Great hammerhead

Sphyrna mokarran is the largest of the hammerhead sharks and belongs to the family Sphyrnidae, in the order Carcharhiniformes ^[1],[2]. It lives in tropical seas worldwide and, in the eastern Pacific, its range reaches northern Peru ^[3]. It is told by a tall sickle-shaped dorsal fin and a very wide head ^[2]. Its global category on the IUCN Red List is Critically Endangered ^[4].

Rüppell described it in 1837 under the name Zygaena mokarran; the reference specimen, a stuffed male of 2515 mm held at the Senckenberg museum as SMB 3590, comes from Massaua, in the Red Sea ^[3].

The FAO catalogue lists two later synonyms, Zygaena dissimilis Murray, 1887 and Sphyrna ligo Fraser-Brunner, 1950, and gives great hammerhead as its English name and cornuda gigante as its Spanish one ^[3]. The WoRMS record accepts the name Sphyrna mokarran today ^[1].

It is a large hammerhead with a very wide, short head whose width equals 23 to 27 % of total length; according to the FAO catalogue, in adults the front edge of the head is nearly straight with a notch in the middle ^[3]. The front teeth are stout and strongly serrated along their edges ^[3].

The first dorsal fin is tall and strongly falcate, with its free tip well in front of the pelvic fins; the second dorsal fin is also tall and has a deeply concave rear edge; the pelvic fins are falcate and the anal fin has a deep notch in its rear edge ^[3]. The body is grey-brown above and pale below, with no marks on the fins ^[3].

It reaches gigantic sizes: the recorded maximum is 550 to 610 cm or more, although most adults do not exceed 366 cm ^[3]. Males mature at about 234 to 269 cm and females at about 250 to 300 cm, and pups are born at 50 to 70 cm ^[3].

Similar species

• Sphyrna lewini: Sphyrna lewini has a strongly arched front edge of the head, grooves in front of the nostrils, teeth with smooth or only weakly serrated edges, a low second dorsal fin and dusky pectoral tips; Sphyrna mokarran has a nearly straight head, almost no grooves in front of the nostrils, strongly serrated teeth and a tall second dorsal fin ^[3].
• Sphyrna zygaena: Sphyrna zygaena has a broadly arched front edge of the head with no central notch at any stage, teeth with very broad cusps and smooth or weakly serrated edges, and a low second dorsal fin ^[3]; it is one of the 6 most landed shark species in Peru, with 15 % of the total between 1996 and 2010 ^[5].

It is a circumtropical species; in the eastern Pacific it is reported from southern Baja California and the Gulf of California to Panama, Ecuador and northern Peru ^[3]. It appears in the updated checklist of chondrichthyans of the Peruvian sea ^[6].

It is coastal and semi-oceanic, found both close to shore and well offshore, over continental shelves and island terraces, at depths from the surface to more than 80 m, and it often favours coral reefs ^[3]. According to a recent study, it tends to choose shallow shelf waters less than 30 m deep, although it makes long migrations into pelagic waters ^[7].

It is viviparous, with a yolk-sac placenta; litters hold 13 to 42 pups, gestation may last at least 7 months and pups are 50 to 70 cm long at birth ^[3].

An article about an animal tagged off Florida, citing earlier studies, notes that the female reproduces only every 2 years and that ages of up to 44 years have been estimated, among the oldest known for elasmobranchs, which makes the species vulnerable to overfishing ^[2].

It takes a wide variety of prey, but its menu leans toward rays of the stingray group and their relatives, serranid fishes such as groupers, and marine catfishes; it also eats other sharks, among them smooth-hounds of the genus Mustelus, as well as crabs and squid ^[3]. Animals have been found with about 50 stingray spines stuck in the mouth, throat and tongue, and this and other large hammerheads were the first to reach newly baited longlines, which points to a keen sense of smell ^[3].

In a Florida nursery area, among animals of 63 to 287 cm fork length, the young depended on bay stingrays, supplemented by bony fishes, and around 125 cm fork length, that is about 2 years of age, they shifted in part to coastal prey ^[7].

It appears to be nomadic and migratory, and some populations move poleward in summer ^[3]. A 250 cm male fitted with a satellite tag travelled at least 1200 km in 62 days, into international waters of the northwest Atlantic ^[2].

Its global category on the IUCN Red List is Critically Endangered ^[4]. It is listed in Appendix II of CITES ^[5].

The large fins of hammerheads are highly sought after by the fin trade, which is considered a major driver of the worldwide decline of sharks, and in the Atlantic hammerhead stocks, this species included, have been estimated to have fallen by more than 89 % between 1986 and 2000 ^[2]. In the US bottom longline fishery an at-vessel mortality of more than 90 % has been reported, and confusion between hammerhead species and imprecise fishery records make it hard to assess its status ^[2].

In Peru it appears in the table of sharks reported in the country's fishery, but with no landings recorded, since landing statistics covered only 18 species ^[5]. That paper states that, according to the Peruvian government (cited as 2014), no shark species of its waters was categorized as threatened ^[5]. Misidentifications are common at port and likely show up in official reports ^[6].

It is regularly caught in the tropics with longlines, fixed bottom nets and hand lines; its meat is used fresh, frozen, dried-salted or smoked, its fins for soup, its hide for leather, its liver oil for vitamins and its carcasses for fishmeal ^[3].

It is considered dangerous to people, but hardly any attack can be attributed to it with certainty because the large hammerheads are hard to tell apart; without bait it has approached divers without aggression ^[3]. Peru's landing records for 1996 to 2010 list no catches of this species ^[5].

The 2015 checklist of Peruvian chondrichthyans includes this species for the Peruvian sea without giving specific localities ^[6]. The shark landing statistics for 1996 to 2010 recorded no catches of it, so that source gives no places where it is found ^[5].