Hammerhead sharks are large predators that warrant caution, but the studies in this guide describe how they feed and behave around divers rather than how often they bite people. The scalloped hammerheads (Sphyrna lewini) that divers meet in schools hunt mainly squid and fish (opens in a new tab) and seldom approach divers, while the great hammerhead (S. mokarran) hunts fishes, rays, and other sharks (opens in a new tab). Hammerhead dives can also be demanding: in the northern Galápagos the schools favor the strongest currents (opens in a new tab), and at Rangiroa in French Polynesia great hammerheads are seen on a plateau 45–60 m (150–200 ft) deep (opens in a new tab).
This guide is written by MMF co-founder Dr Simon J Pierce, whose work in the Galápagos brings him 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 dangerous to humans?
The research in this guide cannot put a figure on the risk, because it describes diet and behavior rather than bites. What is clear is that a large hammerhead is a powerful predator and warrants the same caution as any big shark.
What the studies do show is what each species hunts. Scalloped hammerheads feed on a mixture of fish, crustaceans, and cephalopods (squid and their relatives), with squid as a primary prey item (opens in a new tab). In the Galápagos they leave the islands at dusk on presumed foraging excursions (opens in a new tab). Throughout one tracking study at Wolf Island, none was ever seen feeding at the island (opens in a new tab), which is where divers see them by day. Great hammerheads, in contrast, feed primarily on bony fishes and on other sharks and rays, from small rays to large sharks (opens in a new tab). The feeding chapter covers what each species eats.
Hammerhead species also differ greatly in size. Five of the nine species on the IUCN Red List grow no longer than a tall person. The scalloped bonnethead (opens in a new tab) reaches 92 cm (3 ft); the bonnethead (opens in a new tab) (S. tiburo), scoophead (opens in a new tab), and smalleye hammerhead (opens in a new tab) reach 150 cm (5 ft); and the winghead (opens in a new tab) (Eusphyra blochii) reaches 186 cm (6 ft). The species divers usually meet are larger. Scalloped hammerheads tagged in the Galápagos schools were estimated at 1.2–2.4 m (4–8 ft) long (opens in a new tab). The largest great hammerhead caught in a 2009–2021 survey of South Florida’s coastal sharks measured 4.5 m (15 ft) (opens in a new tab), and the size chapter weighs that against the great hammerhead’s reported maximum of 6.1 m (20 ft) (opens in a new tab).
Encounters between divers and hammerheads are routine where the sharks gather. At Cocos Island, off Costa Rica, dive guides logged 34,342 dives between 1993 and 2019 and recorded scalloped hammerheads on 77.7% of them (opens in a new tab). The Galápagos islands of Darwin and Wolf are likewise a global scuba diving and conservation hotspot (opens in a new tab) because of their sharks. None of these studies records how often encounters go wrong, however, so the sections below describe what is known: how hammerheads behave around people, and how people should behave around them.
Do hammerhead sharks attack humans?
The hammerhead attacks described in detail in the studies cited here are on prey. In two detailed accounts of great hammerheads killing rays near people, the sharks showed no aggression toward the observers; such accounts describe behavior but do not measure how often hammerheads bite people.
In May 1988, at a sunken barge 12 km (7 miles) east of North Bimini in the Bahamas, two snorkelers and a scuba diver watched a female great hammerhead of about 3 m (10 ft) hunt. The shark chased a southern stingray, drove it into the seabed with its head, and pinned it there while biting into its fins (opens in a new tab). It swam close to the snorkelers before diving onto the ray, and the authors suspected that the people in the water briefly kept it from returning to its kill (opens in a new tab). Ten years later, a few kilometers south of South Bimini, a female great hammerhead of 3.5–4.0 m (11.5–13 ft) removed a spotted eagle ray’s pectoral fin in what appeared to be a single bite. It then swam slowly to within 1 m (3 ft) of a researcher who had entered the water to watch. Comparing the two events in the Bulletin of Marine Science in 2002, Demian Chapman and Samuel Gruber noted that both sharks chased off blacktip sharks trying to scavenge their kills. Yet, they wrote, “neither hammerhead exhibited any aggression towards human observers in the immediate vicinity.”
Hammerheads and people do compete for fish. Off South Andros in the Bahamas, a great hammerhead of about 2 m (6.5 ft) was drawn onto a sand flat less than 0.9 m (3 ft) deep by a bonefish struggling on a fly line (opens in a new tab). It chased a lemon shark that was following the same fish, then abandoned the chase, apparently because it made contact with the boat (opens in a new tab), and moved slowly off the flat into deeper water.
Are hammerhead sharks aggressive?
Great hammerheads are aggressive toward prey and toward sharks that try to steal it, but the detailed accounts cited here record none toward people nearby; they cannot, however, measure risk. Scalloped hammerheads are social sharks that seldom approach divers.
By day, scalloped hammerheads school on the upstream side of Darwin and Wolf islands, where they rest in the water flow, attend cleaning stations, and socialize, before leaving on night-time foraging trips. Cleaning, in which small fish pick parasites from the sharks’ skin, was observed frequently at Shark Point on Wolf Island and at Darwin’s Arch (opens in a new tab). Charles Darwin Foundation researchers at Wolf have also filmed hammerheads rubbing their mouths, fins, undersides, and even claspers against a sand patch at 11–15 m (35–50 ft) (opens in a new tab); claspers are the males’ reproductive organs. The behavior was seen on 24 occasions, and the authors suggest that parasites are the likely cause. These observations describe resting, cleaning, and social behavior at the daytime aggregation sites; the hunting appears to happen elsewhere, at night.
Great hammerheads behave differently. They are generally solitary (opens in a new tab), and they hunt with bursts of speed: on shallow flats, researchers have documented a technique they called “grasp-turning”, in which a shark made up to 22 tight turns while holding its prey (opens in a new tab). The aggression in the Bimini accounts was aimed at blacktip sharks competing for a kill, which is a good reason to give any feeding shark plenty of room.
Why are scalloped hammerheads so shy around divers?
The usual explanation is the noise and bubbles of scuba, although it is presumed rather than tested. MMF’s account of shark tourism at Darwin and Wolf describes scalloped hammerheads as a shy species that will seldom approach divers, presumably due to the breathing noise and streams of bubbles.
Several research teams have worked among the schools without conventional scuba. Dr. Alex Hearn’s team fitted acoustic tags to Galápagos hammerheads by free diving among the sharks (opens in a new tab). The researchers who filmed hammerheads chafing at Wolf worked on closed-circuit rebreathers and with remote underwater video (opens in a new tab). In Japan, a free diver tagged scalloped hammerheads by diving into the schools from the surface, partly because of concerns about capture stress and post-release mortality in hammerheads (opens in a new tab).
Bubbles can mean something different to a fed shark. At a baited great hammerhead site in Bimini, the divers kneel in a line that creates a wall of air bubbles (opens in a new tab). The study’s authors suggest the bubbles could serve as a conditioned stimulus (opens in a new tab), a cue the sharks learn to link with food, for sharks that return there each winter. The bubbles presumed to keep scalloped hammerhead schools at a distance may, in other words, draw a provisioned great hammerhead in.
How do divers see hammerheads at Darwin and Wolf islands?
Divers see them by going to the up-current corner of each island, where the schools hold position, and then staying still. Guided divers wait among the boulders for the sharks to pass.
Dr. Alex Hearn, a professor at the Universidad San Francisco de Quito and its Galápagos Science Center, led the tracking study that mapped this. At Wolf Island, the hammerhead aggregation was confined to the southeastern corner (opens in a new tab), around the dive site known as Shark Point; the schooling chapter gives the tracking detail. For divers, the practical points are which side of the island to dive and how strong the current runs there. Surveys with paired video cameras recorded hammerheads on 92% of transects (survey paths) at southeastern Darwin and 59% at southeastern Wolf, but only 9% on the northwestern sides of both islands (opens in a new tab). Tracking also showed that the sharks prefer the parts of the northern islands where currents are strongest, above 0.8 m per second (1.6 knots) (opens in a new tab).
The diving protocol follows these habits. Small groups of tourist divers are briefed before each dive and led by trained guides to viewing areas. There, they take up static positions among the large boulders and stay away from the open-water cleaning areas. Hearn’s census teams used much the same approach, hanging in mid-water at 15–20 m (50–65 ft) with their backs to the reef (opens in a new tab) while they counted. MMF scientists who study whale sharks at Darwin Island with the Galápagos Whale Shark Project work at the same hotspots, where whale sharks, Galápagos sharks, and hammerheads share a similar seasonal pattern (opens in a new tab).
Where can you dive with hammerhead sharks?
Scalloped hammerhead schools are a fixture of the remote islands of the Eastern Tropical Pacific. Great hammerheads are seen regularly at a few sites in the Bahamas, French Polynesia, and the Cayman Islands, and they are among the most common sharks in research surveys off South Florida.
Besides Darwin and Wolf, the Eastern Pacific sites include Cocos Island, where tagged sharks were most resident at Alcyone, a shallow seamount 3.6 km (2.2 miles) offshore (opens in a new tab). At Malpelo Island off Colombia, the largest schools occur from January to March (opens in a new tab). At San Benedicto Island in Mexico’s Revillagigedo archipelago, tagged adults were detected mainly by day, and their residency may be linked to cleaning stations tended by butterflyfish, hogfish, and angelfish (opens in a new tab). These islands also share some of their sharks, and the schooling chapter follows those movements.
Scalloped hammerheads also aggregate well beyond the Eastern Pacific. At Mikomoto Island off southeastern Japan, four local dive companies specialize in shark diving, without bait, because the schools are reliably seen every summer and fall (opens in a new tab). The trips that MMF scientists host include Banda Sea crossings that visit seamounts known for schooling hammerheads, and a Galápagos citizen-science liveaboard hosted by Dr. Simon Pierce.
For great hammerheads, well-studied sites include Bimini in the Bahamas, renowned for reliable and frequent encounters with the typically solitary great hammerhead (opens in a new tab). At Rangiroa in French Polynesia, research dives at the atoll’s passes, mostly in Tiputa Pass, recorded great hammerheads on 64.0% of 225 dives (opens in a new tab). In the Cayman Islands, the species appears to be present throughout the year, except perhaps in August (opens in a new tab). In South Florida, the great hammerhead was the fifth most frequently caught of 15 shark species in a 2009–2021 survey (opens in a new tab).
When is the best time to see hammerheads?
The best time is by day and in season. Scalloped hammerheads school at their islands in daylight, many moving offshore at night, and at every site described below the sharks are scarcer in some months than in others.
At Wolf Island, tagged sharks showed a significant preference for daylight hours in both the warm and cool seasons (opens in a new tab), and many left the island after dark. The schooling chapter gives the daily timing at each island, and the feeding chapter follows the night trips.
In the Galápagos, the best season is contested. A review by Hearn and colleagues found hammerheads at the Darwin and Wolf hotspots more abundant in the cooler months, from May through October, with the large schools rarely seen in the warmer months (opens in a new tab). Hearn’s earlier diver counts at Wolf, by contrast, were highest in January and lowest in May (opens in a new tab) in 2008. The studies agree on a dip from March to May: tagged sharks resided at both islands throughout the year, but their numbers dropped consistently from March to May (opens in a new tab). The schooling chapter sets out the disagreement. At Cocos, tagged sharks were also absent for long periods, notably from March through May (opens in a new tab), whereas Mikomoto’s season runs predominantly from August to November (opens in a new tab). Great hammerheads are winter visitors at Bimini, where residency peaks from January to March (opens in a new tab) and off-season baiting brings only very limited success and unreliable sightings (opens in a new tab). At Rangiroa, female great hammerheads are seen mainly in the austral summer, from November to May (opens in a new tab), and over the two study seasons sightings peaked two days before the full moon (opens in a new tab).
Ocean conditions also shift these patterns from year to year. At Cocos, far fewer scalloped hammerheads are counted in strong El Niño conditions than in strong La Niña conditions (opens in a new tab), as the schooling chapter details. The authors warn that if, as they suspect, the sharks are shifting their distribution, climate change and stronger El Niño events will have consequences for both dive tourism and citizen science at Cocos (opens in a new tab). How climate change affects sharks and rays covers this case study.
Is it safe to dive with great hammerheads?
The research cannot say, because no study cited here measures the risk of diving with great hammerheads. What it does describe is how great hammerhead dives at two studied sites are run, and how feeding shapes the behavior of these large predators where they are provisioned.
A study published in 2021 examined a baited dive site about 1 km (0.6 miles) off South Bimini, where great hammerheads had been fed during commercial dives every winter since 2012 (opens in a new tab). Over one season, from December 2016 to May 2017, researchers working with a local operator recorded great hammerheads on 97% of 104 dives, with up to nine mature sharks attending a single dive (opens in a new tab). Divers kneel on the sand in a line perpendicular to the current while the divemaster hand-feeds the sharks (opens in a new tab). The sharks ate 1,189 kg (2,620 lb) of bait over the season (opens in a new tab). The study’s model linked each additional dive boat at the site to a 14% increase in the bait each shark consumed during a dive (opens in a new tab). Bioenergetic models, which estimate an animal’s energy needs, suggest that some of the sharks could fully fuel their daily energy requirements from bait alone (opens in a new tab). The authors believe the dives are localizing the sharks to the western edge of Bimini in winter but do not seem to be affecting their annual migrations (opens in a new tab).
Rangiroa offers a different model. French Polynesia has protected all sharks in its waters since 2012 and prohibited wildlife provisioning since 2017 (opens in a new tab), according to a 2023 study, so great hammerheads there are seen without bait. At Rangiroa and neighboring Tikehau, researchers identified at least 55 great hammerheads over two austral summers, 54 of them female (opens in a new tab), and two were sighted over a period of up to 12 years (opens in a new tab). Local divers know the “Hammerhead plateau” in Rangiroa’s Tiputa Pass, 45–60 m (150–200 ft) deep (opens in a new tab), as the best place to see them. Research dives there are timed for the incoming tide, because outgoing tides bring dangerously strong currents with poor visibility (opens in a new tab).
Do hammerhead sharks come into shallow water?
Some do, and some live close to cities. Great hammerheads have been documented hunting on tidal flats less than 1.5 m (5 ft) deep, and young hammerheads of several species grow up in shallow coastal bays.
A 2016 study compiled six cases of great hammerheads using flats less than 1.5 m (5 ft) deep in Florida, the Bahamas, and Palau, five of them involving prey capture (opens in a new tab). Its authors speculate that great hammerheads’ use of shallow tidal flats is likely common (opens in a new tab). For anglers and waders on the flats, the practical lesson from the South Andros case is that a struggling hooked fish can draw a great hammerhead in (opens in a new tab).
Miami shows how closely hammerheads and people can share water. Young great hammerheads use a probable nursery in Biscayne Bay less than 7 km (4.3 miles) from downtown Miami (opens in a new tab), and two of the juveniles caught there still had recreational fishing hooks in their jaws (opens in a new tab). Tagged great hammerheads concentrated off South Beach and other urban sites in the bay in 2017 (opens in a new tab), and their space use was consistent with that of “urban adapters”, species that make partial use of urban areas (opens in a new tab). On Florida’s Gulf coast, most of the 35 great hammerheads caught in a survey of west-central Florida’s coastal waters, mainly in St. Joseph Sound, were immature (opens in a new tab), and their capture hot spots were on seagrass flats (opens in a new tab). The aerial and boat surveys of MMF’s Florida Manta Project along the South Florida coast have also recorded hammerheads.
Scalloped hammerheads use shallow bays as well. In Western Australia, drone surveys counted up to 62 hammerheads per survey in a clear bay no deeper than 8 m (26 ft), 50 km (31 miles) south of Perth (opens in a new tab). Those measured from the air were an estimated 1.08–1.84 m (3.5–6 ft) long and likely immature (opens in a new tab). Underwater photographs donated by snorkelers who swam with the aggregation confirmed scalloped hammerheads, although smooth hammerheads may also be present (opens in a new tab). The authors recommend precautionary measures to reduce negative impacts of public interaction with the species (opens in a new tab). The reproduction chapter describes the nurseries where young hammerheads spend their first years.
How to dive with hammerheads responsibly
Let the sharks set the distance, respect the current, and choose sites and operators that keep hammerheads protected.
In the water, follow the guide’s briefing, hold position among the rocks rather than swimming out toward a school, and keep clear of cleaning stations. That is the approach used at Darwin and Wolf to minimize disturbance while divers watch the sharks’ natural behavior. Plan for strong currents, open water, and, at sites such as the Rangiroa plateau, depths of 45–60 m (150–200 ft) (opens in a new tab). At baited sites the operator’s protocol governs the encounter. At Bimini, some great hammerheads became conditioned to the provisioned site, attending regularly and taking enough bait to fuel their daily ration (opens in a new tab).
Finally, report what you see. The dive logs kept by guides at Cocos for 27 years became the dataset behind studies of the long-term trend and the El Niño effect (opens in a new tab). At Rangiroa and Tikehau, videos recorded by citizen scientists over 15 years helped researchers identify 30 additional great hammerheads (opens in a new tab). Dr. Hearn and colleagues have also developed deep-learning software, a form of artificial intelligence trained on example footage, that detects and tracks hammerheads in video and could help scientists monitor their relative abundance (opens in a new tab).
Why it matters for conservation
Diving with hammerheads can also help protect them. Scientists reviewing the large hammerheads note that scalloped hammerheads aggregate at islands and seamounts, which makes it feasible to protect them through well-placed marine protected areas and shark-diving tourism that values them alive (opens in a new tab). Hammerhead schools are an important draw for the dive industry at Cocos (opens in a new tab), and Darwin and Wolf, valued for their economic importance to the dive tourism industry (opens in a new tab), have been fully protected from fishing since March 2016 (opens in a new tab). Hammerheads are also popular: hammerhead was the most popular choice of favorite shark among citizen scientists on research trips, at 22% of 1,800 responses (opens in a new tab).
That protection matters because hammerheads fare badly when they meet people with fishing gear. Hammerheads are in general more sensitive to capture than other shark species (opens in a new tab), and both the scalloped and great hammerheads are assessed as Critically Endangered on the IUCN Red List (scalloped (opens in a new tab), great (opens in a new tab)). Florida lists both as prohibited species, yet post-release mortality rates can still be high (opens in a new tab) for those caught and let go. The threats chapter and the overfishing page explain what drives their decline and what is being done about it, and the hammerhead hub and guide contents link every chapter.
In the Galápagos, MMF takes part in a regional silky shark tracking study led by Dr. Alex Hearn at the Universidad San Francisco de Quito, with MMF’s Dr. Chris Rohner leading MMF’s part. The project page explains how to support that research.
