Shark of the month: Great Hammerhead shark

The great hammerhead shark (Spyrna Mokarran) belongs to the hammerhead shark (Sphyrnidae) family and is its largest member. Great hammerheads can grow up to about 6 meters and weigh up to 580 kilo’s, this in contrast to other hammerhead shark which are generally not longer than 4 meters. The average lifespan in the wild is 20 to 30 years. Great hammerheads occur in tropical and subtropical coastal waters, near continental shelves, coral reefs and in deep waters. Great hammerheads can be recognized by their tall first dorsal fin and of course their hammer shaped head which almost has a straight front margin. These strong apex predators generally live solitary and live of a varied diet. They use their hammer shaped head to pin winged fish, like stingrays, against the sea floor.

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Anatomy and appearance

Great hammerheads have an unmistakable identity with their hammer shaped head that derives its name and the very large first dorsal fin that is strongly falcate. The second dorsal fin is also large but not as large as the first and has a strongly concave rear margin. They are a dark brown or olive color dorsally and this fades into white ventrally. Juveniles are around 60-70cm when they are born and adults can grow to around 6 meters, although most grow to around 3-4 meters.

Habitat

The great hammerhead is a solitary, nomadic predator and is found throughout the Atlantic and Indian Oceans and Pacific oceans and also in the Mediterranean sea. The great hammerhead is predominately a costal shark that is found over continental shelves and in lagoons. They do migrate seasonally, in the summer months they head towards the poles in search of colder water and during the winter they will head back towards the equator in search of warmer water. They have been found to inhabit water from a depth range of 1 meter all the way down to around 300 meters.

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Diet

Great hammerheads are opportunistic, solitary apex predators. They feed on a wide range of prey from invertebrates like crabs, lobsters, octopus and squid and bony fish such as groupers, catfish, tarpons, sardines and porcupine fish. However their favorite prey are rays, especially stingrays. They attack stingrays by using their hammer shaped head to pin the stingrays to the bottom of the ocean. Interestingly great hammerheads seem to be unaffected by the stings of rays and catfish and they are commonly found with spines sticking out of their skin and jaws. They have also been known to prey on other sharks, even on other smaller species of hammerhead sharks.

Population

Female great hammerheads reach maturity at around 2 meters with males growing a little larger at 2.5 meters before reaching maturity. The gestation period is 11 months and during this time eggs are nourished by a yolk-sac placenta. Then, during the spring and summer months parturition occurs with litters of somewhere between 6-40 pups. At birth the pups measure between 56-70 centimeters in length. Immediately after birth they leave their mother and completely rely on themselves. Unlike a lot of shark species, great hammerheads mate routinely throughout the water column and have even been known to mate at the surface. Unfortunately great hammerheads do not reproduce often, they bring forth young once every two years. It’s not known how many great hammerheads are left worldwide, however the population is decreasing rapidly.

Conservation

The official IUCN Red List conservation status of the great hammerhead is ‘Endangered’ (http://www.iucnredlist.org/details/39386/0). This means that great hammerheads are only two steps away from being extinct in the wild. The estimated decline of the total population over the past 25 years is 80%. Great hammerhead sharks are very vulnerable to overfishing and moreover their fins are very valuable and used for shark fin soup. The skin of the sharks can also be used for leather production and its liver for its rich vitamin oils. It is endangered in the North West Atlantic, the gulf of Mexico and in the South West Indian Ocean. Along the west coast of Africa it is critically endangered.

http-_en.wikipedia.org-1024px-Hammerhead_shark,_Cocos_Island,_Costa_Rica

Relationship to humans

The great hammerhead is the only hammerhead species considered a possible danger to humans because of its large size and rather aggressive nature. However the great hammerhead rarely attacks humans and it sometimes only behaves curiously towards divers. Most important great hammerheads, like all sharks, should always be treated with respect, after all you are entering their habitat.

Originally written by Chris Facey and edited by Isabelle Walter.

 

 

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