[Photo credit Peter Verhoog/Dutch Shark Society]
Great hammerhead (Sphyrna mokarran)
Hammerhead Sharks are amongst the most astonishing fish in the sea because of their very distinctive odd shaped head. There are, in fact, seven species of Hammerhead sharks, but not all of them can be found in the Dutch Caribbean. The largest member of the Hammerhead family, the Great Hammerhead shark (Sphyrna mokkaran) has been sighted in the Dutch Caribbean. But increasing fishing pressure and a very low reproductive output, mean that globally populations have declined by 80% in the last 25 years. Hammerheads are now considered endangered species on the IUCN Red List.
A question that most people ask is why they have developed such a particular wide head shape. In addition to the advantage of having an increased number of electric sensors on their snout, with which they can detect their prey, the shape appears to make it less strenuous to swim because it lessens the drag of the water. Additionally, scientists have also found that Hammerheads usually swim slightly tilted to further reduce the drag.
We don’t know how many Hammerhead sharks make their home in the Dutch Caribbean, but they may just be passing through as they are considered a vagrant species. Tracking studies from the United States have demonstrated that Great Hammerheads are highly migratory and undertake journeys of hundreds of miles. A recent study on Great Hammerhead sharks in Bimini showed that they can make large scale return migrations of over 3000km.
Not much is known about the biology of Great Hammerheads. We do know they can grow as big as six meters, although a length of four meters is more common. They can be spotted close inshore over the reef, as well as migrating further offshore over deeper waters depending on their life stage and the time of year. It takes a young Hammerhead shark many years to reach maturity, which doesn’t happen until they’re about 2,5 meters long. After that, females start giving birth every year or two. They can have up to 6-42 pups growing in at 50-70 cm. The diet of Hammerhead consists of shellfish, squid, and other fish, including southern stingrays.
Hammerhead shark fins are relatively large and therefore highly valued on the Asian market as an ingredient for shark fin soup. To keep feeding this demand, Hammerheads are increasingly targeted by fisheries. Research shows that about 5% of all shark fins on the Hong Kong market are Hammerheads, including specimens from the Western Atlantic Ocean.
Also, despite their impressive size, Hammerhead sharks are very prone to stress, which makes them highly vulnerable, even to catch-and-release fishing practices.Sources: IUCN Red List of Threatened Species Red List status: ENDANGERED SPAW Hammerhead sharks: factsheet