Great hammerhead shark (Sphyrna mokarran)

TAXONOMY AND NAMING

Class : Chondrichthyes
Subclass : Elasmobranchii
Order : Carcharhiniformes
Clade : Neoselachii
Family : Sphyrnidae
Genus/species : Sphyrna mokarran
Common names :
English : Great hammerhead shark
Spanish : Tiburón martillo liso
French : Grand requin marteau

Great hammerhead shark (Sphyrna mokarran)©Karl Dietz

ANATOMY AND MORPHOLOGY

Sphyrna mokarran is the largest hammerhead shark. 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 teeth of this hammerhead are triangular and strongly serrated unlike S. lewini’s oblique cusps.

Hammerhead shark diagram©Johan Boshoff

Great hammerhead sharks are viviparous with a reported maximum total size of 550 to 610 cm, though 450 cm is more common for a mature adult. Litter size ranges from 6 to 33 (maximum 42) and pups are born after 11 months gestation with females breeding only once every two years, thus increasing the species’ susceptibility to population depletion. Great hammerheads have one of the oldest reported ages for any elasmobranch (44 years) but grow at relatively similar rates to other large hammerhead species. 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).

BEHAVIOUR AND LIFE HISTORY

The great hammerhead shark is not usually found in aggregations like other members of the Sphyrnidea family, but rather it is nomadic and migratory in its worldwide coastal-pelagic tropical range. A recent study revealed that during a 62 day journey an individual travelled 1,200 km from the coast of South Florida (USA) to the mid-Atlantic off the coast of New Jersey (USA). The evidence that great hammerhead sharks are capable of travelling such large distances in a relatively short time also indicates that the species could potentially be migrating into international waters. In the Bahamas, the species has been observed using designated locations or stop-offs along what are believed to be migratory paths for these animals. They are also seasonal residents in local areas (up to 5 months) and have high levels of site fidelity, since many individuals yearly return to the same sites in the Bahamas and Florida waters and the Northern Gulf of Mexico.

DISTRIBUTION AND ABUNDANCE

The habitat of S. mokarran ranges widely throughout the tropical waters of the world, from latitudes 40°N to 35°S. It is a coastal-pelagic and semi-oceanic species of hammerhead found throughout the world’s oceans in depths ranging from 1-300 m. It occurs close inshore and well offshore, over the continental shelves, in coastal zones near island terraces, and in passes and lagoons of coral atolls, as well as over deep water near land where it co-exists with the scalloped hammerhead, also an inhabitant of the tropic, and the smooth hammerhead, which favours cooler waters. Inshore areas are utilized by early life-stages of the species.

Great hammerhead shark geographic range (Sphyrna mokarran)©IUCN red list

THREATS

It has been shown that sharks on reefs in the Wider Caribbean Region mostly occur in areas with low human population density or in a few places where strong fishing regulations or conservation measures have been implemented. Coastal ecosystems that serve as nurseries for multiple species of sharks including hammerheads face both environmental and anthropogenic threats to their integrity. Environmental threats include fluctuations in temperature and salinity due to rising water temperatures and other climate change factors. In addition, fishing practices as well as habitat degradation and loss caused by human activities are among the major man made threats to coastal shark populations. This decline of great sharks from coastal ecosystems has caused trophic cascades with marked ecological consequences.

In particular, S. mokarran is targeted and bycaught in fisheries. In fact, their morphology, in particular the large body and the laterally expanded head, facilitate its capture by nets. In addition, Sphyrnids are highly vulnerable to stress, and often die after capture, even if they are returned to the water alive. Many countries are documented to consume hammerhead meat (usually salted or smoked) Great hammerheads are also highly prized in the recreational sector, particularly for those interested in obtaining records. Hammerhead sharks, S. mokarran in particular, have been noted as a favoured target species due to the size of their fins. Traders stated that hammerhead fins were one of the most valuable fin types on the market.

ABUNDANCE AND CONSERVATON STATUS

There is very little information on the global population size of great hammerhead sharks, with only occasional mentions in historical records. Also for the Wider Caribbean Region, data on past and present great hammerhead shark abundance is scarce. The Great Hammerhead shark was estimated to have decreased dramatically in global population size with a reduction above 80% in the last 3 generations. Furthermore, in most long-term shark fisheries evaluation studies worldwide, hammerhead shark declines were among the most drastic of any species assessed, and hammerheads declined the fastest of any species. However the Atlantic population of the species has increased since protection measures were introduced in 2005.

As a result of these fishing pressures, and in response to significant population declines, the IUCN recognizes great hammerheads as “Critically endangered” worldwide with a “decreasing” population trend. Regionally, the species is Endangered in the Northwest Atlantic, Gulf of Mexico and Critically Endangered in the Eastern Atlantic - while the South Atlantic stock data, which the IUCN didn’t use due to low catch rates and large confidence intervals, showed a 61.7% decline in CPUE from 1998-2008 of all hammerheads (Sphyrna spp.). This information is based on the most recent pelagic longline surveys conducted by NOAA.

PROTECTION STATUS

There is little regulation of Sphyrnidae trade, and the extent of both illegal trade activities is poorly known. Most Regional Fisheries Management Organisations’ (RFMO) regulations and some national laws prohibit finning sharks at sea (discarding the carcass and transshipping the fins at sea). Moreover, the International Commission for the Conservation of Atlantic Tunas’ (ICCAT) members are prohibited from retaining onboard, transshipping, landing, storing, selling, or offering for sale any part or whole carcass of great hammerhead sharks.

SPAW Protocol : yes since 2017 _ Annex III

CITES : yes _ Annex II

CMS : yes

Memorandum of Understanding on the Conservation of Migratory Sharks (MoU) : yes _ Annex I

List of sharks species prohibited from harvest in Florida state waters Florida Fish and Wildlife Conservation Commission (FWC) : yes since 2012

Highly Migratory Species, of the UN Convention on the Law of the Sea : yes since 1982 _ Annex I

Bibliography
Proposal for the uplisting of great hammerhead shark from Annex III to Annex II of the SPAW Protocol