Scalloped Hammerhead Shark: Habitat, Diet & Conservation

The scalloped hammerhead shark (Sphyrna lewini) is a wide-ranging predator found in warm coastal and open-ocean waters around the world, recognizable by the distinctive scalloped indentations along the front edge of its broad, flattened head. It was listed as Critically Endangered by the IUCN in 2018 after decades of steep population decline driven largely by the shark fin trade, and its biology makes recovery unusually slow. Understanding where this species lives, how it hunts, and what threatens it reveals both an extraordinary animal and a conservation challenge that current policy has not yet solved.

Where Scalloped Hammerheads Live

Scalloped hammerheads occupy tropical and warm-temperate seas worldwide, ranging from inshore bays to the open ocean. Adults spend most of their time in warm surface waters above about 22°C but regularly descend into cold, deep water to feed. Satellite tags on sharks near the mid-Atlantic’s Saint Peter and Saint Paul Archipelago showed they stayed in shallow, warm layers most of the time yet dove deeper than 150 meters on occasion, mostly at night, reaching depths as great as 728 meters where temperatures dropped to around 5.6°C.¹ That combination of surface living and deep foraging dives defines the species’ habitat use throughout its range.

Juveniles, however, live in a very different world. Newborn scalloped hammerheads rely on shallow, murky, soft-bottomed coastal areas near river mouths. A multi-year survey along the central Mexican Pacific found juveniles concentrated within about two kilometers of river outflows, in water shallower than 20 meters. Tagged young sharks stayed near those river mouths for up to nearly a month, with home ranges averaging just a few square kilometers.² These nursery zones offer small sharks protection from larger predators and access to prey in the turbid water. Because births occur during the warmer, rainy months when rivers discharge the most sediment, the nursery areas are tied closely to seasonal rainfall patterns.

What the Hammer-Shaped Head Actually Does

The flattened, laterally expanded head of a hammerhead shark, called a cephalofoil, has prompted plenty of speculation. One long-standing idea was that it works like a forward wing on a canard aircraft, letting the shark steer with greater agility. Research comparing juvenile scalloped hammerheads and bonnetheads to a conventional-headed sandbar shark found that the hammerheads were indeed more maneuverable, turning more tightly and more frequently. But the mechanism was not aerodynamic lift from the head. The hammerheads did not roll their bodies during turns the way an animal using a wing surface would. Instead, they showed greater lateral bending through the trunk, likely because of the cross-sectional shape of their vertebral column rather than anything the head itself was doing during the turn.³

The head’s more clearly supported advantage is sensory. All sharks detect the weak electrical fields produced by prey through specialized pores called ampullae of Lorenzini, scattered across the snout. Spreading those pores across a much wider head gives the scalloped hammerhead a broader lateral search area. Experiments on juvenile scalloped hammerheads and sandbar sharks showed that the hammerhead’s head shape did not make it more electrically sensitive per pore but did widen the zone it could scan on each pass over the seafloor.⁴ Think of it as the difference between searching a room with a narrow flashlight versus a wide floodlight. The hammerhead’s “floodlight” means it encounters buried prey more often, even if each individual sensor is no sharper than those of other sharks.

Deep Diving and the Breath-Holding Strategy

Scalloped hammerheads are warm-water sharks with no internal heating system of their own. Yet adults repeatedly plunge from surface water around 26°C to depths beyond 800 meters, where temperatures can drop to 5°C. Biologgers attached to adult sharks revealed something surprising: the sharks maintained warm muscle temperatures throughout the deepest part of each dive. Cooling only kicked in during the later stages of the ascent, and once it started, it happened fast.⁵

The explanation appears to be a form of “breath holding.” Sharks exchange heat with the surrounding water primarily through their gills, where blood passes close to the environment. By suppressing gill function during the descent and bottom phase of a dive, the shark dramatically slows heat loss to the frigid deep water. It essentially closes the main thermal window.⁶ The cost is reduced oxygen supply to the muscles, which means the shark’s swimming muscles shift toward anaerobic metabolism during the dive and need a recovery period in warm surface water afterward. This cycle of intense, oxygen-limited deep dives followed by surface rest gives the scalloped hammerhead access to a mesopelagic food web that most tropical sharks cannot reach.

Schooling at Seamounts

One of the scalloped hammerhead’s most striking behaviors is its tendency to gather in large groups at seamounts and underwater ridges during the daytime. In the Gulf of California, tracking of tagged individuals at a seamount called El Bajo Espiritu Santo showed that sharks congregated at the site during daylight hours and then dispersed individually into the surrounding open ocean at night. Departures clustered around dusk and arrivals around dawn, tightly linked to the light-dark cycle. Sometimes multiple sharks arrived or left within a short window during the day, suggesting coordinated group movements.⁷

Exactly why they school in this pattern is still debated. The daytime gatherings are not feeding events: the sharks appear to forage when they leave the seamount at night. Hypotheses range from social signaling and predator defense to using the seamount as a navigational landmark and staging ground for nocturnal forays. Whatever the reason, these predictable aggregations have made certain seamounts world-famous dive sites and, unfortunately, have also made the sharks easier for fishers to target.

Diet and Foraging

Scalloped hammerheads are generalist predators whose diet shifts as they grow. Juveniles in shallow nursery zones eat small fish, shrimp, and crabs available near river mouths. As they mature and begin venturing into deeper water, their diet broadens to include reef fish, squid, and stingrays. Adults that make the deep dives described earlier are tapping into a mesopelagic prey base of lanternfish, deep-water squid, and other organisms that inhabit the ocean’s twilight zone between roughly 200 and 1,000 meters depth.

The wide, electrically sensitive head gives the species a particular advantage over buried and benthic prey. Stingrays, which hide under sand, are a well-documented food source for hammerheads across their range. The hammerhead’s broader sensory sweep lets it locate hidden prey more efficiently than sharks with narrower heads, and its lateral flexibility helps it pin struggling rays against the seafloor. That combination of a wide electrical search field and superior turning ability paints a picture of a predator finely adapted for hunting elusive, bottom-dwelling prey, even though it also takes plenty of midwater fish and squid.⁴

Why Populations Have Crashed

The scalloped hammerhead’s conservation crisis stems from a mismatch between how fast the species reproduces and how heavily it is fished. Like most large sharks, scalloped hammerheads grow slowly, mature late, and produce relatively few offspring per litter. That low rate of population growth means even moderate fishing pressure can push numbers downward, and the pressure has been far from moderate. High rates of hooking mortality in longline fisheries, combined with that slow reproductive pace, are believed to have caused severe declines in U.S. Atlantic populations.⁸

Globally, the picture is similar. The species was listed as Critically Endangered by the IUCN in 2018, driven primarily by unsustainable fisheries pressure. While hammerhead shark meat is generally not favored because high urea content gives the flesh a bitter, acidic taste, the fins are extremely valuable. Scalloped hammerhead fins are large and have an especially high count of the internal structural fibers (ceratotrichia) prized in shark fin soup, making them among the most desirable on the market.⁹ This means the species is often caught specifically for its fins even when the meat is discarded.

The Fin Trade and CITES Enforcement

International regulation of hammerhead fin trade has been attempted through CITES (the Convention on International Trade in Endangered Species), which listed the scalloped hammerhead under Appendix II in 2013, requiring exporting countries to demonstrate that trade is sustainable. In practice, enforcement has been spotty. An analysis of trade data from 2015 to 2021 found that minimal trade in listed shark species was officially reported during that period. Yet fins from four of the listed species, including the scalloped hammerhead, were consistently found in the world’s largest shark fin market in Hong Kong throughout those years, indicating substantial ongoing illegal trade. About 81% of shark-fin-exporting nations had never reported any trade of these species at all.¹⁰

Genetic analysis of market fins made the gap even more obvious. A study of fins from one listed species traced them back to six distinct populations of origin, but only three of those populations came from regions where any trade had been officially reported. The rest were effectively untracked, slipping through regulatory gaps. Researchers have identified three overarching challenges for CITES in protecting this species. Listing commercially valuable species is politically difficult because economic interests collide with conservation goals. Many countries have inadequately implemented the required legislation, so compliance is low. And CITES struggles to manage species from the Global South where shark fishing is legal and communities depend on it for income, leaving little local incentive to comply with trade restrictions.¹¹

The policy debate is not a simple narrative of wealthy nations pressuring poorer ones, though. A recent analysis found limited evidence that CITES implementation was being used as a tool of Global North pressure on the Global South. The more pressing reality is that weak enforcement everywhere risks letting populations fall so low that regulation becomes irrelevant.⁹

Why Marine Reserves Alone Are Not Enough

Scalloped hammerheads are protected inside several high-profile marine reserves, including those around the Galápagos Islands, Cocos Island off Costa Rica, and Malpelo Island off Colombia. Tracking data from Cocos Island show that sharks tagged there travel between these oceanic island systems, covering hundreds of kilometers of open water. The problem is that the sharks are only safe while inside the reserve boundaries. The migratory corridors connecting these islands, sometimes called “swimways,” have no protection. Researchers tracking movements in the eastern tropical Pacific have argued that a regional approach protecting these corridors and the coastal nursery areas is needed for conservation to work.¹²

Nursery protection is a particularly important missing piece. Because newborn scalloped hammerheads depend on shallow, nearshore habitat close to river mouths, those areas represent a bottleneck in the species’ life cycle. Protecting adults at offshore seamounts does little if juvenile survival in unprotected coastal zones is low. Coastal nurseries also face threats beyond fishing, including pollution, habitat degradation, and mangrove loss, all of which degrade the turbid, shallow conditions that young sharks need.

Genetic Populations and Connectivity

Genetics research has shown that scalloped hammerheads are not one seamlessly connected global population. Molecular clock analyses point to an origin in the Indo-West Pacific, with late Pleistocene radiations into the central Pacific (Hawaii) and the eastern Pacific (Central America), plus a more recent spread into the Atlantic Ocean via southern Africa.¹³ The Indo-Pacific retains the highest genetic diversity and the least population differentiation, consistent with it being the ancestral range. Moving outward from there, populations become progressively more genetically distinct and less diverse, with the Gulf of Mexico and eastern Pacific populations showing the most isolation.¹⁴

An interesting wrinkle is how the sexes disperse differently. Analyses combining mitochondrial DNA (inherited from mothers) with nuclear microsatellite markers (inherited from both parents) revealed that females tend to return to their natal regions to give birth, creating strong maternal population structure. Males, by contrast, roam more widely and mix between populations. This male-mediated gene flow keeps populations from diverging completely, but it also means that a local population hit hard by fishing cannot easily be replenished by females immigrating from elsewhere. For conservation, the implication is stark: each regional population of scalloped hammerheads needs to be managed separately, because losing one is not something the rest of the species can simply fill back in.

Disappearing from Dive Sites

Some of the most visceral evidence of decline comes not from stock assessments but from the people who have spent decades in the water with these sharks. A study in the southwestern Gulf of California compiled observations from local divers at two well-known seamount aggregation sites, El Bajo and Las Ánimas. At El Bajo, divers reported encountering an average of roughly 150 scalloped hammerheads per dive in the 1970s. By the 2010s, that number had fallen to about five, a decline of around 97%. At Las Ánimas, groups that once averaged about 100 sharks had dropped to zero.¹⁵

The study also documented a clear shifting baseline. Divers who first visited these seamounts in later decades consistently described smaller numbers as “very abundant” compared with divers who had been going since the 1970s. Someone who began diving El Bajo in the 2000s might consider seeing 10 hammerheads a great day, unaware that 30 years earlier the same spot hosted more than ten times that many. That perceptual drift matters because it shapes what people consider normal and worth protecting. If the public benchmark for a healthy hammerhead aggregation keeps sliding downward, political will to impose costly conservation measures slides with it. For a species already listed as Critically Endangered, that kind of quiet normalization of decline is one of the less obvious but more dangerous threats it faces.