Scalloped Hammerhead

fieldmarks: A large hammerhead with a broad, narrow-bladed head, anterior margin of head very broadly arched in adults and with a prominent median indentation, teeth with moderately broad cusps and smooth to weakly serrated edges, moderately falcate first dorsal fin with origin over or behind pectoral insertions and free rear tip in front of pelvic origins, low second dorsal fin with weakly concave posterior margin, long posterior margin about twice fin height, and free rear tip nearly or quite reaching upper caudal origin, non-falcate pelvic fins, a deeply notched posterior anal margin, and dusky or black-tipped pectoral fins. Expanded prebranchial head hammer- or axe-shaped and very wide but longitudinally narrow, its width 24 to 30% of total length (mostly above 26%), distance from tip of snout to rear insertions of posterior margins of expanded blades less than half width of head; anterior margin of head very broadly arched, with prominent medial and lateral indentations; posterior margins of head long, angled posterolaterally, and generally broader than mouth width. Well-developed prenarial grooves present anteromedial to nostrils. Preoral snout about 1/5 to 1/3 of head width. Rear of eyes slightly anterior to upper symphysis of mouth. Mouth rather broadly arched. Anterior teeth with moderately long stout to slender cusps, smooth or weakly serrated, posterior teeth mostly cuspidate and not keeled and molariform. Pelvic fins not falcate, with straight or slightly concave posterior margins. First dorsal moderately falcate, origin above or slightly behind pectoral insertions, free rear tip well anterior to pelvic origins. Second dorsal fin low, less than anal height, with a shallowly concave posterior margin; inner margin long, about twice fin height, and ending almost opposite upper caudal origin. Anal fin larger than second dorsal fin and rather long, base 4.3 to 6.4% of total length, origin well ahead of second dorsal origin, posterior margin shallowly concave to nearly straight. Total vertebral centra 174 to 209. A large hammerhead, to over 3 m. Colour gray-brown above, white below, with dusky to black pectoral fin tips.

Maximum total length about 370 to 420 cm, males maturing at 140 to 165 cm and reaching at least 295 cm, females maturing at about 212 cm and reaching at least 309 cm; size at birth 42 to 55 cm.

Probably the most abundant hammerhead, a coastal-pelagic,semioceanic warm-temperate and tropicalspecies occurring over continental and insular shelves and in deep water adjacent to them, often approaching close inshore and entering enclosed bays and estuaries. Ranges from the intertidal and surface down to at least 275 m depth. Young sharks primarily occur close inshore.Forms large true polarized schools at different stages of its life-history, though solitary individuals of both young and adults also occur. This species is apparently highly mobile and in part migratory, and forms huge schools of small migrating individuals that move polewards in the summer in certain areas such as off Natal, South Africa. Elsewhere, as in the East China Sea, it may not migrate and is thought to form large resident populations. Adult males and females may segregate during certain phases of their life-cycle. Off southern Baja California, in the Gulf of California, polarized schools of scalloped hammerheads of mixed sexes with females predominating and sizes from immatures of slightly less than a meter to adults over 3 m have been intensely observed underwater by A. Peter Klimley and Donald R. Nelson. These congregate offshore over seamounts and near islands, and show a considerable range of behaviours including lateral tilting of the body (possibly to enhance the shark's view of divers when approached from above and behind them); accelerated swimming variants with headshaking, thrusting the midsection while swimming rightside up or upside down, and corkscrew swimming with rotation around their longitudinal axes; hitting other hammerheads with their snouts; jaw opening; and clasper flexion. Some of these displays may involve aggression or courtship. Many females bear apparent courtship scars, but a smaller proportion of males have them too. The function of these schools is uncertain: reproduction is thought unlikely because of the presence of juveniles in the schools; defence unlikely because of the absence of possible predators on the hammerheads; and grouping for attaining a swimming advantage in the strong currents that are common in these places unlikely because the sharks school when currents are absent. Feeding advantages may occur for the sharks to cluster near food resources or even for social feeding, but so far this is hypothetical because the sharks have never been seen to feed in the daytime when observations can be made though they may do so at night. Sharks have been tracked and may wander off from the schooling area. Viviparous, with a yolk-sac placenta; number of young in a litter 15 to 31. Off Hawaii adults move inshore in Kaneohne Bay, Oahu to drop young and mate. The smallest young are found close inshore in the bay but these move into deeper water as they grow, to eventually depart for open water. The scalloped hammerhead takes a wide variety of fish prey, but also invertebrates (especially cephalopods ). Food items include sardines and herring, anchovies, ten-pounders (Elopidae), conger eels, milkfish, sea catfish, silversides, halfbeaks, mullet, lizardfish, barracuda, bluefish, spanish mackerel, jacks, porgies, mojarras, cardinal fishes, goatfish, grunts, damselfishes, parrotfishes, wrasses, butterfly fishes, surgeonfish, gobies, flatfish, sharpnose sharks (Rhizoprionodon ), blacktip reef sharks, angel sharks, stingrays, squid, octopus, cuttlefishes, sea snails, shrimp, mantis shrimp, crabs, lobsters and isopods .

This is probably the commonest hammerhead in the tropics and is readily available in abundance to inshore artisanal and small commercial fisheries as well as offshore operations; it is caught with pelagic longlines, fixed bottom longlines, fixed bottom nets, and even bottom and pelagic trawls ; the young are easily caught on light longline gear. Only 2 t of catches in 1994 and 12 t in both 1995 and 1996 have been reported to FAO for this species, all by Guinea-Bissau in Eastern Central Atlantic (area 34). The total catch reported for this species to FAO for 1999 was 10 t. The countries with the largest catches were Guinea-Bissau (10 t). The meat is utilized fresh, fresh-frozen, dried salted and smoked for human consumption; the fins are used to prepare shark-fin soup base; the hides are prepared into leather; the oil used for vitamins; and carcasses for fishmeal. Impact of fisheries Scalloped hammerhead sharks are commonly taken in coastal fisheries around the world. Because this species is known to form very large aggregations, it is easy for fishermen to occasionally take extremely large catches in a single fishing event. Purse seiners have been reported to catch as much as 35 t of scalloped hammerhead sharks in a single set (Stretta et al. 1997). Pirate fishermen are known to take advantage of the gregarious nature of scalloped hammerheads and illegally fish large quantities of this species in the Galápagos Islands Marine Reserve (Lavenberg et al. 1994, Camhi 1994). There are virtually no catch statistics for this species throughout the world, but Chen et al. (1988) report that off NE Taiwan (Province of China) where this is the most commercially important shark, at least 500 t/y were fished in the mid 1980s. Logbook data from the Atlantic US longline fisheries (Cramer 1998) shows CPUE of hammerhead sharks (3 different species, of which scalloped hammerhead sharks are likely the major part) decreasing from about 2.7 sharks/1000 hooks in 1986 to 0.35 in 1997. Brown (1998) reports non-standardised CPUE for unclassified hammerheads in the sport fishery off Virginia-Massachusetts that show wide yearly variations with an overall downward trend between 1986-97. However, scalloped hammerhead sharks are poorly represented in this sport fishery, as they tend to have a more southerly distribution. Scalloped hammerheads are also under pressure from heavy fishing in their nursery areas because these commonly occur in shallow coastal waters where heavy fishing takes place. Large quantities of newborns and juveniles of this species are taken by fisheries in nursery areas in the Gulf of México (Bonfil 1997), Mauritania (Ducroq 1998), and probably elsewhere throughout their range. Conservation Status : Smith et al. (1998) found that scalloped hammerheads have one of the lowest intrinsic rebound potentials among 26 shark species they analysed. This means that fisheries for scalloped hammerheads should be conducted under very tight management and monitoring to avoid stock collapses. The worldwide distribution and known high abundance of this shark offers a relative protection against the threat of extinction, however the occurrence of local depletion is a very real threat. Mooney-Seus and Stone (1996) consider scalloped hammerhead sharks as Reduced in US Atlantic waters and Data Deficient elsewhere. Additional information from IUCN database Additional information from CITESdatabase