The previously, monotypic genus Manta was re-evaluated with two species identified, Reef Manta Ray (Manta alfredi) and Giant Manta Ray (Manta birostris) (Marshall et al. 2009). Genetic evidence supported the separation (Kashiwagi et al. 2012). Both species have broad global distributions and are sympatric in some locations and allopatric in others (Kashiwagi et al. 2011, Lawson et al. 2017). A third species of manta ray has also been proposed (Marshall et al. 2009) with increasing genetic support (Hinojosa-Alvarez et al. 2016, Kashiwagi et al. 2017, Hosegood et al. 2018). Genetic evidence has also resulted in the genus Manta being subsumed within the genus Mobula (Poortvliet et al. 2015, White et al. 2018).

Due to the recent taxonomic changes, both present day and historical reports can often be unclear and without adequate descriptions, photographs or geographic locations, it can be difficult to conclusively attribute fisheries data to a single species. Care should be taken when using reports or accounts of the Giant Manta Ray. Melanistic (black) and leucistic (white) colour morphs occur in all species of manta ray (Marshall et al. 2009). Variant colour morphs often contribute an added degree of confusion when attempting to visually discriminate between species of manta rays in the field or in photographs, especially when close examination is not possible. It should be noted that these colour morphs could be a source of error, resulting in misidentifications in past or future studies or surveys of distribution.

The Giant Manta Ray (Mobula birostris) is a large (to 700 cm disc width) ray with a circumglobal distribution in tropical and temperate waters throughout the Pacific, Indian, and Atlantic Oceans in coastal and pelagic waters from the surface to a depth of 1,000 m. Within this broad range, aggregations are sparsely distributed and while individuals are capable of long-distance movements they do so infrequently and exhibit a degree of philopatry resulting in a high likelihood of local depletion. The global population size is not known, but local and regional abundance has been estimated and is mostly small, numbering less than 500 individuals, except for Ecuador where abundance is estimated at more than 2,000 individuals. The species has an extremely slow life history, producing only 1 pup on average every 4–5 years, and consequently is likely to have one of the lowest maximum rates of population increase (median 0.032 per year) of any elasmobranch. Giant Manta Rays are targeted or taken as bycatch in artisanal small-scale fisheries, as well as taken as bycatch in large-scale tuna fisheries. The meat is consumed locally, and the species is traded internationally due to the rapid rise of the valuable gill plate trade. Where Giant Manta Ray are protected (in over a dozen countries and territories), and hence where they are not being fished, the sighting trends appear stable. Elsewhere, however, very rapid declines have been noted in sightings records and landings where they are targeted or caught as bycatch; these range from 71 to 95% declines over 13- to 21-year periods (all less than one generation length of 29 years). It is suspected that the Giant Manta Ray has undergone a population reduction of 50–79% over the past three generation lengths (87 years), with further population reduction suspected over the next three generation lengths (2018–2105) due to current and ongoing levels of exploitation, and a reduction in area of occupancy due to suspected local and regional extinctions, and it is assessed as Endangered A2bcd+A3d.

The global population size of the Giant Manta Ray is difficult to assess, but abundance trajectories have been estimated based on long time series of sightings at diving sites. Generally, divers encounter the Giant Manta Ray less frequently than the Reef Manta Ray (Mobula alfredi) and this is thought to be due to their more oceanic habitat preferences and behaviour. Locally, abundance varies substantially and may be based on food availability and the degree that they were, or are currently, being fished. In most regions, Giant Manta Ray population sizes appear to be small (less than 1,000 individuals). Photo-identification studies at specific aggregation sites have yielded minimum estimates of 42 to 500 individuals over almost a decade of monitoring in most locations, including: Mozambique, Thailand, Myanmar, Indonesia (Holmberg and Marshall 2018), Japan (Kashiwagi et al. 2010), Brazil (Luiz et al. 2008), and Mexico (Rubin 2002). A 6-year study has catalogued more than 2,000 individuals in a single site, off mainland Ecuador (Holmberg and Marshall 2018).

Giant Manta Ray aggregation sites are widely separated and the lack of genetic substructuring indicates occasional large-scale movements have occurred. Crossing referencing of regional photo-identification databases has not detected inter-region individual movements (e.g. across ocean basins) (Holmberg and Marshall 2018), indicating a low degree of interchange between ocean basins. Unlike the Reef Manta Ray, no significant genetic substructuring has been detected within the Giant Manta Ray (Stewart et al. 2016, Hosegood et al. 2019). Long-term studies, including those which have incorporated telemetry, have shown low resighting rates but a degree of philopatry.

The trend of the number of individuals varies widely across the range of the Giant Manta Ray, but trends appear stable where they are protected and declining rapidly where fishing pressure is greater. The sighting trends appear stable where they receive some level of protections, such as Hawaii (Ward-Paige et al. 2013) and Ecuador (Holmberg and Marshall 2018), although individuals sighted in Ecuador seasonally migrate to Peru (A. Marshall unpubl. data 2019) where directed fishing occurs (Heinrichs et al. 2011). Elsewhere, the number of individuals is likely to be declining in places where the species is targeted or caught regularly as bycatch. There are two estimates based on sightings-per-unit-effort. In southern Mozambique, a 94% decline in diver sighting records occurred over a 15-year period in a well-studied population (Rohner et al. 2017). In Cocos Island, Costa Rica there has been an 89% decline in diver sighting records of Giant Manta Rays over a 21-year period (White et al. 2015). These steep declines have occurred in less than one generation length (29 years).

The Giant Manta Ray is a neritic and oceanic pelagic ray that occurs in places with regular upwelling along coastlines, oceanic islands, and offshore pinnacles and seamounts (Marshall et al. 2009). The Giant Manta Ray can exhibit diel patterns in habitat use, moving inshore during the day to clean and socialize in shallow waters, and then moving offshore at night to feed to depths of 1,000 meters (Hearn et al. 2014, Burgess 2017). It can spend long periods of time offshore without visiting shallow coastal waters (Stewart et al. 2016, A. Marshall unpubl. data 2019). The Giant Manta Ray may be the largest living ray species attaining a maximum size of 700 cm disc width (DW) with anecdotal reports up to 910 cm DW (Compagno 1999, Alava et al. 2002). Males mature at 350–400 cm DW and females mature at 380–500 cm DW (White et al. 2006, Last et al. 2016, Stevens et al. 2018). Reproduction is aplacental viviparous with a single large pup of 122–200 cm DW (White et al. 2006, Rambahiniarison et al. 2018). Reproductive periodicity is unknown, but assumed to be 4–5 years, similar to the closely related Reef Manta Ray (M. alfredi). Female age-at-maturity is estimated as 8.6 years of age but first pregnancy may be delayed by up to 4 years (making first age of pregnancy 12 years) depending upon food availability (Rambahiniarison et al. 2018). The maximum age is estimated as 45 years, based on the longevity of the Reef Manta Ray; generation length is therefore estimated as 29 years. Based on this life history, the maximum intrinsic rate of population increase could range between 0.019 and 0.046 per year (median 0.032 per year) (J. Carlson unpubl. data 2019, following methods in Dulvy et al. 2014). The species is among the longest-living rays and has an extremely conservative life history; the average Giant Manta Ray may produce only 4–7 pups during its estimated lifespan, which would contribute to its slow recovery from population reductions due to over-exploitation or other threats.

Mobulid rays, including the Giant Manta Ray, are both targeted and caught incidentally in industrial and artisanal fisheries (Couturier et al. 2012, Croll et al. 2016, Stewart et al. 2018). These rays are captured in a wide range of gear types including harpoons, drift nets, purse seine nets, gill nets, traps, trawls, and longlines. Manta rays are also caught in bather protection nets (Cliff and Dudley 2011, Department of Agriculture and Fisheries 2018). Their coastal and offshore distribution, and tendency to aggregate, makes mobulid rays particularly susceptible to bycatch in purse seine and longline fisheries and targeted capture in artisanal fisheries (Croll et al. 2016, Duffy and Griffiths 2017). In particular, Giant Manta Rays are easy to target because of their large size, slow swimming speed, tendency to aggregate, predictable habitat use, and lack of human avoidance (Couturier et al. 2012).

Mobula rays, including Giant Manta Rays, are caught in at least 13 targeted artisanal fisheries in 12 countries. Some of the largest documented fisheries have been in Indonesia, the Philippines, India, Sri Lanka, México, Taiwan, Mozambique, Palestine (Gaza strip), and Peru (Couturier et al. 2012, Ward-Paige et al. 2013, Croll et al. 2016), where sometimes thousands of manta rays are landed per annum (Alava et al. 2002, Dewar 2002, White et al. 2006, Lewis et al. 2015). While many artisanal fisheries have grown to meet international trade demand for gill plates, some still target these rays mainly for food and local products (White et al. 2006, Essumang 2010, Rohner et al. 2017).

Mobula rays, including Giant Manta Rays, are caught incidentally as bycatch throughout their ranges in at least 21 small scale fisheries in 15 countries and 9 large-scale fisheries in 11 countries (Croll et al. 2016). Despite being unintentionally caught, mobulid rays are typically retained because of their high trade value. Even when discarded alive, e.g. from tuna purse seine fisheries, they are often injured and have high post-release mortality (Tremblay-Boyer and Brouwer 2016, Francis and Jones 2017). Many fisheries remain open and active even after dozens of national fishing bans and international listings on the appendices of both Convention for the Conservation of Migratory Species of Wild Animals (CMS) and the Convention on International Trade in Endangered Species of Wild Flora and Fauna (CITES) (Lawson et al. 2017, Lawson and Fordham 2018).

Global landings of mobulid species, including Giant Manta Ray, have been increasing steadily due in large part to the recent rise (from the 1990s onwards) in demand for gill plates (Croll et al. 2016, O’Malley et al. 2017). Many former bycatch fisheries have become directed commercial export fisheries (Dewar 2002, White et al. 2006, Heinrichs et al. 2011, Fernando and Stevens 2011). Between 2000 and 2007, total landings of ‘Mantas, devil rays nei’ ('nei' refers to 'not landed elsewhere') increased from 900 tonnes to over 3,300 tonnes according to the FAO Fishstat Capture Production database (Lack and Sant 2009). This equates to an average of 1,593 metric t being landed per annum with this average increasing to 4,462 metric t per annum from 2008 to 2017 (Oakes and Sant 2019); reported landings are likely to estimate only a fraction of total fishing-related mortality (Ward-Paige et al. 2013).

In the markets of Guangzhou, China, where 99% of mobulid products are routed, mobulid products are sourced from over 20 countries and regions (O’Malley et al. 2017). The source locations for the largest amounts of product are Indonesia, Sri Lanka, India, China, and Vietnam (O’Malley et al. 2017). Demand for products has driven up the price and traded volume of these products in recent decades. Between 2011 and 2013, there was an increase from 60 to 120 t of mobulid product moved through shops in Guangzhou (O’Malley et al. 2017).

In the Western Indian Ocean, Romanov (2002) estimated that between 253 and 539 manta rays and devilrays were being caught per year as bycatch in purse seine fisheries, and between 2003 and 2007, 35 manta rays were observed in purse seine bycatch, most of which were likely Giant Manta Ray (Amandè et al. 2012). In the Western and Central Pacific, from 2010 to 2015, observed bycatch of Giant Manta Rays in purse seine fisheries was 4,176 individuals, and in longline fisheries was 226 individuals (based on distribution, some of these were likely Giant Manta Ray) (Tremblay-Boyer and Brouwer, 2016). While a few individuals were released in good condition, post-release mortality is likely to be high as most were released alive but injured, or dead (Francis and Jones 2017). The Eastern Pacific purse seine fisheries show a substantial increase in the bycatch of mobulid rays, including Giant Manta Rays, from 20 tonnes per year before 2005 to 150 tonnes per year by 2006, which then reduced to 10 tonnes per year by 2009 (Hall and Roman 2013). The Inter-American Tropical Tuna Commission (IATTC) purse seine vessels operating from 1993 to 2015 reported an average catch of approximately 135 Giant Manta Rays per year (Miller and Klimovich 2017). A susceptibility analysis indicated that negative interactions with fishing gear and unintended mortality will continue to be an issue with this species as these fisheries coincide with high productivity areas where Giant Manta Rays are likely to aggregate for feeding (Duffy and Griffiths 2017, Duffy et al. 2019).

While the overwhelming cause of population reduction is fishing mortality, sublethal effects and lower levels of mortality occur from numerous lesser threats, such as entanglement in nets, recreational foul hooking, and vessel strikes (Marshall and Bennett 2010, Deakos et al. 2011, Couturier et al. 2012, Stewart et al. 2018). While there is no direct evidence, there are concerns for effects of climate change, ocean acidification, oil spills, and other forms of pollution and contaminants (e.g., heavy metals) (Essumang 2010, Ooi et al. 2015, Stewart et al. 2018). Furthermore, shallow water lagoon nursery habitats are subject to habitat loss and degradation that pose threats to juvenile Giant Manta Rays (Stewart et al. 2018).

Mobulids are widely used for their meat, skin, liver oil, and gill plates (Couturier et al. 2012). The gill plates fetch high prices in Asia and are used for Chinese health tonics (O’Malley et al. 2017). The meat from mobulids is often used for food and shark bait or attractant, and the skin of mobulids is sometimes used for leather products (shoes, wallets, and knife handles). Giant Manta Rays are sometimes caught and transported to aquariums for use in display tanks. Some of these captive animals have been released into the wild.

The Giant Manta Ray is protected in over a dozen countries and territories (Lawson et al. 2017, Lawson and Fordham 2018, Booth et al. 2020). Local, national, and territory-level restrictions and/or protections for manta rays are currently in place in the USA (Hawaii), Australia and its territories, the Maldives, the United Arab Emirates, the Federated States of Micronesia (Yap), Thailand, Mozambique, Indonesia, and the Philippines. Even with legislative protection in some areas, fisheries for manta ray persist in others and there are more locations still where they are incidentally captured as bycatch in artisanal as well as small and large fisheries. With little to no effort to monitor or regulate mobulid fisheries in many countries, these current paper regulations are proving inadequate and misleading.

Manta rays have been listed in the appendices of a number of international conventions. The Giant Manta Ray was listed on Appendix I and II of the Bonn Convention for the Conservation of Migratory Species of Wild Animals (CMS) in 2011, however, many CMS Parties have inadequate protections (Lawson and Fordham 2018). As a threatened species whose wide-ranging migratory tendencies put it at risk, this listing acknowledges the fact that this species would benefit from the implementation of cooperative international agreements to manage and protect its populations and encourages range states to work together toward this goal. Manta species (i.e., including Giant Manta Ray) were listed on Appendix II of the Convention on International Trade in Endangered Species (CITES) in 2013, requiring fishing states to legitimize all exports through a permitting system based on non-detrimental findings which delimit acceptable, sustainable catches.

Some Regional Fishing Management Organizations (RFMOs) have formally prohibited the retention of manta rays as bycatch, however most regulatory systems doing an inadequate job of protecting mobulids from fishing mortality. In 2015, recognizing the threatened status of mobulids, including the Giant Manta Ray, IATTC adopted recommendations which prohibited retaining, trans-shipping, landing, storing or selling, in whole or in part, any species of Mobula and required going forward that all mobulids be promptly released alive under strict non-detrimental protocols (Lawson et al. 2017, Lawson and Fordham 2018). Since then IOTC (2019), have adopted similar recommendations, and the WCPFC will adopt similar conservation measures in January 2021. Efforts should be put in place to reduce the unintentional capture of these rays and when they are caught, protocols should be in place for their safe release, to reduce the chances of post-release mortality.

Manta ray tourism is managed in some locations but if sustainable tourism industries are indeed to be developed around encounters with these rays, they should be legislated, continuously monitored, and enforced with permitting systems to make sure operators and tourists are adhering to best practice protocols and codes of conduct.