Great hammerhead

The great hammerhead (Sphyrna mokarran) is the largest of about ten species of hammerhead shark. It is a coastal and semi-oceanic shark of tropical and warm-temperate seas, and where the scalloped hammerhead forms large schools, the great hammerhead is generally solitary. The three large hammerheads are told apart by the front edge of the cephalofoil, the hammer-shaped head, which in the great hammerhead is straight. Build differs too: the great hammerhead has a much larger body and a disproportionately large, sickle-shaped dorsal fin than the scalloped and smooth hammerheads. Our guide to the types of hammerhead shark covers the other species, and why hammerheads have a hammer covers the head itself.

The size of great hammerheads deserves care, because the figure most often repeated is the extreme. The 2019 Red List assessment cites a maximum of 550 to 610 centimeters (18 to 20 feet) reported in 1984, but notes that 400 centimeters (13 feet) total length is a more commonly observed maximum. Recent studies have measured great hammerheads well short of the historical extreme, although some exceeded 4 meters (13 feet). South Florida’s drumline surveys use a baited hook on a line anchored to a submerged weight and marked by a surface buoy. The largest great hammerhead they caught between 2009 and 2021 measured 450 centimeters (15 feet). The largest of 31 tagged in Queensland and Western Australia measured 455 centimeters (15 feet), and laser measurements of free-swimming sharks in French Polynesia recorded total lengths up to 400 centimeters (13 feet). The “20-foot hammerhead” question is taken further in how big hammerhead sharks get.

Great hammerheads range worldwide throughout tropical and warm-temperate seas, and occur close inshore and well offshore, from near the surface to 300 meters (about 1,000 feet). Genetic studies support two subpopulations, one in the Atlantic and one in the Indo-Pacific. In the Galápagos, the species has not been confirmed. A 2014 review of the marine reserve’s sharks and rays, led by Dr. Alex Hearn, now of the Universidad San Francisco de Quito, noted that the great hammerhead had been listed only on the basis of plausible yet unconfirmed sightings.

Warm water suits them. Accelerometers, which record a shark’s movement, were deployed on nine great hammerheads and estimated an optimal temperature of 29.3 °C (84.7 °F). However, their performance changed less with temperature than that of other large sharks, enabling high performance across a relatively broad temperature range. The authors point out that how performance responds to temperature has implications for changes to distribution, population dynamics, and species interactions as the climate changes, a theme of our climate change page.

How far they travel varies from region to region. In the Atlantic, great hammerheads make long seasonal migrations (below). In Australia, 31 great hammerheads tagged in Queensland and Western Australia, all but one in Queensland, showed limited dispersal. Their mean maximum displacement, the farthest each shark was recorded from where it was tagged, was 105.8 kilometers (66 miles). The authors link this to the rays the sharks specialize on, which appear to be abundant year-round on the shallow flats where the sharks hunt. In French Polynesia, dive surveys at Rangiroa and Tikehau documented a female-biased aggregation of at least 55 great hammerheads, two of them seen at the same atoll over up to 12 years. In Saudi Arabian waters of the Arabian Gulf, all 105 great hammerheads examined from commercial fishery catches were immature. The authors suggest that young sharks may live in shallow habitats there for their first three years.

Tagging at Bimini, in the Bahamas, and at Jupiter, Florida, recorded numerous international movements and provided the first evidence of philopatric behavior (return migrations, seasonal residency, and long-term site fidelity) in the species. Some sharks made return migrations from as far north as Virginia, a round trip of about 3,000 kilometers (1,900 miles). At Bimini, residency peaked in winter, and the three sharks that made long migrations left from April to early May, as the water warmed above 26 °C (79 °F). Not every shark migrates. Of great hammerheads tagged in the Florida Keys, some made repeatable round-trip migrations to the Gulf of Mexico or the Atlantic coast, while others remained in the Keys year-round, a pattern known as partial migration.

In the standardized drumline surveys from Miami to the middle Florida Keys, great hammerheads were the fifth most commonly caught shark, with 238 among 3,398 sharks of 15 species between 2009 and 2021. They were 2.6 times more likely to be caught in ocean than in bay habitats. Off Miami, tagged great hammerheads behaved as “urban adapters”, making partial use of urbanized areas rather than avoiding them. In 2021, researchers reported the first identification of a probable nursery area for young great hammerheads off the Atlantic coast of the United States, in Biscayne Bay, less than 7 kilometers (4.3 miles) from downtown Miami.

Great hammerheads have also become a draw for divers. At a dive site off South Bimini where the sharks are fed to attract them (provisioning), great hammerheads were seen on 97% of 104 dives in the 2016–2017 season. By the authors’ estimates, nine of 12 provisioned sharks were regularly able to fuel their daily energy needs from the bait alone. They believe the dives concentrate sharks at the western edge of Bimini in winter, but that the feeding does not seem to be impacting their annual migration patterns. The diving chapter covers how to see great hammerheads responsibly.

MMF’s Florida Manta Project, which studies juvenile manta rays along the Palm Beach County coast, has recorded hammerheads on its aerial and boat surveys.

Great hammerheads are apex predators that feed primarily on bony fishes and other sharks and rays, from small rays to large sharks. Stingrays dominated the diet of great hammerheads in studies from South Africa, Australia, and Florida. Off eastern Australia, chemical tracers in tissue showed that large great hammerheads are apex predators primarily relying on other sharks and rays for their diet, with a preference for cownose rays in summer. Young sharks eat differently. In Saudi Arabian waters of the Arabian Gulf, juveniles under three years old fed mainly on bony fishes, which made up 93.8% of their diet by an index of relative importance. That index combines the number, weight, and frequency of each prey type. Females shifted toward rays in their second and third years.

The head plays a direct part in catching rays. In 1988, divers off Bimini watched a great hammerhead thrust its head down onto a southern stingray, sending the ray into the bottom with great force. It then pivoted on top of the ray to bite off part of a pectoral fin. The authors knew of no earlier report of a shark using the underside of its head to restrain a ray on the bottom while pivoting to bite. In a second case near Bimini, described by Chapman and Gruber in 2002, a great hammerhead bit off a spotted eagle ray’s pectoral fin. It then pressed the ray against the bottom with its head and pivoted on top of it, a sequence the authors named “pin and pivot”. A 2016 study of great hammerheads hunting on tidal flats cites both cases as support for specialized foraging behavior in the species.

Great hammerheads also hunt in very shallow water. Six cases from Florida, the Bahamas, and Palau documented great hammerheads on tidal flats less than 1.5 meters (5 feet) deep. The sharks pursued a school of permit (a coastal fish), a lemon shark that was chasing a hooked bonefish, and a nurse shark. In three cases the shark made up to 22 tight turns before the prey was consumed, a behavior the authors termed grasp-turning. They suggest that it may help a hammerhead keep prey in its mouth and maneuver it for swallowing. Other sharks take the threat seriously: drone footage off southeast Florida shows adult blacktip sharks fleeing into the shallow water next to the beach when chased by a great hammerhead. The feeding chapter compares the diets of all the hammerheads.

Great hammerheads give birth to live young. The 2019 Red List assessment gives litters of 6 to 42 pups and a gestation of 11 months, with a likely two-year breeding cycle. Pups measure 50 to 70 centimeters (20 to 28 inches) total length at birth. Males mature at 225 to 269 centimeters (7.4 to 8.8 feet) total length and females at 210 to 300 centimeters (6.9 to 9.8 feet). The maximum ages recorded are 44 years in the Northwest Atlantic and 39 years in the Western Central Pacific.

Pups grow quickly at first. In the Arabian Gulf, the mean size at birth, back-calculated from vertebrae, was 71.2 centimeters (28 inches), and young sharks there were estimated to grow 83.3 centimeters (33 inches) a year just after birth. In the probable Biscayne Bay nursery, one recaptured juvenile grew 25 centimeters (10 inches) in 146 days. The Red List assessment notes that a long lifespan and breeding only once every two years, combined with high bycatch mortality, make the great hammerhead susceptible to depletion where it is taken in unmanaged fisheries. Our chapter on baby hammerhead sharks covers nurseries across the family.

There are no data on the global population size of the great hammerhead. The 2019 Red List assessment combined regional trends into median global reductions of 50.9 to 62.4% over three generations (71.1 to 74.4 years), with the highest probability of a reduction of more than 80%. The regions with no trend data, such as the tropical Indian Ocean and West Africa, are likely to be under intensive fishing pressure. The assessors therefore took a precautionary approach, inferring a global population reduction of more than 80% and listing the species as Critically Endangered. In the Indian Ocean, a 2020 analysis of catch rates in South Africa’s bather-protection nets from 1978 to 2003 estimated an annual decline of 6.6%. Off Queensland, the average size of great hammerheads caught in the state’s shark-control program declined by 22%, from 274 to 215 centimeters (9.0 to 7.1 feet), between 1997 and 2017.

The Northwest Atlantic shows what management can do. The US population declined dramatically through the early 2000s, but time series there show it began to increase soon after management measures were introduced after 2005. A model using North Atlantic catch-rate data updated to 2017 estimated recovery at 6.6% a year, although with high uncertainty. The global analysis of oceanic sharks that we contributed to likewise found hammerhead shark populations rebuilding in the Northwest Atlantic, owing to strictly enforced quotas throughout their US range.

The great hammerhead is caught globally as target and bycatch in longline, purse-seine, and gillnet fisheries, and is often retained for its fins. Hammerheads are in general more sensitive to capture than other shark species. For great hammerheads, at-vessel mortality, the share already dead when the gear is hauled, is estimated at 56% on US shark bottom longlines and 30.8% on Western Australian demersal longlines. The species’ fins are valuable. Hong Kong traders recognize a market category of large, light-colored fins that mainly derive from great hammerheads, and hammerheads are one of the three groups of premium species with the highest-value fins in Hong Kong. Recreational fishing matters too: two of the nine juveniles caught in the probable Biscayne Bay nursery still carried recreational fishing hooks in their jaws. Staying close to home carries its own risk: off Queensland, the sharks’ small home ranges and reliance on shallow inshore habitats may render them particularly vulnerable to localized depletion.

Protection has built up since 2010. That year, the International Commission for the Conservation of Atlantic Tunas prohibited retaining, landing, or selling great hammerheads in its fisheries. Developing countries may keep them for local consumption if they cap catches, report catch data, and keep the fins out of international trade. The General Fisheries Commission for the Mediterranean banned retention in 2012. The species was added to Appendix II of the Convention on International Trade in Endangered Species (CITES) in 2013, with the listing in force from September 2014. It was added to Appendix II of the Convention on Migratory Species in 2014. In Florida, great hammerheads are a prohibited species in state waters. A 2018 review noted that prohibiting their catch in US waters would protect the Northwest Atlantic population from fishing across over 90% of its core habitat.

The next step is national rules elsewhere: a 2022 review found landings prohibited nationally in only 8 of the 129 countries where great hammerheads occur. The Red List assessors recommend that all great hammerhead retention and landings be prohibited while the species is classified as Critically Endangered or Endangered. More on these threats is in why hammerhead sharks are endangered and on our overfishing page, and you can support our shark research.