giant Manta Ray) are taken in significant numbers as bycatch in the Pakistani, Indian and Sri Lankan gillnet fisheries, where they are used as shark bait, for human consumption and their branchial filaments are sold to Asian buyers (Anderson et al. 2010, P. Hilton pers. comm. 2011, G. Stevens unpubl. data 2010).

Artisanal Fisheries
Manta rays are caught in artisanal fisheries in Tanzania on longlines and in trawls. The meat is typically dried and consumed (Bianchi 1985, Iddawi and Stanley 1999, N. Iddawi, S. Yahya and S. Semesi pers. comm. 2010, A. Marshall unpubl. data 2011).

Both species of Manta (but predominately the Reef Manta Ray) are caught in artisanal fisheries in southern Mozambique for consumption. Manta rays are typically harpooned but also caught in nets with motorized boats. Approximately 50 individuals are taken per annum from a 50 km stretch of coastline (A. Marshall unpubl. data).

Isolated reports of fishing for mantas have continued in the Gulf of California. Artisanal pelagic gillnet fishermen throughout the Gulf of California have been observed to retain mantas as bait as well as utilize landed specimens for personal consumption and sale.

There is a seasonal fishery for manta rays along the Ghanaian coastline, particularly in Dixcove. Manta rays are targeted in this region for local food (Essumang 2010).

Bycatch fisheries
Manta rays are caught in gillnet and purse seine fisheries as well as netting programs throughout their distribution. Specific cases are outlined below:

Giant Manta Rays are caught in small numbers as bycatch in the European purse seine tuna fishery operating in the Atlantic Ocean. Observer data from 2003-2007, which corresponded to 2.9% coverage, recorded 11 individuals landed, a number that represented 17.8% of the total ray bycatch (Amande et al. 2010).

Incidental catches of manta rays in the protective shark nets off the beaches of KwaZulu-Natal, South Africa, peak in the summer months (49% of the total annual catch), although manta rays are caught throughout the year (Young 2001). Manta rays (both species but predominately the Reef Manta Ray) comprised 16.9% of the total historical batoid catches from these nets, with a mean annual catch of 60 individuals and an overall 33.7% mortality rate (Young 2001).

Giant Manta Rays are caught as bycatch or are killed in fisheries along the west coast of Thailand and Myanmar, including within the Similans National Park where evidence suggests that a high proportion of individuals visiting the area have been entangled by fishing line or nets. Incidental kills have also been reported in fishing nets, tackle and ghost nets (A. Marshall unpubl. data 2011).

Giant Manta Rays are not generally directly targeted in Ecuador, although shark and ray catch data collected by the Subsecretaria de Recursos Pesqueros show occasional incidental capture and one small directed fishery since the mid-eighties. Many manta rays in the aggregation site around Isla de la Plata show damage received from fishing equipment, which occurs when artisanal fishermen use trawling tackle illegally within the Machalillia National Park boundaries to fish for seasonal aggregations of Wahoo (Acanthocybium solandri) which coincide with the seasonal aggregation of Giant Manta Rays (M. Harding unpubl. data 2010).

Although manta rays are not directly targeted by fisheries in southeastern Brazil, several reports of Giant Manta Rays being captured as bycatch show that local fishing poses a threat to manta rays (Zerbini and Kotas 1998). The Brazilian government is currently promoting a policy to boost commercial fisheries in the area, through financial incentives, raising concerns on the future of that manta ray population. Reports of individuals entangled within discarded fishing gear (e.g., ‘ghost nets’) are not uncommon (G. Kodja unpubl. data 2010). The main aggregation site for the Giant Manta Ray is located close to the Port of Santos, Latin America’s largest seaport, increasing the risk of ship strikes.

Surveys made of the bycatch from 52 sets from the shark drift net fishery off Georgia and east Florida, USA from 1992–1995 included 148 rays, 14 of which were recorded as being the Giant Manta Ray (Trent et al. 1997). Another study of the bycatch in the directed shark drift gillnet fishery off the east coast of Florida and Georgia, which was set 4.8 km offshore in EEZ waters from 1998–1999, revealed that manta rays are still occasionally caught in this fishery (Carlson and Lee 2000).

Fisheries bycatch data collected from the U.S. tuna purse seine fishery in the central-western Pacific in 1999 listed the Giant Manta Ray amongst the species caught with a set frequency of 1.5%. A total of 18 mantas were caught (1.14 t) during the observed period, 100% of which was discarded (Coan et al. 2000).

 

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