Smooth hammerhead

The smooth hammerhead (Sphyrna zygaena) is one of about ten species of hammerhead shark and one of the three large ones, alongside the great and scalloped hammerheads. The 2019 Red List assessment describes it as a large coastal and semi-oceanic shark with a maximum size of 370 to 400 centimeters (12 to 13 feet). It is also the least known of the three. Of the large hammerheads, the scalloped hammerhead is by far the most well studied, followed by the great hammerhead and the smooth hammerhead. The Red List assessment notes that despite its widespread occurrence, biological data on this species is limited.

The front edge of the cephalofoil, the hammer-shaped head, is one rounded lobe, without the prominent central indentation of the scalloped hammerhead, and the great hammerhead’s head is straight. Where smooth and scalloped hammerheads overlap, researchers also check the fins: on a smooth hammerhead, the bases of the anal fin and the second dorsal fin are of similar size. Our guide to the types of hammerhead shark covers the other species, and why hammerheads have a hammer covers what the head may be for.

The smooth hammerhead occurs worldwide in temperate seas and, in some regions, in tropical seas. It tolerates cooler water than the scalloped and great hammerheads, reaching as far north as Nova Scotia and the British Isles. It lives on the continental shelf to at least 200 meters (660 feet) deep, and possibly 500 meters (1,640 feet). The 2019 Red List assessment describes it as the most oceanic of the hammerhead species, commonly leaving coastal habitats at 2 to 3 years of age. A newer, regional estimate sits at the low end of that range. A 2023 mercury-isotope study of juveniles caught off the Pacific coast of Baja California Sur, Mexico, estimated that young smooth hammerheads there rely on coastal food for about two years before migrating offshore. Juveniles appear to prefer coastal waters, and adults offshore waters.

Despite that oceanic habit, populations in different oceans are genetically distinct. A global study of mitochondrial DNA, which is passed down the maternal line, found very strong population structure among the Atlantic, Indian, and Pacific Oceans. It concluded that the species is made up of six populations worldwide and recommended that each be managed separately. Past climate probably helped shape that pattern. Modeling suggests that the two main lineages diverged about 7 million years ago and were isolated in separate glacial refuges, areas where populations persisted through ice ages, in the Atlantic and the Indo-Pacific. Structure appears at smaller scales too. In South Africa, juveniles from the warm-temperate south coast were genetically different from those on the subtropical east coast. Young sharks sampled in Mossel Bay, which the authors call a putative, or suspected, pupping ground, differed genetically between the two years sampled. In the northern Mexican Pacific, by contrast, a 2024 study of 92 smooth hammerheads found no population structure in neutral genetic markers, those not thought to be shaped by natural selection. The authors suggest that this may be associated with the high dispersal capacity of the species. Markers putatively under selection, however, revealed a pattern of adaptive variation between sharks from the Gulf of California and those on the Pacific coast. Sharks within one well-connected population may therefore still be locally adapted.

In the Eastern Pacific, smooth hammerheads share the mainland coast of South America with scalloped hammerheads. A 2014 review of the Galápagos Marine Reserve’s sharks and rays, led by Dr. Alex Hearn, now of the Universidad San Francisco de Quito, reported that the smooth hammerhead was the fourth most commonly landed shark at Ecuador’s mainland ports, at 11%. Chemical tracers in sharks landed at the port of Manta showed that the two species have low overlap in their feeding niches, suggesting they use different feeding areas even though they eat similar prey. Farther south, young smooth hammerheads use three nursery areas off northern Peru whose feeding niches differ enough that the authors recommend managing each nursery area as a unique juvenile stock.

Satellite tracking has shown that smooth hammerheads can travel far. Seven satellite-tagged sharks in the tropical Northeast Atlantic covered 131.7 to 6,610 kilometers (82 to 4,107 miles). The longest journey was made by an adult female that traveled 6,610 kilometers (4,107 miles) over 150 days south along the west coast of Africa. It was the first trans-equatorial migration ever recorded for the species and, when the study was published in 2018, the longest migration recorded for a smooth hammerhead. The authors suggest she might have been drawn by feeding in the Benguela system, one of the most productive marine systems, and note that she moved progressively into cooler water. The tagged sharks spent most of their time near the surface, at a mean depth of 13.6 meters (45 feet). They made occasional dives to a maximum of 260.9 meters (856 feet) and a minimum temperature of 12.8 °C (55 °F).

Off the US East Coast, six juvenile females tagged off Maryland were tracked for 49 to 441 days, with track distances of 1,345.8 to 7,319.5 kilometers (836 to 4,548 miles). They used a summer core area in the New York Bight and a winter core area off Cape Hatteras, North Carolina, moving between the two in autumn. Track distance sums the distances between successive estimated positions, so the longest track, accumulated over 441 days of these seasonal movements, is not directly comparable with the single southward journey off Africa. For the four sharks tracked into the following winter and spring, 315 of 326 locations fell within the Mid-Atlantic Shark Area off North Carolina, which was then closed to commercial bottom longlines from January 1 to July 31. The authors suggest starting that closure on December 1 instead, and considering both core areas for Essential Fish Habitat designation, the US designation for waters a fish species needs to breed, feed, or grow to maturity. The schooling and migration chapter compares these journeys with those of the other hammerheads.

Stomach contents show that the major prey of smooth hammerheads is cephalopods, mainly ommastrephid (flying) squid, and small schooling fishes. Off southern Africa, cephalopods of the continental shelf (neritic species) and of the open ocean made up 55.81% and 21.31% of the wet mass of prey in stomachs that contained food. In the eastern Pacific, the diet is dominated by jumbo flying squid and Patagonian squid. The feeding chapter compares their diet with that of the other hammerheads.

Smooth hammerheads are both predator and prey for marine mammals. In New Zealand’s Bay of Islands in 2002, killer whales were seen catching a smooth hammerhead about 1 meter (3 feet) long, the first report of the species as killer whale prey. Off Rio de Janeiro, Brazil, an aerial survey in 2012 recorded a group of about 100 pantropical spotted dolphins in water 2,400 meters (about 7,900 feet) deep. The dolphins were being pursued by at least three smooth hammerheads, estimated at 3.0 to 3.5 meters (10 to 11 feet) long. The authors noted that hammerheads appear to rarely include dolphins in their diets, but that they could still shape the behavior of open-ocean dolphins.

Smooth hammerheads give birth to live young. The 2019 Red List assessment gives litters of 20 to 50 pups and a gestation of 10 to 11 months, although how often females breed is unknown. Pups measure 49 to 63 centimeters (19 to 25 inches) total length at birth. Males mature at 250 to 260 centimeters (8.2 to 8.5 feet) total length and females at about 246 to 265 centimeters (8.1 to 8.7 feet). Female age at maturity is estimated at 15 years. The maximum ages observed in the age study the assessment cites were 24 years for females and 25 years for males. None of the sharks in that study was close to full size, however, so these are unlikely to be the true maximum ages.

Despite that slow maturity, the smooth hammerhead has an estimated population growth rate of 0.225 per year, among the fastest known for pelagic sharks and the fastest of the hammerhead species. Young sharks grow up in coastal nurseries such as those off northern Peru, and the baby hammerhead sharks chapter covers nurseries across the family.

The 2019 Red List assessment found that the smooth hammerhead has undergone steep historic declines in the Atlantic, but the introduction of management measures may be allowing slow recovery. It found lesser declines in the South Pacific and increases in the Indian Ocean. In the Northwest Atlantic, modeled abundance fell by 7.0% a year from 1981 to 2005. However, a pelagic longline observer time series for 1992–2017 showed the population had slowly begun to increase after management was introduced after 2005. Catch rates in South Africa’s bather-protection nets showed a slight, but not significant, increasing trend over 1978–2014.

Weighing these trends, the assessors inferred a global population reduction of 30 to 49% over three generations (72.3 years) and listed the species as Vulnerable. Part of the reason it fares better than the great and scalloped hammerheads is geography: its generally more temperate distribution leaves less of its range exposed to intense small-scale fishing. The global analysis of oceanic sharks that we contributed to also set the smooth hammerhead apart. Of the 18 oceanic sharks and rays with abundance data, all apart from the smooth hammerhead decreased in abundance over the past half-century. That window is shorter than the three generations (72.3 years) behind the Red List’s inferred decline, so the two figures are not directly comparable.

The smooth hammerhead is caught globally as target and bycatch in longline, purse-seine, and gillnet fisheries, and is often retained for its fins. In some areas, such as Peru, that catch includes pregnant females and juveniles. Capture is often fatal. At-vessel mortality, the share of sharks already dead when the gear is hauled, was estimated at 71% on Portuguese longlines in the Atlantic. In the beach-protection nets of New South Wales, Australia, mortality was about 95% before 2010, with nearly all of the catch juveniles.

Its fins are prominent in the trade. In Hong Kong’s retail market in 2014–2015, smooth hammerhead made up an estimated 1.7% of fin trimmings, ranking fourth among all species. In 2015, it accounted for 35% by weight of the CITES-listed shark fins legally imported into Hong Kong. Those documented imports were so small compared with the hammerheads’ share of the retail market that the authors suggest listed species were often imported without CITES documentation that year. In the Galápagos, when the cargo ship Fu Yuan Yu Leng 999 was detained inside the marine reserve in 2017, a team including Hearn identified 67 smooth hammerheads (7.2%) among 929 sampled carcasses. Of the 46 smooth hammerheads measured, 43 were judged sexually mature from their length. Contaminants are a further concern. Muscle from four male smooth hammerheads caught in the Ionian Sea in July 2001 held 8.55 to 21.07 micrograms of mercury per gram (wet weight), most of it as methylmercury. The authors also measured other metals and PCBs, a group of industrial pollutants, and judged the overall pollutant load in these sharks considerable.

Protection has grown since 2013. The smooth hammerhead 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. In 2018 it was added to Annex 1 of the Convention on Migratory Species’ Memorandum of Understanding for Migratory Sharks. In August 2013, Ecuador banned its industrial fleet from retaining scalloped or smooth hammerheads. In 2016, Peru introduced a seasonal fishing ban to protect juvenile smooth hammerheads in its small-scale fisheries. In interviews with 88 gillnet fishers, 76% were in favor of the ban. The fishers also proposed changes, such as minimum sizes, that the authors argue a participatory process could take up. Elsewhere, national landing bans were rare as of 2022: a review that year found landings prohibited nationally in only 5 of the 122 countries in the species’ range.

More on these threats is in why hammerhead sharks are endangered and on our overfishing and climate change pages, and you can support our shark research in the Galápagos.