Introduction
The megamouth shark Megachasma pelagios Taylor et al., Reference Taylor, Compagno and Struhsaker1983 (Lamniformes: Megachasmidae) is the only extant representative in both its genus and family. This species was first described based on a specimen caught in 1976, off Hawaii (Taylor et al., Reference Taylor, Compagno and Struhsaker1983). It can be easily identified by its characteristic short, but broadly rounded, snout, very large and long head, numerous small, hooked teeth, and a huge terminal mouth that extends behind the eyes. Although the behavioural ecology of this species remains largely unknown, this species is described as an epi- and mesopelagic slow swimming shark with a flaccid body and fins, which feeds by filtering plankton (e.g. euphausiids, copepods and jellyfish) in a similar way of engulfment feeding that is typically seen in the rorqual and humpback whales (Compagno, Reference Compagno2001; Nakaya et al., Reference Nakaya, Matsumoto and Suda2008; Nakaya, Reference Nakaya2010; Watanabe & Papastamatiou, Reference Watanabe and Papastamatiou2019). According to the results published in an acoustic tracking study, this species shows vertical movements to depths near 200 m during the day with a shallower night phase (Nelson et al., Reference Nelson, McKibben, Strong, Lowe, Schroeder, Lavenberg and Sisneros1997).
Currently the species is assessed as ‘Least Concern’ on the IUCN Red List (Kyne et al., Reference Kyne, Liu and Simpfendorfer2019). The level of threat is primarily related to the increasing number of catches in the north-west and central western Pacific and is largely explained by the development of a driftnet fishery targeting sharptail mola Masturus lanceolatus in the deeper waters off eastern Taiwan since 2000 (Kyne et al., Reference Kyne, Liu and Simpfendorfer2019).
Since the holotype was described in 1983, a number of bycaught and stranded individuals have been documented from around much of the world. According to Rodriguez-Ferrer et al. (Reference Rodriguez-Ferrer, Shivji, Wetherbee, Schärer, Lilyestrom and Zegarra2017), about 100 confirmed specimens of this rare shark had been caught until 2017, and 65 reported with documented positions, whilst the Florida Museum of Natural History reported on 65 documented records from 1976–2016 (Weichao et al., Reference Weichao, Mehmet, Roslynna, Batuhan and Rosemary2016). Other sources (Yu et al., Reference Yu, Joung, Hsu, Lin, Hsieh, Liu and Yamaguchi2021) inform from 236–250 records until 2020 and 2021, respectively, most of which were associated with the Kuroshio Current, including from the Philippines, Japan, and especially near Taiwan's waters (146 catches), which seems to be a hotspot for the species (Liu et al., Reference Liu, Joung, Yu, Hsu, Tsai and Liu2018). By contrast, until now only four catches from the Atlantic Ocean have been documented in scientific publications; one from Senegal in 1995, two from Brazil in 1995 and 2009, and one stranded on a beach in Puerto Rico in 2017 (Séret, Reference Séret1995; Amorim et al., Reference Amorim, Arfeli and Castro2000; Lima et al., Reference Lima, Rennó and Siciliano2009; Rodriguez-Ferrer et al., Reference Rodriguez-Ferrer, Shivji, Wetherbee, Schärer, Lilyestrom and Zegarra2017), and only seven specimens from the Indian Ocean, including from off Western Australia (Berra & Hutchins, Reference Berra and Hutchins1990), South Africa (Smale et al., Reference Smale, Compagno and Human2002), north of Sumatra (White et al., Reference White, Muhammad Adrim and Sumadhiharga2004) and Sri Lanka (Fernando et al., Reference Fernando, Perera and Ebert2015) from the coast of Komodo Island (2017), and three specimens in the eastern Indian Ocean near Manado (1998) and the south of the Bay of Bengal (2002).
Most of the juveniles of this species have been recorded off the Pacific coast of Mexico, but immature individuals have also been caught in Atlantic waters (Amorim et al., Reference Amorim, Arfeli and Castro2000; Castillo-Géniz et al., Reference Castillo-Géniz, Ocampo-Torres, Shimada, Rigsby and Nicholas2012; Hsu et al., Reference Hsu, Ebert, Joung, Liu, Yu and Lin2015) and in the Indian Ocean (White et al., Reference White, Muhammad Adrim and Sumadhiharga2004). This work aims to provide new data on fishing events with the presence of M. pelagios obtained through the observer programme in the Spanish tuna purse seine fleet operating in the tropical area of the Atlantic and Indian oceans.
Materials and methods
The observer programme onboard the Spanish (and associated flags) tropical tuna purse seine fleet started in 2003. This programme focuses on discards and bycatch, and its observation coverage has progressively increased from around 10%, in its initial stage, up to nearly 100% in recent times, mainly thanks to the implementation of a volunteer programme by the fleet.
This observation coverage means that 23,903 and 16,146 fishing operations have been monitored in the Atlantic and Indian Oceans, respectively, during the period 2003–2020, in which observers recorded all species caught and their fate, in addition to other bridge and environmental data such as position, date and time, set type (free school set (FSC) or Fish Aggregating Device (FAD) set), sea surface temperature (SST) and wind speed.
Results
Two M. pelagios were caught by the tropical tuna purse seine fleet in the tropical eastern Atlantic fishing grounds, near the Gabonese and Mauritanian coasts, in July 2016 and August 2018, respectively. A third individual was also caught in December 2005 by this fleet in the western Indian Ocean, between the Maldives and Chagos archipelagos (Figure 1). Both specimens caught in the Atlantic Ocean were recorded as being female, but no information on the sex was available for the other.
Observed fishing operations, showing the areas fished by the tropical tuna purse seine fleet (green dots), and location of the three megamouth sharks (red dots) bycaught by these fleets.
The megamouth sharks caught in the Indian Ocean and off the Gabon coast were released alive, but the individual bycaught in Mauritania was discarded dead.
Detailed information by set was obtained from the reports provided by the observers onboard the vessels (Table 1). There are only pictures available for the specimen caught on the Mauritanian coast; according to its length it was presumed to be a juvenile (Compagno, Reference Compagno2001) (Figure 2).
Photos of the 180 cm juvenile female megamouth shark caught in 2018 near the coast of Mauritania.
Summary of the set characteristics of the three M. pelagios bycaught by the purse seiner fleet
Discussion
Of the only four previously reported catches of megamouth sharks in the Atlantic Ocean, two were juveniles measuring 184–190 cm total length (Séret, Reference Séret1995; Amorim et al., Reference Amorim, Arfeli and Castro2000), and the other two were a 529 cm male and a 457 cm total length female (Lima et al., Reference Lima, Rennó and Siciliano2009; Rodriguez-Ferrer et al., Reference Rodriguez-Ferrer, Shivji, Wetherbee, Schärer, Lilyestrom and Zegarra2017).
The megamouths caught near the Gabon and Mauritania coasts were females of around 180 cm while the one caught in the Indian Ocean was a specimen of 300 cm in total length. According to the life-history parameters males become mature at about 400 cm total length and females at about 500 cm, therefore these three individuals can all be considered as juveniles (Compagno, Reference Compagno2001; Nakaya, Reference Nakaya2010). Both sharks in the Atlantic, although in different years, were caught at similar times of the year (July and August). Noteworthy are the few reported catches or stranded individuals of megamouth in the Atlantic Ocean. There are not many mentions of juveniles of this species, but it is important to point out that they have been found in the three oceans and it is also remarkable that four of the six records reported until now in Atlantic waters have been juveniles. In this sense, the juveniles caught by Séret (Reference Séret1995) and Amorim et al. (Reference Amorim, Arfeli and Castro2000), and the two reported in this paper were all caught by commercial vessels targeting pelagic fish, operating in areas relatively near the coast, while the shark reported by Lima et al. (Reference Lima, Rennó and Siciliano2009) was an adult stranded on a beach on the south-western coast of Puerto Rico.
On the other hand, of the seven records in the Indian Ocean, four were juveniles of less than 350 cm, two adults of around 500 cm and the last one could not be estimated in length.
According to a recent revision of the habitat distribution of this species, megamouth shark can be found in all oceans at latitudes from 40°N to 40°S (Watanabe & Papastamatiou, Reference Watanabe and Papastamatiou2019). Using records reported until 2010, Nakaya (Reference Nakaya2010) found that specimens smaller than 2.5 m were seemingly confined to lower latitudes between 27°39′N and 27°08′S that are in the latitudinal range of the individuals reported in this paper.
The interaction of the purse seine fleet with this species has been documented in various sources, with confirmed specimens caught by purse seiners in the Atlantic and Japanese waters (Séret, Reference Séret1995; Yano et al., Reference Yano, Yoshitaka, Tanaka, Tsukada, Furuta, Seret and Sire1999; Sawamoto & Matsumoto, Reference Sawamoto and Matsumoto2012).
However, other studies related to the bycatch of the tropical tuna purse seine fleet did not record M. pelagios. In this sense, Amandé et al. (Reference Amandé, Ariz, Chassot, Chavance, De Molina, Gaetner, Murua, Pianet and Ruiz2010) studied the bycatch for the European (i.e. Spanish and French fleets) purse seine tuna fishery operating in the Atlantic Ocean during the period 2003–2007, and Hall & Roman (Reference Hall and Roman2013) reviewed the information available in published papers, or documents presented at the tuna regional fishery management organizations (RFMOs) related to bycatches in the tropical tuna purse seine fisheries of the world. Two of the specimens were caught in free school sets, while the third was caught associated with a floating object (FOB) set. In general, higher shark bycatches are found associated with FOB sets than free school sets although this can vary depending on the species (Clavareau et al., Reference Clavareau2020). Regarding the rest of the companion species in the fishing sets, we found other elasmobranchs and tuna, and/or billfishes in all cases. Among the elasmobranchs, manta rays (Mobula spp.), shortfin mako Isurus oxyrhinchus and silky shark Carcharhinus falciformis were the species identified, in addition to two pelagic stingrays Pteroplatytrygon violacea, and one unidentified carcharhiniform shark, which were caught in the Indian Ocean. These species are among the most common bycatch elasmobranch species in the tropical purse seine fleet (Amandé et al., Reference Amandé, Ariz, Chassot, Chavance, De Molina, Gaetner, Murua, Pianet and Ruiz2010; Hall & Roman, Reference Hall and Roman2013; Clavareau et al., Reference Clavareau2020). The catches of elasmobranch and billfishes were limited in all instances presented here, no more than two individuals/set, while the catches of the target species Thunnus albacares and Katsuwonus pelamis were between 2 and 21 tons.
Several international organizations (Institut de Recherche pour le Développement (IRD), Instituto Español de Oceanografía (IEO), Seychelles Fishing Authority (SFA), Arrantzuarekiko Zientzia eta Teknologia Iraskundea (AZTI)) work in a coordinated programme that aims to monitor the fishing activity of the European (and associated flags) tropical purse seine fleet operating in the Atlantic and Indian Oceans. This joint work could be very helpful in the future for the identification and bycatch reporting of M. pelagios by the commercial purse seine fleet.
Acknowledgements
We thank Sea Eye, provider company of onboard observers for the Spanish purse seine fleet in the Gulf of Guinea, for providing us with the shark photos.
Author contributions
G. Diez analysed the data, revised literature, and completed the first draft of this manuscript, Ruiz, J. provided data and revised the manuscript draft, and Salgado, A. revised literature and the manuscript draft.
Financial support
The observer programme onboard the tropical tuna purse seine fleet is partially funded through the European Maritime and Fisheries Fund (EMMF), as it is included within the mandatory European Regulation for the Fishery Data Collection (Data Collection Framework Council Regulation (EC) No. 199/2008, and Regulation (EU) 2017/1004). Additionally, since 2015, the monitoring programme has been financed largely by the industry itself.
