Leading edge of hammer curved with well defined central and lateral indentations (scalloping). First dorsal rear margin almost straight. Free rear tip does not reach origin of pectoral fins. Second dorsal fin small and low. Free rear tip long; extending almost to pre-caudal pit. Pelvic fin posterior margins not noticeably falcate. Ventral surfaces of pectoral fins have dusky tips; darker in adults. Body greyish-brown to olive-brown above. Ventral surface pale.
Maximum length approximately 350cm. Reports of a 420cm specimen are possible but unconfirmed. 42-55cm at birth.
Inshore to offshore sea mounts. Adults are more pelagic. Intertidal to 275 meters.
Circumtropical/temperate in coastal and oceanic waters.
All life-stages are vulnerable to capture as both target and bycatch in fisheries: large numbers of juveniles are captured in a variety of fishing gears in near shore coastal waters, and adults are taken in gillnets and longlines along the shelf and offshore in oceanic waters. Population segregation and the species? aggregating habit make large schools highly vulnerable to fisheries and means that high CPUEs can be recorded, even when stocks are severely depleted.
Hammerhead shark fins are more highly valued than other species because of their high fin ray count, leading to increased targeting of this species in some areas. Where catch data are available, significant declines have been documented: both species-specific estimates for S. lewini and grouped estimates for Sphyrna spp combined suggest declines in abundance of 50-90% over periods of up to 32 years in several areas of its range, including South Africa, the northwest and western central Atlantic and Brazil.
Interviews with fishermen also suggest declining trends. Similar declines are also inferred in areas of the species? range from which specific data are not available, but fishing pressure is known to be high. Although S. lewini is relatively fecund compared to other large sharks (with litters of 12-38 pups) the generation period is greater than 15 years in the Gulf of Mexico and its life-history characteristics mean that it resilience to exploitation is relatively low. Given the major declines reported in many areas of this species? range, increased targeting for its high value fins, low resilience to exploitation and largely unregulated, continuing fishing pressure from both inshore and offshore fisheries, this species is assessed as Endangered globally.
Citations and References
Baum, J., Clarke, S., Domingo, A., Ducrocq, M., Lamónaca, A.F., Gaibor, N., Graham, R., Jorgensen, S., Kotas, J.E., Medina, E., Martinez-Ortiz, J., Monzini Taccone di Sitizano, J., Morales, M.R., Navarro, S.S., Pérez-Jiménez, J.C., Ruiz, C., Smith, W., Valenti, S.V. & Vooren, C.M. 2007. Sphyrna lewini. In: IUCN 2012. IUCN Red List of Threatened Species. Version 2012.2. <www.iucnredlist.org>
A viviparous species with yolk-sac placenta. Litter size up to 44 but usually between 10-22.
Diet consists mainly of stingrays, small sharks, a variety of bony fishes and crustaceans.
Although Scalloped hammerheads are encountered individually, they have been well documented swimming in huge schools around sea mounts and offshore drop offs where they approach cleaning stations to rid themselves of parasites.
Why they swim in polarized schools is unclear as it is an unusual behavior among sharks. However, divers have witnessed scalloped hammerheads in coitus (two individuals coupling and crashing into the reef before separating) so their schooling behavior may be primarily related to reproduction.
Reaction to divers
Shy and easily spooked by loud noises such as diver’s bubbles. Very difficult to approach when viewed in schooling behaviour. Divers often have better success when using rebreathers.
Discretely positioning yourself next a cleaning station and trying to breathe slowly generally yields the best results.
Can be aggressive making close passes in baited situations.
Rarely seen by snorkelers.
Schooling scalloped hammerheads can be encountered at numerous popular dive sites around the world. Generally speaking, the encounters are seasonal but stragglers may stay year round, some locations may be good all year. Temperature seems to play an important role i.e. if it is too warm, the hammerheads descend to deeper cooler water and visa versa.
Seamounts and offshore islands in the eastern tropical and temperate Pacific Ocean are particularly good. The northern Galapagos Islands (wolf and Darwin), Malpelo Island off of Panama, Cocos Island in Costa Rica, and the Revillagigedo Archipelago (aka Socorro) south of Baja are all well known hot spots.
All of the above mentioned spots are only accessible by liveaboard.
In the Sea of Cortez, Gorda Bank near Cabo, and El Bajo near La Paz were once very good spots but over-fishing has significantly reduced the shark populations those areas.
In the Central Pacific, Nuku Hiva in the Marquesas Archipelago delivers reliable close encounters.
In the Western Pacific, Japan has some seasonally excellent schooling hammerhead sites such as, Yonaguni Island at the western tip of the Ryukyu Archipelago, and Mikomoto Island on the Izu Peninsula.
Hammerheads are also encountered on deep drop offs near Malapascua Island in the Philippines.
In Florida in the Western Atlantic, shark-feeding operators sometimes inadvertently attract scalloped hammerheads. Although the encounters are unpredictable, this may be thee best way to enjoy extremely close encounters.
Note: El Nino years may see less hammerheads as the water becomes too warm at recreational scuba depths and the sharks stay deeper. Following El Nino years may see the best schooling activity as the sharks are then in dire need of the cleaning stations that they were unable to reach the previous year.
Great hammerhead distinguished by straighter leading edge of hammer, proportionately taller dorsal fin, and often larger size.
Smooth hammerhead distinguished by a leading edge of hammer which has no central indentation.
Carolina hammerhead Indistinguishable in the field. Examination of dead specimens will reveal approximately 10 fewer vertebrae than a scalloped hammerhead. Range restricted to the western Atlantic.