Yes: eight of the nine hammerhead species on the IUCN Red List are threatened with extinction, six of them Critically Endangered. Both the scalloped and great hammerheads are judged most likely to have declined by more than 80% over three generations. Fishing is the cause: hammerheads are caught for their valuable fins and as bycatch, and they are generally more sensitive to capture than other sharks.
This guide is written by the scientists of the Marine Megafauna Foundation, whose work in the Galápagos and Florida brings them into regular contact with hammerheads, and draws on the published research, including Dr. Alex Hearn’s long-term tracking of scalloped hammerheads in the Eastern Tropical Pacific.
Are hammerhead sharks endangered?
Most species are. Of the nine hammerhead species assessed on the IUCN Red List, six are Critically Endangered, one is Endangered, and one is Vulnerable. The ninth is Data Deficient, meaning there is too little information to judge its risk of extinction.
The three large, wide-ranging species were last assessed in November 2018. The scalloped hammerhead (Sphyrna lewini) and the great hammerhead (S. mokarran) are Critically Endangered, and the smooth hammerhead (S. zygaena) is Vulnerable. MMF co-founder Dr. Andrea Marshall co-assessed all three. Each listing rests on how steeply the species has declined over three generations, about 72 years for a scalloped hammerhead, rather than on how many animals remain.
The smaller species, most of them coastal, are faring no better. The winghead shark (Eusphyra blochii) of the Indo-West Pacific is Critically Endangered in an assessment published in 2025, which considers it “possibly extinct” across northern Southeast Asia. Three small hammerheads of Central and South America, the scalloped bonnethead (S. corona), the scoophead (S. media), and the smalleye hammerhead (S. tudes), were all listed as Critically Endangered in assessments published in 2020. The Carolina hammerhead (S. gilberti), a look-alike of the scalloped hammerhead, is Data Deficient. The types chapter introduces each species, quick answers are on the facts page, and the hammerhead hub and guide contents link every chapter.
The bonnethead (S. tiburo) shows how much depends on management. Its 2021 assessment lists it as Endangered overall, yet the population off the United States, the Bahamas, and Atlantic Mexico is stable under management. On the Pacific side, by contrast, the species is no longer present in the Gulf of California.
How many hammerhead sharks are left in the world?
No one knows. The 2019 assessments for both the great and smooth hammerheads state that no data are available on their global population size. Scientists therefore measure trends instead, at places where hammerheads have been counted consistently for years.
At Darwin Island in the Galápagos, a study co-authored by Dr. Alex Hearn of the Universidad San Francisco de Quito combined acoustic tags with underwater surveys. On field trips in September 2011, September 2012, and October 2013, it estimated a resident population of about 500, 670, and 400 scalloped hammerheads. At Cocos Island, Costa Rica, dive guides have counted up to 1,000 scalloped hammerheads on a single dive. The schooling chapter explains where and why they gather.
Where counts go back decades, they have fallen sharply. Dive guides at Cocos Island logged 34,342 dives between 1993 and 2019, and over that period the relative abundance of scalloped hammerheads, an index of numbers rather than a head count, fell by 75%. That result extends an earlier analysis of the same logs, which had found a 45% decline by 2013. In the Galápagos, dive guides interviewed for a study co-authored by Dr. Hearn perceived a 50% decline in hammerheads between the 1980s and the 2010s. At El Bajo seamount in Mexico’s Gulf of California, long-time divers reported that the average number of scalloped hammerheads seen per dive fell from 150 in the 1970s to 5 in the 2010s. In Australia, hammerhead catch rates in Queensland’s shark-control nets fell by 92% between 1962 and 2016.
These local losses fit a global pattern. A global analysis we contributed to found that the abundance of oceanic sharks fell by 71.1% between 1970 and 2018, driven by an 18-fold rise in relative fishing pressure. Scalloped and great hammerheads are among the oceanic sharks now Critically Endangered, although the same data also show recovery where fishing has been controlled (see below).
Why are hammerhead sharks so vulnerable to overfishing?
Their biology makes them easy to catch and slow to replace. The large hammerhead species grow and breed slowly, scalloped hammerheads gather in large schools at predictable sites, and hammerheads die from the stress of capture more readily than most sharks.
The great hammerhead can live at least 44 years and likely breeds only once every two years, which, combined with high bycatch mortality, makes it susceptible to depletion wherever it is taken in unmanaged fisheries. The reproduction chapter covers litters, pregnancy, and nurseries.
In 2014, shark researchers argued that hammerheads are “evolved for extinction”: specialized traits that served the family well have become liabilities under modern fishing, “a de facto evolutionary trap”. The clearest example is schooling. Scalloped hammerheads form partially sex-segregated schools of more than 200 individuals at a few predictable places, including Cocos Island and the northern Galápagos. The behavior is thought to help males find the largest females, but because industrial fishing targets those aggregations, it now makes the species easier to catch.
Hammerheads also cope badly with being caught. Immediately after capture, they show some of the most disturbed blood chemistry recorded in sharks, which the authors link to the burst swimming they use to hunt. Scalloped hammerheads are also obligate ram ventilators, meaning they must keep swimming to push water over their gills, so a shark held on a line is highly vulnerable to dying. Even the hammer works against them: in nets, the wide, flattened head, or cephalofoil, easily becomes entangled. The why-the-hammer chapter explains what that head is for.
Are hammerhead sharks killed for their fins?
Yes. Hammerhead fins are among the most valuable in the shark fin trade. Scalloped and smooth hammerheads are among the most common species in the fin trade of Hong Kong, one of the largest in the world.
In Hong Kong, the three large hammerheads made up about 5.9% of the fin auction trade by weight in 1999–2001, a minimum estimate. Traders sorted great hammerhead fins into a high-value grade of their own. More than a decade later, in 2014–2016, scalloped and smooth hammerheads were the fourth and fifth most common species among more than 80 identified in Hong Kong’s retail fin trimmings. These trimmings are the inexpensive offcuts left when fins are prepared for sale. Hammerheads are one of three groups whose fins command premium prices, and every species with premium-value fins in Hong Kong is now threatened with extinction.
Many of those fins come from the Eastern Pacific. A pilot study that matched fin DNA to its population of origin traced 61.4% of scalloped hammerhead fin trimmings sampled in Hong Kong in 2014–2015 to the Eastern Pacific. The US Endangered Species Act lists that population as Endangered.
Trade rules also need monitoring. In 2015, the first full year of the hammerhead CITES listing, the gap between documented imports and the hammerheads’ share of Hong Kong’s market suggested that listed fins were often imported without CITES documentation. The overfishing page explains how CITES listings work.
What happens to hammerheads caught as bycatch?
Many die. Hammerheads are caught on longlines and in gillnets and purse seines set for other fish, and a large share are dead when the gear is hauled in or die after being released.
In the US Atlantic pelagic longline fishery, observer records from 1995 to 2012 gave an estimated mean survival of only 45.9% for scalloped hammerheads when the line came in, among the lowest of the 12 species monitored. The 2019 assessments report at-vessel mortality, the share of sharks already dead when the gear is retrieved, of 56% for great hammerheads on US shark bottom longlines and 71% for smooth hammerheads on Portuguese longlines. They also cite reports of 100% post-release mortality for scalloped hammerheads released from purse seines. In the Galápagos, Dr. Hearn and his colleagues reported on an experimental longline fishery run inside the marine reserve in 2003. It killed hammerheads at almost three times the rate of silky and Galápagos sharks: 32% of hooked hammerheads died, against 11%.
Distance from shore offers little refuge. A global tracking study we co-authored found that, on average, 24% of the space that tracked pelagic sharks use each month, hammerheads among them, falls under the footprint of pelagic longline fleets. Closer to shore, young great hammerheads use coastal habitat such as a probable nursery in Biscayne Bay, Florida, although where great hammerheads give birth is still unknown. Two of the juveniles caught there still had recreational fishing hooks in their jaws. In the Mexican Pacific, young smooth hammerheads use coastal waters for about two years before moving offshore. In Peru, a seasonal fishing ban was introduced in 2016 to protect juvenile smooth hammerheads in small-scale gillnet fisheries. Of 88 gillnet fishers interviewed, 76% favored the ban, though many found it unfair; the authors argue for designing such rules with fishers.
What did the shark ship seized in the Galápagos reveal?
That industrial fishing in the Eastern Pacific is likely taking threatened sharks on a vast scale, hammerheads among them. In August 2017, the refrigerated cargo ship Fu Yuan Yu Leng 999 was detained inside the Galápagos Marine Reserve carrying 7,639 sharks. One in five of the sharks identified was a hammerhead.
The alert came from the Galápagos National Park Directorate’s monitoring center, and Ecuador’s Navy brought the ship into port. A team led by researchers at the Universidad San Francisco de Quito, including Dr. Hearn, then identified 929 of the carcasses by their appearance and DNA. Of those, 122 (13.1%) were scalloped hammerheads and 67 (7.2%) were smooth hammerheads; 43 of the 46 smooth hammerheads measured were judged sexually mature from their length. Tracking data suggest the ship loaded its cargo from tuna longliners about 2,735 km (1,700 miles) to the northwest, and its empty holds had room for more than 30,000 additional sharks.
It was the largest reported shark seizure in Ecuadorian waters, and it ended in court. The Galápagos provincial court ruled in favor of the Galápagos National Park Directorate and sentenced 20 crew members to between one and three years in prison. It also confiscated the vessel and imposed a fine of US$6.1 million for damage to protected species.
Foreign fleets are not the only pressure. In a 2017 letter to Science, Dr. Hearn and Santiago Bucaram noted that Ecuador’s artisanal fleet of more than 45,000 vessels was allowed to land sharks as bycatch. At least 250,000 sharks were landed each year, they wrote, and much of that effort was concentrated along the marine reserve’s border. Ecuador has also moved to protect hammerheads specifically. Since August 2013, its industrial fleet may not keep scalloped or smooth hammerheads, and artisanal boats may retain no more than five immature hammerheads per trip, each under 150 cm (59 in) total length.
How does climate change affect hammerhead sharks?
Warmer water appears to drive scalloped hammerheads away from their usual sites. At Cocos Island, far fewer are seen in warm years and during El Niño events, and models suggest their suitable habitat in the Eastern Pacific may shift as the ocean warms. The climate change page uses the Cocos Island study below as a case study for sharks and rays.
The clearest evidence comes from the same Cocos Island dive logs, 34,342 dives recorded by 46 dive guides over 27 years and analyzed in a 2021 study of gradual and acute warming. Of the sharks studied, scalloped hammerheads responded most strongly to temperature. Predicted counts fell by 10.3% for every 1 °C (1.8 °F) rise in sea surface temperature, and by 40.3% between 25 and 30 °C (77 and 86 °F). Schools of more than 50 sharks were 42.8% more likely at the cooler temperature. El Niño, a periodic warming of the eastern tropical Pacific, had the sharpest effect: predicted counts were 105.6% higher during strong La Niña conditions, its cool counterpart, than during strong El Niño conditions.
The authors read these temperature effects as likely reflecting the sharks moving elsewhere rather than dying. The decades-long decline, however, continued, suggesting, they write, “impacts of ongoing fisheries exploitation”. And the warming is already under way: the Eastern Tropical Pacific was 0.5–1.0 °C (0.9–1.8 °F) warmer in 2010–2020 than in 1950–1980.
Where do the sharks go? A decade of observations from a tourist submersible at Cocos Island found scalloped hammerheads increasingly often in deep water. On dives to 300–400 m (980–1,310 ft), the modeled chance of seeing them rose from 20% in 2010 to 37% in 2019, and it was higher when the surface was warmer than usual. The authors suggest the sharks’ depth range is being “compressed” between warming surface water above and the low-oxygen water below. They also suggest that better management may have contributed to the increases they recorded, including a radar system introduced in April 2016 to detect vessels inside the national park. Costa Rica’s adoption of CITES and CMS listings may also help, especially for wide-ranging species such as the scalloped hammerhead.
Two studies co-authored by Dr. Hearn look ahead. An expert assessment of 132 sharks, rays, and chimaeras in the Eastern Tropical Pacific found the great hammerhead to be the only one of the region’s six hammerhead species with high vulnerability to climate change. A habitat model for the scalloped hammerhead projected that suitable conditions may shift south toward the Chilean Pacific by 2050, though only one of 952 candidate models met the authors’ criteria. Tracking researchers have argued that protection around the Galápagos should expand during El Niño years, when sharks need to travel farther from the islands to feed.
Are hammerhead sharks protected?
Partly. Protection has been added in stages since 2010: international trade in the three large species requires CITES permits, and the great and scalloped hammerheads are listed under the Convention on Migratory Species. Several fisheries bodies and countries ban keeping them, but a 2022 review found that most countries in their range did not prohibit landings.
The main measures:
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- The International Commission for the Conservation of Atlantic Tunas prohibited keeping, landing, or selling great hammerheads and other hammerheads in its fisheries, with exceptions for local consumption in developing countries.
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- The General Fisheries Commission for the Mediterranean banned retention of great hammerheads.
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- The scalloped, great, and smooth hammerheads were added to Appendix II of CITES, the Convention on International Trade in Endangered Species, in force from September 2014. Exports must be accompanied by permits showing the fins come from legal and sustainable fisheries.
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- The great and scalloped hammerheads were listed on Appendix II of the Convention on Migratory Species. The scalloped hammerhead also became the first shark protected under the US Endangered Species Act, with its Eastern Pacific and Eastern Atlantic populations listed as Endangered and two others as Threatened.
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- Peru introduced its seasonal ban to protect juvenile smooth hammerheads.
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- The winghead shark was listed on CITES Appendix II.
The next gains lie in national fisheries rules. As of a 2022 review, landings were prohibited nationally in 8 of the 129 countries where great hammerheads occur, 7 of 119 for scalloped hammerheads, and 5 of 122 for smooth hammerheads. The Red List assessors stress that treaty listings succeed only when countries implement them at home, and recommend that all great hammerhead retention and landings be prohibited while the species remains Critically Endangered or Endangered.
Do marine protected areas work for hammerheads?
They protect scalloped hammerheads where they school, places where the sharks are very vulnerable to fishing, but tracking shows the sharks leave reserves at night and between seasons. Tracking researchers have therefore argued for larger reserves, with wider protection in El Niño years.
Dr. Hearn’s team tagged more than 100 hammerheads at Darwin and Wolf between 2006 and 2009. The sharks stayed close to the islands by day but left at night, presumably to forage. Preliminary satellite tracks from a small, mostly male sample also suggested that hammerheads spend significant periods outside the Galápagos Marine Reserve. At Cocos Island, a 2019 study noted that the nightly excursions of hammerheads at oceanic islands could exceed 40 km (25 miles). Sharks at Cocos may therefore move in and out of what was then the park’s 22 km (14 mile) radius every day, into waters where longlines generally fish from dusk to dawn. A study of Galápagos dive guides’ accounts likewise found no support for a positive effect of the reserve on hammerhead numbers; the sharks remain exposed to fishing around its borders.
Governments in the region have since enlarged protection. Before 2016, only about 50 km² (19 square miles) around Darwin and Wolf, 0.04% of the marine reserve, was fully closed to fishing. In March 2016, Ecuador created the 40,000 km² (15,400 square mile) no-take Darwin and Wolf Marine Sanctuary. Surveys at the two islands in 2013 and 2014 had recorded the largest reef fish biomass reported at the time, and scalloped hammerheads made up 48% of it. In 2017, Colombia expanded the Malpelo sanctuary from 950 to 26,770 km² (367 to 10,336 square miles), and Mexico established Revillagigedo National Park, the largest no-take area in the waters around the Americas. In 2022, Ecuador declared the Hermandad Marine Reserve, a 60,000 km² (23,000 square mile) expansion that our work helped secure (see MMF in the Galápagos). A review of 20 years of tracking across these islands, co-authored by Dr. Hearn, concludes that scalloped hammerheads, which aggregate at confined hotspots, may be more feasible to protect than sharks that spread out.
Coverage remains the gap. A regional review we co-authored examined Important Shark and Ray Areas, the places identified as critical for these species, across the 12 nations of the Central and South American Pacific. These areas overlap marine protected areas by only 15.6%, and no-take areas by 7.3%. Scalloped hammerheads are already, with whale sharks, the species most often named in the region’s management plans; the task is to extend protection to the places they use.
Can hammerhead populations recover?
Yes, where fishing deaths are brought down. A 2021 global analysis found hammerheads rebuilding in the Northwest Atlantic under strictly enforced US quotas, evidence that enforced fishing limits can support their recovery.
The global analysis we contributed to found that hammerhead populations are rebuilding in the Northwest Atlantic, “owing to strictly enforced quotas throughout their US range”. It also found that the smooth hammerhead was the only one of the 18 oceanic species analyzed whose abundance did not decrease over the past half-century. In the Red List assessments, the scalloped hammerhead shows signs of stabilization and possible recovery in the Northwest Atlantic and Gulf of Mexico. For the great hammerhead, an analysis of updated North Atlantic data to 2017 indicated recovery at 6.6% per year. The same analysis found a 6.6% annual decline in the Indian Ocean, and the assessors, treating the North Atlantic as the exception, listed the great hammerhead as Critically Endangered globally. The smooth hammerhead, which has the highest population growth rate of any hammerhead, may be recovering slowly in the Atlantic under management.
Hammerheads have the reproductive potential to recover if mortality is reduced, so limiting their encounters with fishing gear may be one of the best conservation strategies. Changes to fishing practice may also reduce deaths. Scalloped hammerheads on US Atlantic longlines survived better on deeper hooks, although the authors note that whether fishers can change tactics depends on the effect on their target catch. Most of the great hammerhead’s core habitat in the Northwest Atlantic, the areas the sharks use most, lies within US waters. Recent data suggest that prohibiting their catch there would protect over 90% of that core habitat.
What are MMF and its partners doing for hammerheads?
MMF’s work in the Galápagos and Florida puts its scientists in the water with hammerheads. Its partners at the Universidad San Francisco de Quito help lead the long-term tracking that underpins hammerhead protection in the Eastern Pacific.
Dr. Hearn and his collaborators have tracked scalloped hammerheads at Darwin and Wolf, Cocos, Malpelo, and Revillagigedo in studies spanning some 20 years. MMF works with the same university group on other Galápagos species: silky sharks, yellowfin tuna, and whale sharks. The university leads a regional tracking study of silky sharks across the Galápagos and the other offshore islands of the Eastern Tropical Pacific, with MMF’s Dr. Chris Rohner leading MMF’s contribution. It follows a yellowfin tuna study the same partners completed in the Galápagos Marine Reserve. At Darwin Island, MMF’s Dr. Simon Pierce leads MMF’s part in the Galápagos Whale Shark Project, whose fieldwork brings the team into regular contact with the island’s hammerhead schools. MMF-hosted citizen-science liveaboards to the Galápagos support that project.
In Florida, MMF’s Florida Manta Project aerial and boat surveys have recorded hammerhead sharks alongside the manta rays they set out to count. Beyond the field, an MMF co-founder co-assessed the three large hammerheads for the 2019 Red List, as noted above. MMF scientists also co-authored several of the global and regional analyses of shark declines, fishing overlap, and protected-area coverage cited on this page.
Why it matters for conservation
Hammerheads are among the most recognizable sharks in the ocean, and their decline has a clear cause: fishing, for fins and as bycatch. Warming water also appears to push scalloped hammerheads away from some of their sites. Yet the Northwest Atlantic shows that these sharks can rebuild when fishing deaths are reduced, and the Eastern Pacific shows that tracking data can turn a schooling site into a protected one. Protecting and connecting the places where hammerheads aggregate, cutting bycatch deaths, and enforcing the trade rules already agreed would give them that chance across their range.
MMF’s shark research in the Galápagos, including the silky shark tracking study led by the Universidad San Francisco de Quito, can be supported through that project’s page.