Scalloped hammerhead

Scalloped hammerheads live in warm seas all over the world.

Some swim near the coast. Others swim in deeper water far from land.

Scalloped hammerheads live in warm coastal seas around the world, in the Atlantic, Indian and Pacific Oceans. They swim over the shallow sea floor around continents and islands and in the deeper water nearby.

Adults spend most of their time offshore. Mothers come into shallow coastal waters to give birth, and the young sharks grow up there.

The species is circumglobal in coastal warm-temperate and tropical seas (IUCN). NOAA Fisheries describes a range from New Jersey to Brazil, including the Gulf of Mexico and the Caribbean; from the Mediterranean to Namibia; from South Africa and the Red Sea to Pakistan, India and Myanmar; from Japan and China to New Caledonia, Indonesia, the Philippines and Australia; and from southern California to Ecuador and possibly Peru, plus Hawaii and Tahiti.

It is a coastal and semi-oceanic pelagic shark found over continental and island shelves and nearby deep water, usually from the surface to 275 meters but recorded as deep as 1,043 meters. Adults spend most of their time offshore in midwater, and females move into coastal areas to give birth (IUCN).

Scalloped hammerheads eat fish and squid.

They also eat octopus, crabs and shrimp, and sometimes rays. They often hunt at night.

Scalloped hammerheads eat mostly bony fish, from small schooling fish like sardines to eels and reef fish. They also catch squid, octopus and crustaceans such as crabs and shrimp, and sometimes small sharks and rays.

Many spend the day near the coast and move offshore to hunt at night. The Florida Museum describes the wide head as helping the shark find prey that is well hidden.

The Florida Museum describes a diet mainly of teleost (bony) fishes and invertebrates, including sardines and other small schooling fish, conger eels, reef fishes, squid, octopus and crustaceans, along with smaller elasmobranchs such as blacktip reef sharks, angelsharks and stingrays; dozens of stingray spines are sometimes found in a hammerhead's mouth and gut.

The same source notes that scalloped hammerheads often spend the day closer inshore and move offshore to hunt at night, and that the broad cephalofoil is used to detect hidden prey.

Scalloped hammerheads often swim together in big groups called schools.

Sometimes small fish clean their skin.

Scalloped hammerheads are famous for gathering in large schools, sometimes with many sharks swimming together. Adults often form groups of only males or only females.

They also visit cleaning stations, where small cleaner fish called wrasses pick tiny parasites off their skin and even from inside their mouths. The sharks travel too: large schools have been seen heading north for the summer on seasonal migrations.

Adults spend most of their time offshore and form schools segregated by sex, and large schools can be seen on seasonal migrations (Florida Museum). Genetic studies show that males move across ocean basins, while females move only regionally and do not cross between separate continental coastlines (IUCN).

Parasites such as leeches and copepods are removed at cleaning stations by cleaner wrasses. Regarding people, the Florida Museum, citing the International Shark Attack File, lists 17 unprovoked bites for all hammerhead species combined, and notes that scalloped hammerheads have sometimes made threat displays when divers came close, while at other times showing no aggressive behavior.

There are far fewer scalloped hammerheads than before.

Too many have been caught by fishing boats. People are making new rules to protect them.

The scalloped hammerhead is critically endangered. Fishing boats catch many of them, sometimes on purpose and often by accident while fishing for other fish, and their fins are sold for shark fin soup. Because they swim in schools, many can be caught at once.

There is some hope: in the northwest Atlantic and the Gulf of Mexico, where catches have been controlled, their numbers have stopped falling and may be starting to recover. International trade in their fins is now controlled too.

The IUCN assessed the species as Critically Endangered (A2bd) in 2018 (published 2019), estimating median declines of 76.9 to 97.3 percent over three generations (72.3 years), with the most likely reduction above 80 percent. It is caught as a target and as bycatch in longline, purse seine and gillnet fisheries and kept for its meat and fins, which are among those preferred for shark fin soup.

Only in the Northwest Atlantic and Gulf of Mexico do populations show signs of stabilizing and possibly recovering under management. Protections include an ICCAT ban (2010) on keeping hammerheads in its tuna fisheries, listing on CITES Appendix II, and, in the United States, Endangered Species Act listing of two populations as endangered and two as threatened (NOAA Fisheries, 2014).