Giant Oceanic Manta Ray

Mobulid rays, including the Oceanic 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, Oceanic 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 Oceanic 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 Oceanic Manta Rays, are caught incidentally as bycatch throughout their ranges in at least 21 small scale fisheries in 15 countries and nine 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 Oceanic 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 devil rays 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 Oceanic Manta Ray (Amandè et al. 2012). In the Western and Central Pacific, from 2010 to 2015, observed bycatch of Oceanic 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 Oceanic 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 Oceanic 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 Oceanic 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 Oceanic 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 Oceanic Manta Rays (Stewart et al. 2018).