Geographical Distribution
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.
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.
Biology
Distinctive Features
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.
·Size, Age, and Growth
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). Expected life span of this species is approximately 20-30 years of age.
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).
Food Habits
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.
Hammerheads are highly valued among fin traders and hammerheads are the second-most abundant species in the international fin trade. Hammerheads are generally not a target species but suffer high bycatch mortality. Great hammerheads (Sphyrna mokarran), along with smooth hammerhead (Sphyrna zygaena) and scalloped hammerheads (S. lewini), are caught in a variety of fisheries including artisanal and small-scale commercial fisheries, bottom longlines as well as offshore pelagic longlines. Despite their distinctive characteristics, scalloped and smooth hammerhead sharks are often not distinguished from each other by harvesters. Abundance trend analyses of catch-rate data indicate large declines in abundance.
Source: Prepared by Cathleen Bester for the Florida Museum of Natural History
NOAA Fisheries Service produces an annual report on U.S. implementation the Shark Finning Prohibition Act of 2000. The 2008 Annual Report to Congress is available online (note that this document does not reflect updates from 2009-2010): http://www.nmfs.noaa.gov/ia/intlbycatch/rpts_shark_finning.htm
SHARKS at CITES CoP15
The United States and co-sponsor Palau have submitted a proposal to list
scalloped hammerhead sharks (Sphyrna lewini) in CITES Appendix II, with
the following species listed as “look-alikes” due to similar-looking
fins that are highly valued in the market:
• Great hammerhead sharks (S. mokarran);
• Smooth hammerhead sharks (S. zygaena);
• Dusky sharks (Carcharhinus obscurus); and
• Sandbar sharks (C. plumbeus).
The United States and Palau also submitted a proposal to list oceanic
whitetip sharks (Carcharhinus longimanus) in CITES Appendix II.
In comparison to other marine species, sharks are slow to mature and produce few young, making them especially vulnerable to overfishing. The greatest threats to both the scalloped hammerhead and oceanic whitetip sharks are harvest for the international fin trade and bycatch, which have resulted in significant population declines for both species. Shark fins are highly valued particularly in Asia and Asian markets around the world, where fins are consumed primarily as shark fin soup. The United States has proposed CITES Appendix-II listings for these shark species to ensure that international trade, including trade in their fins, is legal and sustainable.
A CITES Appendix-II listing does NOT prohibit harvest; it requires
regulation of international trade. Should these listing proposals be
adopted, sharks taken within a country’s own Exclusive Economic Zone
(EEZ) for domestic consumption would not be regulated by CITES. Sharks
taken within a country’s own EEZ and subsequently exported, and sharks
taken on the high seas would be subject to CITES requirements.
Link to the US/Palau proposal: http://www.cites.org/eng/cop/15/prop/E-15-Prop-15.pdf
There is also a progress report and recommendations regarding the conservation and management of sharks and stingrays:
