Sphyrna mokarran

Great Hammerhead Shark

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Great Hammerhead Shark Great Hammerhead Shark

 

Description

Common names: hammerhead (English), shark (English), tiburón (Espanol)

Sphyrna mokarran (Rüppell, 1837)

Great hammerhead,     Great hammerhead shark

A hammerhead shark with broad, narrow-blade side extensions on the head forming a hammer (width 23-27% of TL); front margin of head nearly straight in adults, with shallow central and side indentations; front teeth blade-like, with 1 point, lower teeth straight, upper teeth oblique, deeply notched on rear side; rear teeth like front teeth; first dorsal fin very tall and curved, with pointed tip; second dorsal fin large, height > length of 3rd  gill slit; second dorsal and anal fins with strongly notched rear edges, their bases about equal; pelvics large, with concave rear borders; transverse pit above tail base crescent shaped, a pit below tail base; tail fin strongly asymmetrical, notched under tip of top lobe, large lower lobe.

Grey brown on back and sides, whitish below; no prominent markings on fins.

Said to reach 610 cm and 450kg, but uncommon above 350 cm; size at birth 50-70 cm.

A coastal pelagic and semi- oceanic species.

Depth range 1-300 m.

Circumtropical distribution, in the eastern Pacific from southern Baja California and the Gulf of California to northern Peru, Malpelo and the Galapagos.

Geography

Great hammerhead sharks occur in all tropical waters worldwide.

Biogeographic Regions: oceanic islands (Native ); indian ocean (Native ); atlantic ocean (Native ); pacific ocean (Native )

Ecosystem

Sphyrna mokarran is a coastal-pelagic and semi-oceanic tropical hammerhead occurring close inshore and well offshore, over the continental shelves, island terraces, and in passes and lagoons of coral atolls, as well as over deep water near land, at depths ranging from near-surface to over 80 m (Compagno in prep. b). The maximum total size is reported as 550 to 610 cm by Compagno (in prep. b), though 400 cm is more common for a mature adult (Compagno in prep. b, Last and Stevens 1994). Males mature at about 234 to 269 cm, and reach at least 341 cm, and females mature at about 250 to 300 cm and reach 482 to 549 cm (Compagno in prep. b). S. mokarran is viviparous, with a yolk-sac placenta. Litter size ranges from 6 to 42 pups after 11 months? gestation (Compagno in prep. b). Size at birth is 50 to 70 cm. Females breed once every two years (Stevens and Lyle 1989). Pups are born in late spring to summer in the Northern Hemisphere and between December and January off Australia (Compagno in prep. b, Last and Stevens 1994).

The diet includes fish (mainly demersal species), other elasmobranchs, crustacea and cephalopods (Compagno in prep. b). Strong et al. (1990) observed a large (ca 4 m) great hammerhead feeding on a southern stingray Dasyatis americana (disc width 1.5 m).

Systems
Marine

Conservation

Great hammerhead shark populations seem to be stable.

US Federal List: no special status

CITES: no special status

IUCN Red List of Threatened Species: endangered

 

Threats

Due to the distinctive head shape of this genus, it is typical for catches to be reported at the genus level, Sphyrna spp. Therefore, it is rare to find fisheries statistics that are specific to one species of hammerhead shark. Due to the great hammerhead?s preference for warmer waters, it can be expected to make up a greater proportion of tropical catches of hammerheads than more temperate fisheries. Sphyrna mokarran is taken by target and bycatch, fisheries (Dudley and Simpfendorfer 2006, Zeeberg et al. 2006) and is regularly caught in the tropics, with longlines, fixed bottom nets, hook-and-line, and possibly with pelagic and bottom trawls (Compagno in prep). Hammerhead sharks, S. mokarran in particular, have been noted as a favoured target species due to the size of their fins (R.T. Graham pers. comm). Fin prices are rising, driven by the Asian Fin market (R.T. Graham pers. obs).

There was a directed shark fishery operated by Taiwan around the northern coast of Australia that regularly caught great hammerheads up until 1986 (Stevens and Lyle 1989). Other possible threats include sport fishing (Pepperell 1992) and capture in anti-shark measures around the beaches of Australia and South Africa (Paterson 1990, Cliff 1995). Bonfil (1994) gives an overview of global shark fisheries. This species is mentioned specifically with reference to fisheries in Brazil, East USA and Mexico, however Sphyrna spp. are mentioned in the majority of tropical fisheries cited.

West Africa
Mainly taken by drift gillnets, bottom gillnets and on longlines, hook and line, pelagic and bottom trawls (Schneider 1990). This species is a bycatch in both the industrial and artisanal fisheries but a specialised artisanal fishery for charcharhinid and sphyrnid species was introduced in Sierra Leone in 1975, and since then fishing pressure has not decreased (M. Seisay pers. obs). The Subregional workshop for sustainable management of sharks and rays in West Africa, 26-28 April 2000 in St Louis, Senegal (Ducrocq 2002) noted the high threat to sharks in the west African region and a noticeable decline in the CPUE of total sharks and rays. This workshop identified S. mokkarran as particularly threatened. The subsequent sub-regional plan of action for sharks of West Africa (member states of the Sub Regional Fishing Commission) states that landings of S. mokarran have collapsed and lists this as one of the four most threatened species, deserving the greatest attention in the whole region (Ducrocq 2002).

Previously observed from Mauritania to Angola, reportedly abundant from November to January in Senegal, and in October in Mauritania (Cadenat and Blache 1981). However, recent scientific trawl surveys off Guinea-Bissau, Mauritania, Senegal, Gambia and Guinea-Conakry between 20 to 1,000 m have failed to record it, except in very low numbers off Guinea-Conakry and one record from Senegal in 1995 (FISA unpublished data). Anecdotal evidence from interviews with fishermen in Senegal, Guinea-Bissau and Guinea suggest there was a large decline in all shark species during the 1990s and that S. mokarran is almost extirpated from these areas (M. Ducrocq pers. obs.).

Data are lacking as there is little species specific data collection in the region, however this is a very distinctive species with a large dorsal fin which is highly valued for the shark fin trade. Increased targeting of sharks began in the 1970s, when a Ghanaian fishing community settled in the Gambia and established a commercial network throughout the region, encouraging local fishermen to target sharks for exportation to Ghana. By the 1980s many fishermen were specialising in catching sharks, resulting in a decline in overall shark populations (Walker et al. 2005). There has been rapid growth in the shark fin market in this region, for export to the Far East, and yearly production of dried fins exported from Guinea-Bissau alone is estimated at 250 t (dry weight) (Walker et al. 2005). Sphyrna species combined represented 42% of bycatch in the European industrial pelagic trawl fishery off Northwest Africa (Zeeberg 2006).

Although there are very little species specific data available, the absence of recent records and region-wide recognition of the extent of the decline, give cause to suspect that the population has decreased by least 80% in the past 25 years. Fisheries in this region remain largely unmonitored and unmanaged, leading to an assessment of Critically Endangered in the Eastern Atlantic.

Southwest Indian Ocean
This species is widely distributed in the SW Indian Ocean and is a summer migrant to KwaZulu-Natal (KZN) (east coast of South Africa), where the annual catch in the KZN shark nets is 11 sharks (1978 to 1999), consisting mainly of adolescents and adults. Over this period there has been a significant decline in annual catch (18 to 4 sharks) and catch rate (0.5 to 0.2 sharks.km-net-1.yr-1 (p = 0.000) (Dudley 2002). A continued decline in catch rate was reported for the period 1978 to 2003 (Dudley and Simpfendorfer 2006). Over this period, regression of catch and catch rate against year revealed a significant decline in annual catch from 18 to two sharks (89%) and in catch rate from 0.44 to 0.09 sharks.km-net-1.yr-1 (79%) (S. Dudley pers. obs. 2006). It is uncertain whether these declines reflect highly localized stock depletion or whether they reflect a general decline in the Southwest Indian Ocean, but large numbers of longline vessels have been reported to be operating illegally in coastal waters of the western Indian Ocean where they are targeting primarily hammerhead sharks and giant guitarfish Rhynchobatus djiddensis (IOTC 2005 in Dudley and Simpfendorfer 2006). This species is generally regarded as solitary, and is therefore unlikely to be abundant wherever it occurs. This is in contrast to other large hammerheads, such as Sphyrna lewini which forms large schools. Sphyrna mokarran, like other hammerheads, readily takes baited hooks and is sought after for its fins. Based on these characteristics, together with the decline of 79% in catch rates in the KZN shark nets, this species is assessed as Endangered in the southwest Indian Ocean.

Northwest Atlantic
This species is caught primarily as a bycatch in the pelagic longline, bottom longline and net fisheries along the northwest Atlantic and Gulf of Mexico. It is also caught in the recreational fishery. The species represents 0.7% of the species catch and suffers from greater than 90% at-vessel fishing mortality in the U.S. bottom longline fishery (Commercial Shark Fishery Observer Program unpubl. data). The U.S. pelagic fishery logbook data has shown a decline close to 90%, however this data-set is known for inaccurate data reporting (Beerkircher et al. 2002). There is probably a lack of reporting of the catch of great hammerheads because this species is routinely finned and discarded, which is illegal in the US Atlantic Federal Waters (Commercial Shark Fishery Observer Program unpub. data). Both the pelagic and bottom longline observer programs have recorded a 2 to 3:1 ratio for S. Lewini to S. mokarran. The meat is not valuable but the fins are high grade and bring in a good price, thus finning still occurs in the U.S. fishery. Interview with shark fishermen in Belize indicate that hammerheads (S. mokarran in particular) are a favoured target species for their large fins (R.T. Graham pers. obs.). Fin prices are rising above US$50/lb in the neighbouring countries of Guatemala, driven by Asian buyers, according to these interviews (R.T. Graham pers. obs). This species is probably caught in other fisheries but is usually placed in a combined ?hammerhead? category. Species identification (S. mokarran vs. S. lewini) a large obstacle in the proper assessment of this species. The high at-vessel fishing mortality for both species of hammerhead makes the threat of fishing even greater for this species. In the Pacific Ocean off of Guatemala this species is caught as by-catch in the commercial longline fishery.

There appear to be little data for landings and catch effort for this species in Central America and the Caribbean. Off the coast of Belize hammerheads were fished heavily by longline in the 1980s and early 1990s. Interviews with fishermen indicate that the abundance and size of Sphyrnids has declined dramatically in the past 10 years as a result of over exploitation, leading to a halt in the Belize based shark fishery (R.T. Graham pers. obs). However, the pressure is still sustained by fishers driving into Belizean waters from Guatemala (R.T. Graham pers. obs). The Cuban directed shark fishery (longline) between 1983 and 1991 recorded S. mokarran (subadults and juveniles) as one of 23 species caught. Since 1992 small increases in mean sizes were noted, indicating partial recovery of the species. In Mexico between November 1993 and December 1994 (Tamaulipas, Veracruz, Tabasco, Campeche and Yucatan) 901 vessels were monitored every day. Sphyrna mokarran represented 86% of the total catch.

The difficulty in species identification and accurate recording make an assessment of this species very difficult. However, low survival at capture makes this species very vulnerable to fishing pressure, whether directed or incidental. This species is listed as Endangered in the Northwest Atlantic under criterion A2 based on a suspected decline of at least >50% over the past 10 years. The decline is poorly documented and has not been curtailed.

Australia
There has been a large increase in the illegal, unregulated and unreported (IUU) fishing in northern Australia in the last few years (J. Stevens pers. obs.). Several initiatives are underway to identify which species are being taken and in what quantities. Hammerheads are known to feature in the catches, and are suspected targets for their large valuable fins, although no specific data are available. Some domestic boats are also suspected to be targeting species for their fins in the Northern Territory, and this likely includes hammerheads (J. Stevens pers. obs). It is not a productive species and is coming out at the ?high-risk? end in recent Risk Assessments of northern Australian elasmobranchs (J. Stevens pers. obs). There is concern that this species is being increasingly targeted, and therefore an urgent need to obtain data to form an accurate assessment of the population in this region.

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