Fish Facts » Hammerhead Shark
Description
The great hammerhead is a very large shark with the characteristic hammer-shaped head from which it gets its common name. The font margin of the head is nearly straight with a shallow notch in the center in adult great hammerheads, distinguishing it from the smooth hammerhead and scalloped hammerhead. The first dorsal fin is very tall with a pointed tip and strongly falcate in shape while the second dorsal is also high with a strongly concave rear margin. The origin of the first dorsal fin is opposite or slightly behind the pectoral fin axil with the free rear tip falling short to above the origin of the pelvic fins. The rear margins of the pelvic fins are concave and falcate in shape, not seen in scalloped hammerhead (S. lewini). The posterior edge of the anal fin is deeply notched. Within the hammerhead family, several species are differentiated from each other by variations within the cephalophoil. The scalloped hammerhead (S. lewini) is distinguished as a smaller species with a rounded anterior margin and notch on the head. The smooth hammerhead (S. zygaena) has a broad, flat unnotched head. The smaller bonnethead (S. tiburo) is much easier to identify with a shovel-shaped head. Another distinguishing characteristic of the great hammerhead is the curved rear margins on the pelvic fins while the scalloped hammerhead has straight posterior edges. The dorsal side of the great hammerhead is dark brown to light grey or even olive in color fading to white on the underside. The fins lack markings in adults while the apex of the second dorsal fin may appear dusky in juveniles.
Habitat
This large coastal/semi-oceanic shark is found far offshore to depths of 300 m as well as in shallow coastal areas such as over continental shelves and lagoons. The great hammerhead migrates seasonally, moving poleward to cooler waters during the summer months.
Diet
Great hammerheads are active predators, preying upon a wide variety of marine organisms, from invertebrates to bony fishes and sharks. A favorite prey item is the stingray, which is consumed along with the tail spine! Invertebrate prey include crabs, squid, octopus, and lobsters while commonly consumed bony fish are groupers, catfishes, jacks, grunts, and flatfishes. Great hammerheads have also been reported as cannibalistic, eating individuals of their own species. It feeds primarily at dusk along the seafloor as well as near the surface using its complex electro-sensory system to located prey.
Fishery
Fished both commercially and recreationally, great hammerheads are highly valued for their fins while the meat is rarely consumed by humans. They are also provide liver oil used in vitamins, hides for leather, and carcasses ground into fishmeal. Although not often a targeted species, the great hammerhead is regularly caught in tropical regions with longlines, bottom nets, hook-and-line, and trawls. Recreationally fished with surface bait, the great hammerhead provides a good fight and exciting sport to fishers
Size
As the largest of the hammerheads, the great hammerhead averages over 500 pounds (230 kg). The world record great hammerhead was caught off Sarasota, Florida (US) weighing 991 pounds (450 kg). The largest reported length of a great hammerhead is 20 feet (6.1 m). In waters off Australia, males reach maturity at a length of 7.4 feet (2.25 m) corresponding to a weight of 113 pounds (51 kg) and females are mature at a total length of 6.9 feet (2.10 m) corresponding to a weight of 90 pounds (41 kg) (source: Stevens and Lyle 1989).
Range
Circumtropical in distribution, the great hammerhead is found in coastal warm temperate and tropical waters within 40°N - 37°S latitude. In the western Atlantic Ocean, it ranges from North Carolina (US) south to Uruguay, including the Gulf of Mexico and Caribbean regions, while in the eastern Atlantic Ocean, this species ranges from Morocco to Senegal, including the Mediterranean Sea. Distribution of the great hammerhead includes the Indian Ocean and the Indo-Pacific region from Ryukyu Island to New Caledonia and French Polynesia. The eastern Pacific range is from southern Baja, California (US) through Mexico, south to Peru. The great hammerhead is considered a highly migratory species within Annex I of the 1982 Convention on the Law of the Sea.
Source(s)
Florida Museum of Natural History
Source(s) on the web
http://www.flmnh.ufl.edu/fish/Gallery/Descript/GreatHammerhead/GHammerhead.html