Scalloped hammerhead

Sphyrna lewini is a hammerhead shark of the family Sphyrnidae, described by Griffith and Smith in 1834 ^[1]. FAO gives its Spanish name as «cornuda común» and regards it as probably the most abundant hammerhead: a coastal and semi-oceanic shark of tropical and warm-temperate seas ^[2]. The IUCN Red List classes it as Critically Endangered, because its global population has probably fallen by more than 80 % ^[3].

It is a large hammerhead. The head is very wide and short from front to back, its width between 24 and 30 % of total length; its front edge forms a broad arch with a well-marked notch in the centre ^[2]. The first dorsal fin is moderately falcate and starts over the rear of the pectorals or slightly behind; the second dorsal is low and its inner margin, about 2 times its height, reaches almost to the start of the tail fin; the pelvic fins are not falcate and the anal fin has a deeply notched rear edge ^[2]. It is grey-brown above and white below, with dusky or black tips to the pectoral fins ^[2].

Similar species

• Sphyrna zygaena: The smooth hammerhead has no notch in the centre of the head's front edge, and the free tip of its second dorsal sits well ahead of the start of the tail fin ^[2].
• Sphyrna mokarran: In the great hammerhead, the front edge of the head is almost straight in adults; its first dorsal is also markedly falcate, its pelvic fins are tall and falcate, and its teeth are strongly serrated at every size ^[2].

It lives in nearly all the world's tropical and warm-temperate coastal seas; in the eastern Pacific, FAO recorded it from southern California and the Gulf of California to Panama and Ecuador, and marked northern Peru with a question mark ^[2]. An Ecuadorian study places its southern limit in the eastern Pacific in northern Peru ^[4]. The 2015 updated checklist of chondrichthyans of the Peruvian sea, with 115 species, includes it among the 6 Sphyrna species recorded for the country ^[5].

It occupies continental and island shelves and the deep water next to them; it often comes very close inshore and enters bays and estuaries ^[2]. It has been found from the surface down to at least 275 m, and young animals live mostly near the coast ^[2].

It is viviparous, with a placenta formed from the yolk sac, and bears litters of 15 to 31 pups that are born at 42 to 55 cm ^[2]. Males mature at 140 to 165 cm and females at around 212 cm; maximum size is about 370 to 420 cm ^[2]. Off Hawaii, adults enter Kaneohe Bay to give birth and mate, and the pups move out into deeper water as they grow ^[2].

In Golfo Dulce, on the Pacific coast of Costa Rica, 315 hammerheads caught by small-scale fishers between May 2010 and May 2011 were analysed: all were pups or juveniles, 46 to 140 cm in total length with a mean of 74.3 cm, and there were 1.2 males per female ^[6]. Catch per unit effort rose in July and August, when mean size was smallest (64 cm), and the authors conclude that pregnant females probably arrive to give birth in the first months of the rainy season ^[6].

In the Ecuadorian Pacific, the stomachs of 116 sharks landed at Manta during 2004 were examined: they held 34 prey types, 15 of them molluscs (mostly cephalopods), 18 fishes and 1 crustacean ^[4]. By the index of relative importance, the main prey were squids of the genus Histioteuthis (22.7 %) and the jumbo squid Dosidicus gigas (21.9 %); the authors see it as a hunter of open-ocean prey, although juveniles also eat bottom-dwelling fishes and crustaceans near the coast ^[4].

It forms large schools at different stages of life; in the Gulf of California, groups of both sexes with a majority of females have been watched over seamounts and beside islands, with behaviours such as head-shaking and corkscrew swimming; what these schools are for remains unclear ^[2]. At Wolf Island in Galápagos, hammerheads fitted with transmitters spent a disproportionate share of their time at the island's southeastern corner, the same spot where censuses found the most tunas, jacks and other species; the authors propose using hammerheads as a sign of these focal points of pelagic life ^[7].

Its global category on the IUCN Red List is Critically Endangered: the worldwide population has probably fallen by more than 80 % and is highly fragmented ^[3]. The main cause is fishing, which takes it both as a target and as bycatch on longlines, in purse seines and in gillnets, and keeps its meat and fins ^[3].

It is listed in CITES Appendix II, together with Sphyrna zygaena and Sphyrna mokarran ^[8]. In Golfo Dulce, where the small-scale fishery catches mostly pups and juveniles, closing the fishery from June to August, the season when pups are born, at the sites with the highest catches was recommended ^[6].

Not medical advice. For a sting, a poisoning or an allergy, seek medical care.

In muscle from sharks from 4 states on Mexico's Pacific coast (Baja California Sur, Sinaloa, Nayarit and Colima), mean concentrations were 0.06 µg/g of cadmium, 0.94 µg/g of selenium and 1.56 µg/g of mercury, by dry weight; mercury and cadmium increased with size ^[9]. Hazard quotients and indices were below 1, so the authors see no risk to people who eat it, although the molar ratio between mercury and selenium indicates that meat from Baja California Sur sharks might not be beneficial ^[9].

FAO regarded it as probably dangerous to people, though without certainty, because for a long time it was confused with Sphyrna zygaena and several incidents could only be put down to hammerheads in general ^[2]. With bait in the water these sharks approached divers and soon left, and the schools of adults in the Gulf of California proved shy and hard to approach on scuba ^[2].

It is probably the most common hammerhead in the tropics; both inshore artisanal and small commercial fisheries and offshore operations take it, with pelagic and bottom longlines, fixed bottom nets and bottom and pelagic trawls, and the pups are easily taken on light longlines ^[2]. Its meat is eaten fresh, frozen, dried and salted or smoked; the fins go into soup, the skin into leather, the oil into vitamins and the remains into fishmeal ^[2].

An analysis of shark landings in Peru from 1950 to 2010 counts it among the 33 shark species that interact with Peru's small-scale fishery, although its table gives it no recorded landings; instead, from 1996 to 2010, 98 % of landings came from 6 other species: blue shark, shortfin mako, smooth hammerhead, smooth-hound, common thresher and angel shark ^[8].

Published information on the species in Peru is scarce: the 2015 chondrichthyan checklist includes it for the country on the strength of a 2001 catalogue ^[5], the 1984 FAO catalogue marked northern Peru with a question mark ^[2], and a 2009 study places the southern limit of its eastern Pacific range in northern Peru ^[4].