| Common names: hammerhead (English), shark (English), tiburón (Espanol), cornuda (Espanol) |
Sphyrna zygaena (Linnaeus, 1758)
Smooth hammerhead, Smooth hammerhead shark
A large hammerhead shark with broad, narrow-blade lateral extensions on the head; width of hammer 26-29% of TL; front margin of head broadly convex, with prominent side indentations, but no central indentation; front teeth blade-like, with 1 point, lower teeth straight, upper teeth oblique, deeply notched on rear side, weakly serrate in adults; rear teeth like front teeth; nostrils short, before eyes on head plane; 5 gill slits, last one over front of pectoral fin; 2 dorsal fins, first moderately large and erect, its rear margin concave; second dorsal and pelvic fins low, rear edge of pelvic ~ straight; anal fin base ~ equal to base of second dorsal fin, its rear margin prominently notched; origin of second dorsal fin slightly behind origin of anal fin; 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.
Dark olive to dark grey-brown on back and sides, white below; undersides of pectoral fin tips dusky.
Grows to 500 cm and 400kg; size at birth 50-61cm.
A coastal-pelagic and semi-oceanic hammerhead often sighted near the surface.
Depth: 0-200 m.
Circumglobal in tropical and temperate seas; in the eastern Pacific from California to Chile and the Galapagos and Malpelo. Most common in cooler temperate waters.
Global Endemism: All species, TEP non-endemic, Circumtropical ( Indian + Pacific + Atlantic Oceans), "Transpacific" (East + Central &/or West Pacific), All Pacific (West + Central + East), East Pacific + Atlantic (East +/or West), Transisthmian (East Pacific + Atlantic of Central America), East Pacific + all Atlantic (East+West)
Regional Endemism: All species, Eastern Pacific non-endemic, Tropical Eastern Pacific (TEP) non-endemic, Continent + Island (s), Continent, Island (s)
Climate Zone: North Temperate (Californian Province &/or Northern Gulf of California), Northern Subtropical (Cortez Province + Sinaloan Gap), Northern Tropical (Mexican Province to Nicaragua + Revillagigedos), Equatorial (Costa Rica to Ecuador + Galapagos, Clipperton, Cocos, Malpelo), South Temperate (Peruvian Province )
Western Atlantic: From Nova Scotia to Florida, USA and Virgin Islands to southern Argentina (Compagno in prep, Domingo in prep, Last and Stevens 1994).
Eastern Atlantic: From the UK and as a vagrant in the North Sea, southwards, including the Mediterranean Sea, where has been reported as more common in the western basin (Buencuerpo et al. 1998), to Mauritania, Senegal, Cape Verde Islands, Guinea, Ivory Coast, and Angola (Compagno in prep, Last and Stevens 1994).
Indian Ocean: South Africa and southern Mozambique, Comoros Islands, southern India, Sri Lanka, and Australia (Western Australia, South Australia, Victoria, Tasmania) (Compagno in prep, Last and Stevens 1994).
Western Pacific: From Viet Nam (Gulf of Tonkin) to Japan and southern Russia in the Northwest Pacific. Australia (New South Wales), New Zealand, Lord Howe and Kermadec Islands in the Southwest Pacific (Compagno in prep, Last and Stevens 1994).
Eastern Pacific: From northern California, USA, to Gulf of California, Mexico, Panama, and from Ecuador to Chile, including Galapagos Islands (Compagno in prep, Last and Stevens 1994).
Also occurs off the Hawaiian Islands, USA and possibly off Samoa (Compagno in prep, Last and Stevens 1994).
Regularity: Regularly occurring
Type of Residency: Year-round
Inshore/Offshore: Offshore, In & Offshore, Inshore
Water Column Position: Mid Water, Near Bottom, Water column only
Habitat: Water column
FishBase Habitat: Bentho-Pelagic
Habitat and Ecology
The Smooth Hammerhead is a coastal-pelagic and semi-oceanic and occurs on the continental shelf, to 200 m depth (Ebert 2003). Smale (1991) reported that large individuals were commonly found over deep reefs on the edge of the continental shelf. The species has also been observed in freshwater in Indian River, Florida, USA (G. Burgess pers. comm.) and in the Rio de la Plata estuary in Uruguay (Doño 2008, Domingo in prep.). In the demersal gillnet fishery in southern Western Australia, juvenile S. zygaena are caught on the bottom in depths from the shore to at least 60 m (C. Simpfendorfer pers. comm.). Compagno (in prep.) reports that this species occurs at or near the surface in the East China Sea, whereas S. mokarran and S. lewini range into deeper water in this area. The nursery habitat of this species is smooth sandy substrate in shallow waters, down to depths of 10m. Large schools of juvenile S. zygaena have been reported from off South Africa (Bass et al. 1975). Off southern Brazil in South America, S. zygaena occurs at depths of 10-100 m (Kotas 2004, Vooren et al. 2005). Nursery areas and juveniles have been reported in coastal waters off Uruguay from mid-Rio de la Plata to the Atlantic Ocean bordering with Brazilian waters from November to March at temperatures of 16-23°C and salinities of 12-27 ups (Doño 2008, Domingo in prep.).
There are only limited published biological data on S. zygaena, despite its widespread occurrence. Compagno (1984, in prep.) reported that the species reaches a maximum size of 370-400 cm total length (TL). Stevens (1984) reported that off the east coast of Australia males mature at about 250-260 cm TL and females at about 265 cm TL. Castro and Mejuto (1995) reported gravid females between 220 and 255 cm fork length (FL), but gave no relationship between fork and total length. Bass et al. (1975) reported a female S. zygaena from South Africa that appeared to have recently mated in February and another female caught in November that contained full-term embryos. Stevens (1984) reported that off the east coast of Australia parturition occurs between January and March, with ovulation at about the same time. The gestation period off eastern Australia would appear to be 10-11 months. Castro and Mejuto (1995) reported 21 gravid females with a mean litter size of 33.5 from the waters of western Africa. Off eastern Australia Stevens (1975) reported litter sizes between 20-49 (mean 32). The sex ratio of embryos is 1:1 (Stevens 1984, Castro and Mejuto 1995). Compagno (1984, in prep) gave the size at birth as 50-61 cm. Smale (1991) and Doño (2008) reported juveniles with open umbilical scars from South Africa at sizes between 59 and 63 cm, and from Uruguay between 49 and 55 cm TL, respectively. Possible pupping grounds and nursery areas for this species include the northern Gulf of California and shallow coastal waters off southern Brazil and Uruguay (Vooren 1997, 1999, Vooren and Klippel 2005, Doño et al. in prep). In Uruguay pupping grounds and nursery areas were reported in coastal waters by Doño (2008).
Although maximum age has yet to be determined for this species, it is thought that the lifespan of the smooth hammerhead may be 20 years or longer (FLMNH 2008). Further information is required on the biology and life-history parameters of this species.
Squid and teleosts are the most common prey. Based on specimens caught by recreational anglers off New South Wales, Australia, Stevens (1984) reported that 76% of specimens with food in their stomachs contained squid and 54% teleosts. For S. zygaena less than 2 m in length from the waters off South Africa, Smale (1991) reported that the diet was dominated by inshore squid (mostly Loligo v. reynaudii), with teleosts such as Hake, Horse Mackerel and Ribbonfish also important. Crustaceans and elasmobranchs have also been reported from stomach analyses (Bass et al. 1975, Compagno 1984, Smale 1991, Last and Stevens 1994). Compagno (1984) reported that sharks and rays were a favoured food, presumably of larger specimens. However, of 145 S. zygaena from South Africa examined by Dudley and Cliff (1993) only 0.7% contained elasmobranch prey.
Habitat Type: Marine
Recorded at 200 meters.
Water temperature and chemistry ranges based on 46 samples.
Depth range (m): 0 - 2430
Temperature range (°C): 3.050 - 22.758
Nitrate (umol/L): 0.441 - 19.278
Salinity (PPS): 34.213 - 36.255
Oxygen (ml/l): 2.346 - 5.965
Phosphate (umol/l): 0.215 - 1.546
Silicate (umol/l): 1.104 - 23.567
Depth range (m): 0 - 2430
Temperature range (°C): 3.050 - 22.758
Nitrate (umol/L): 0.441 - 19.278
Salinity (PPS): 34.213 - 36.255
Oxygen (ml/l): 2.346 - 5.965
Phosphate (umol/l): 0.215 - 1.546
Silicate (umol/l): 1.104 - 23.567
Note: this information has not been validated. Check this *note*. Your feedback is most welcome.
Non-Migrant: No. All populations of this species make significant seasonal migrations.
Locally Migrant: No. No populations of this species make local extended movements (generally less than 200 km) at particular times of the year (e.g., to breeding or wintering grounds, to hibernation sites).
Locally Migrant: No. No populations of this species make annual migrations of over 200 km.
Diet: mobile benthic crustacea (shrimps/crabs), octopus/squid/cuttlefish, bony fishes, sharks/rays
Life History and Behavior
Molecular Biology and Genetics
Barcode data: Sphyrna zygaena
Below is a sequence of the barcode region Cytochrome oxidase subunit 1 (COI or COX1) from a member of the species.
See the BOLD taxonomy browser for more complete information about this specimen and other sequences.
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Download FASTA File
Statistics of barcoding coverage: Sphyrna zygaena
Public Records: 26
Specimens with Barcodes: 77
Species With Barcodes: 1
CITES: Not listed
IUCN Red List Assessment
Red List Category
Red List Criteria
The Smooth Hammerhead (Sphyrna zygaena) is one of the larger hammerhead sharks, found world-wide in temperate and tropical seas, with a wider range than other members of its family. It is semipelagic and occurs on the continental shelf. Although few data are available on the Smooth Hammerhead's life-history characteristics, it is a large hammerhead shark and presumably at least as biologically vulnerable as S. lewini. This species is caught with a wide variety of gears in both coastal and oceanic fisheries, as bycatch and a target. Therefore in some areas, all size classes and reproductive stages are susceptible to capture. The Smooth Hammerhead's large fins are highly valued for their high fin ray count and they are being increasingly targeted in some areas in response to increasing demand for the fin trade. Few species-specific data are available to assess population trends because catches of hammerhead sharks are often grouped together under a single category. Very often these sharks are finned and the carcasses discarded. This species has sometimes been confused with S. lewini in the tropics and these two species are probably misidentified with each other in some areas. Time series data on population trends in hammerhead sharks, including S. zygaena, are available from the Northwest and Western Central Atlantic and the Mediterranean Sea. In the Northwest and Western Central Atlantic, where S. zygaena is outnumbered by S. lewini by about ten to one, analysis of U.S. pelagic longline logbook data estimated that Sphrynidae (including S. lewini, S. mokarran and S. zygaena) declined in abundance by 89% since 1986. In the Mediterranean Sea, where S. zygaena outnumbers S. lewini, compilation and meta-analysis of time series abundance indices estimated that Sphrynidae (including S. lewini, S. mokarran and S. zygaena) declined by >99% in abundance and biomass since the early 19th century. While very steep declines have been recorded in these areas, the species is afforded some refuge in other areas of its range, such as southern Australia, where it is abundant and fishing pressure is low. The species is currently assessed as Vulnerable globally and further investigation into threats, population trends, catches and life-history parameters throughout its range is required to determine whether it may warrant a higher category in the future.
National NatureServe Conservation Status
Rounded National Status Rank: NNR - Unranked
NatureServe Conservation Status
Rounded Global Status Rank: GNR - Not Yet Ranked
This species' fins are highly valued and they are being increasingly targeted in some areas in response to increasing demand for shark fins. Hammerhead shark species S. zygaena and S. lewini were found to represent at least 4-5% of the fins auctioned in Hong Kong, the world's largest shark fin trading center (Clarke et al. 2006a). Hammerhead shark fins are generally high value compared to other species because of their high fin ray count (S. Clarke unpubl. data). It is estimated that between 1.3 and 2.7 million S. zygaena and/or S. lewini are represented in the shark fin trade each year or, in biomass, 49,000 to 90,000 mt (Clarke et al. 2006b).
Catches of Sphyrnidae have been reported only from the Atlantic Ocean since 1991 and these landings are undoubtedly under-reported. The catch was near 2,200 tonnes in 2004 (Maguire et al. 2006). Only S. zygaena and S. lewini are reported as individual species in the Food and Agriculture Organisation (FAO) fisheries statistics, but hammerhead catches are often grouped in one category as, Sphyrna species, which makes identification of actual catches of S. zygaena difficult.
Northeast Atlantic and Mediterranean Sea
The Smooth Hammerhead appears to be less common in the central Mediterranean, in comparison to the western regions of this sea. Buencuerpo et al. (1998) reported that 757 specimens were captured during their pelagic longline and gillnet survey from July 1991-July 1992 off southern Spain. Most of these specimens were presumed to be part of the Atlantic population. In the Northeast Atlantic and Mediterranean Sea this species is mainly caught by longlines and gillnets, particularly as bycatch in tuna and swordfish fisheries. Despite a ban on driftnetting in Mediterranean waters, this practice continues illegally (WWF 2005). A recent study of the Moroccan driftnet fleet operating in the Alboran Sea (southwest Mediterranean) and around the Strait of Gibraltar by Tudela et al. (2005) indicates that pelagic fishing pressure in this area is beyond the reproductive capacity of several other semi-oceanic shark species that were previously caught with S. zygaena (such as Alopias vulpinus). Some of these vessels appear to be increasingly targeting sharks by deploying driftnets closer to shore (Tudela et al. 2005). Sphyrna zygaena is not mentioned in this study. However, this species was reported in Buencuerpo et al.'s (1998) survey of longline and gillnet landings from the same area in 1992. Buencuerpo et al. (1998) report the highest catches of Sphyrna zygaena in the Spanish swordfish fishery from the western African coasts and near the Strait of Gibraltar.
Ferretti et al. (2008) compiled nine time series of abundance indices from commercial and recreational fishery landings, scientific surveys and sighting records, to reconstruct long-term population trends of large sharks in the northwestern Mediterranean Sea. Of the taxa for which there were enough data to investigate, hammerhead sharks (Sphyrna spp) declined the fastest; they appeared to disappear from coastal waters after 1963 and catches declines consistently in pelagic waters in the early 1980s in all sectors. Meta-analysis showed an average instantaneous rate of decline of -0.17 (CI 95%: -0.34, -0.003; time range 178 years) in abundance and -0.36 (CI 95%: -0.56, -0.16; time range: 107 years) in biomass, which translated into an estimated species decline of 99.99% in both cases. Walker et al. (2005) also report that the species has virtually disappeared from the central-southern Mediterranean Sea since 1986.
Mejuto et al. (2002) estimate landings of 240 tons of Sphyrna spp. And 1.1 ton of Sphyrna zygaena in the North Atlantic for the Spanish Surface longline fleet in 1999. De la Serna et al. (2002) reported only eight specimens of S. zyagaena (0.05%) in a total 17,759 sharks caught during a survey of the Spanish Mediterranean Fisheries from 1997-1999. This is significantly lower when compared to results of the same fishery along the west African coast and Iberian peninsula (where 757 specimens in period July 1991-July 1992 were caught). Megalofonou et al. (2000) only recorded four specimens during their survey of shark bycatches and discards in Mediterranean large pelagic fisheries in 1998-1999 (one in the Adriatic, two in the Ionian Sea and one in Spanish Mediterranean waters). There are few recent records of Sphyrna species in the eastern Mediterranean Sea. A total of 16 records of S. zygaena were collected in the eastern Adriatic from the 19th century to the 1950s, including reported catches were distributed throughout whole of the eastern coast. A higher number of records were reported during the 19th century in comparison to the 20th century (10 vs. 6, respectively) and the species has not been reported in this area since 1956 (Soldo and Jardas 2002). Although it occurs in open waters of southern Adriatic, it is only caught very rarely (Bello 1999). There were only 13 records of S. zygaena in the Northern Tyrrhenian and Ligurian Seas from the 1960s-1995 and there are no reports of this species during the last five years (F. Serena pers. comm.).
Longline fleets exert intense fishing pressure throughout the Northwest Atlantic (Baum et al. 2003). Baum et al. (2003) estimated that hammerhead sharks (grouped data for S. lewini, S. mokarran and S. zygaena) have declined in abundance by 89% since 1986 (95% confidence interval (CI): 86 to 91%) in their analysis of U.S. pelagic longline logbook data. This group is primarily composed of S. lewini; in Virginia Institute of Marine Science sampling programmes since 1973, S. lewini outnumbered S. zygaena by more than ten to one (Ha 2006).
In the Southwest Atlantic S. zygaena faces two main threats related to fisheries: catches of juveniles and pups along the continental shelf by different fishing gears (mainly bottom gillnets and trawlers (Vooren and Lamónaca 2003, Kotas and Petrere 2002, Doño 2008); and adult and juvenile catches by gillnets and longlines throughout the continental shelf and oceanic environments for the international fin market (Kotas et al. 2001, Kotas and Petrere 2002ab, Kotas and Petrere 2003, Zerbini and Kotas 1998, Domingo in prep). The species therefore faces intensive fishing pressure at all points in its life-cycle in this area. Off southern Brazil; from 1983-2005, indices of abundance of neonates of S. zygaena varied from 1-5% of the abundance of those of S. lewini in the same area (Vooren et al. 2005). Catches of neonates, juveniles and adults are taken by longline fleets based in Itajaí, which operate along the coast of southern Brazil (Kotas and Petrere 2002). The species is also a known bycatch of domestic, as well as Taiwanese, Japanese and other international longline fishing fleets operating throughout the South Atlantic Ocean (Joung et al. 2005, Matsushita and Matsunaga 2002). Juveniles and adults are also taken as bycatch by longline fleets operating in shelf and oceanic waters off Uruguay and Brazil (200-3000m). Given the declining trends apparent in other areas of the species' range where it is heavily fished, for example the Mediterranean Sea, the population in the Southwest Atlantic may be unable to withstand continued fishing pressure.
Sphyrna zygaena is taken as both target and bycatch in artisanal and industrial fisheries along the eastern Pacific coast of the Americas. Landings of this species in Guatemala, Costa Rica, Panama and Mexico have been reported as 1% of the total chondrichthyan catch. Seasonal surveys of fisheries on the eastern coast of Mexico, conducted during 1998-1999, showed that S. zygaena was a common component of the landings of directed shark fisheries. The majority of those landed were juveniles in many areas (Bizzaro et al. 2007). This study reports that large sharks, such as S. zygaena were usually targeted in directed drift gillnet, or to a lesser extent, surface longline fisheries. Márquez-Farías (2000) report an overall decline in catch rates for all shark species in the Gulf of California. This species has also been reported as bycatch in longline, bottom trawl and gillnets in the landings of other Central and South American countries, down to Peru. In Ecuador, S. zygaena represents 11% of the total chondrichthyan landings. There is a high proportion of juveniles in these Ecuadorian landings; 91% of female (measuring S. zygaena represented 12.5% of total chondrichthyan landings over the period 1996-2005 (Romero, M. PANtiburones).
An artisanal fishing fleet targeting sharks south of Tres Marias Islands in the Central Mexican Pacific operated out of La Cruz Huanacaxtle from the early 1990s to 1997, after which they moved to Yavaros in the Central Gulf of California, apparently following the migratory movements of some of the shark species (Pérez-Jiménez et al. 2005). Pérez-Jiménez et al. (2005) monitored the shark landings of this fleet from 1995-1996. Sphyrna zygaena was the most important of 10 species recorded in 607 sets taken south of the Tres Marias Islands, composing 35% of 2,004 sharks recorded. This artisanal fleet has not caught sharks south of the Tres Marias Islands or in the Central Gulf of California since the late-1990s (Pérez-Jiménez et al. 2005).
Illegal fisheries target sharks for their fins around the Galapagos Islands. There are no specific data for these fisheries, but given the high value of its fins, it is very likely that it is targeted in illegal finning activities. Illegal fishing in this area is not only practiced by fishermen from the Galapagos, but also by the industrial and artisanal fleets from continental Ecuador and international fleets (Coello 2005). Divers and dive guides in the Galapagos have noted a severe decrease in shark number and schools of hammerhead sharks (P. Zarate pers comm.). The Ecuadorian Government issued a decree in 2004 prohibiting fin export from Ecuador, in an attempt to help stop illegal finning in the Galapagos. Unfortunately the Decree had the reverse effect of establishing illegal trade routes with fins now being exported mainly via adjacent Peru and Colombia, where there is no regulation banning finning. Interviews within fishermen and traders in both Ecuador and Peru suggest that there are illegal trade routes for fins transported both from continental Ecuador and directly from the Galapagos to Peru, and illegal finning activity is ongoing (Sáenz 2005, WildAid 2005).
Hammerhead sharks, including S. zygaena, are also caught by international purse seine fleets targeting tunas in the high seas of the Eastern Pacific, particularly those associated with Fish Aggregating Devices (FADs) (Román-Verdesoto and Orozco-Zöller 2005). IATTC conducted a one year species sampling programme to determine the accuracy of identification by fisheries observers on these vessels. Sphyrna zygaena was found to represent 1.7% and S. lewini, 3.6% of the sharks recorded during this programme, respectively.
In the West Australian demersal gillnet fishery Sphyrna zygaena is an important bycatch species, but appears not to be impacted by this low fishing pressure. The species is reasonably abundant around the northern North Island of New Zealand, and the most abundant shark species recorded in aerial surveys along the northwest coast. In New Zealand, neonates and small juveniles are a common bycatch in commercial gillnet fisheries for flatfish, and are also commonly taken by trawlers and Danish seiners. They are usually discarded, although juveniles discarded from gillnets are usually dead. Adults are mainly taken by gamefishers as bycatch when targeting marlin with live baits, and consequently the annual catch is small. Many of the sharks taken by game fishers are released alive (C. Duffy, pers. comm.). S. zygaena is abundant in the region, and significant numbers of adults do not appear to be taken in commercial fisheries.
Sphyrna zygaena, along with S. lewini and S. mokarran, are exploited by the commercial line fishery and recreational game fishery off New South Wales. The majority of the hammerhead catch in commercial and recreational fisheries is composed of S. zygaena and S. lewini (Scandol et al. 2008). Total commercial hammerhead landings peaked in 1993/94 at 15/7 t. Annual commercial catches have averaged ~3 t in the last five years. Most hammerhead landings come from the Ocean Trap and Line Fishery (74%), but they are also taken in the Estuary General and Ocean Trawl Fisheries.
Information on catches of S. zygaena by recreational anglers is very limited due to the lack of species identification. Total catch of hammerhead sharks by recreational gamefishing and protective beach meshing remained at about 250 sharks per year between the 1970s and 2001, with 95% of recreational gamefishing catches being tagged and released. It is likely that S. zygaena is caught in recreational fisheries off temperate and subtropical coasts. This species has also been reported from nets set in the New South Wales beach protection programme (Krough 1994). This employs large mesh gillnets to catch large sharks as a measure to protect beach users from shark attack. In the New South Wales programme, hammerheads (mostly S. zygaena) made up nearly 50% of the catch of 4,715 sharks in the period from 1972-73 to 1989-90 (Reid and Krough 1992). The large mesh nets used by shark control programmes appear to be very efficient at catching hammerhead sharks, including S. zygaena, while catches are very low on the large baited lines used in some programmes (Simpfendorfer 1993).
Hammerhead sharks, including S. zygaena, are taken in a variety of artisanal and commercial fisheries in this region as both target and bycatch, however, no reliable species specific data are available because catches tend to be grouped into "Sphyrna spp.", if recorded at all. Pelagic fisheries have operated in the Indian Ocean for more than 50 years; Japanese long-liners in the eastern Indian Ocean since 1952 and in the western region since 1956. Russian, Taiwanese and South Korean vessels have fished there since 1954-1966 (Gubanov and Paramonov 1993). The Spanish swordfish longline fishery, which also targets sharks, is also active across the Indian Ocean (ICCAT 2005). Large hook and line and in mesh gillnet fisheries are known to target sharks off India, where this species occurs (Anderson and Simpfendorfer 2005). Finning and discarding of carcasses has also been reported, especially in offshore and high seas fisheries (Anderson and Simpfendorfer 2005). A recent review of fisheries in the Indian Ocean (Young et al. 2006) reported that sharks in this area are considered fully to over-exploited. Large numbers of longline vessels have also been reported to be operating illegally in coastal waters of the western Indian Ocean, primarily targeting hammerhead sharks and giant guitarfish (IOTC 2005, Dudley and Simpfendorfer 2006).
Japanese data on hammerhead species are limited, but reported landings in Japan's coastal ports totaled 11-34 mt annually between 2000 and 2004 with an average of 24 mt per annum. No CPUE trends are available (Japan Fisheries Agency 2006).
The adoption of shark finning bans by fishing states (e.g., USA, Australia, Brazil), regional entities (EU) and regional fisheries organisations (e.g. ICCAT, IOTC, IATTC, WCPFC) is accelerating and should increasingly prevent the capture of oceanic sharks for their fins alone. Strict enforcement of these bans is required if they are to be effective.
Management plans, fishing regulation, and monitoring programs are needed throughout this species? range. Estimates of acceptable catch rates should be viewed with precaution until there is more certainty in age and growth parameters and further research on the species? life-history characteristics is required. Protection of known adult aggregation sites and coastal nursery areas is also recommended.
In the USA, this species is included in the Large Coastal Shark complex management unit on the US Highly Migratory Species Fishery Management Plan, however there are no management measures specific to this species and no stock assessments have been undertaken for it. Enforcement of Brazilian laws restricting the length of pelagic gillnets and banning trawl fishing at a distance of less than 3 nm from shore has been difficult and therefore trawling in inshore nursery grounds has persisted.
Relevance to Humans and Ecosystems
The smooth hammerhead (Sphyrna zygaena) is a species of hammerhead shark, and part of the family Sphyrnidae. This species is named "smooth hammerhead" because of the distinctive shape of the head, which is flattened and laterally extended into a hammer shape (called the "cephalofoil"), without an indentation in the middle of the front margin (hence "smooth"). Unlike other hammerheads, this species prefers temperate waters and occurs worldwide at medium latitudes. In the summer, these sharks migrate towards the poles following cool water masses, sometimes forming schools numbering in the hundreds to thousands.
The second-largest hammerhead shark after the great hammerhead shark, the smooth hammerhead can measure up to 5 m (16 ft) long. It is an active predator that takes a wide variety of bony fishes and invertebrates, with larger individuals also feeding on sharks and rays. As in the rest of its family, this shark is viviparous and gives birth to litters of 20–40 pups. A relatively common shark, it is captured, intentionally or otherwise, by many commercial fisheries throughout its range; its fins are extremely valuable for use in shark fin soup. This shark is potentially dangerous and has likely been responsible for a few attacks on humans, though it is less likely to encounter swimmers than other large hammerhead species due to its temperate habitat.
Taxonomy and phylogeny
The Swedish natural historian Carl Linnaeus, known as the "father of taxonomy", originally described the smooth hammerhead as Squalus zygaena in the 1758 tenth edition of Systema Naturae, without designating a type specimen. The name was later changed to Sphyrna zygaena. The specific epithet zygaena originates from the Greek word zygòn, meaning "yoke", referring to the shape of its head. The Greek name zýgaina had already been used for the hammerhead shark by Aristotle in the second book of his History of Animals. Other common names for this species include common hammerhead, common smooth hammerhead, round-headed hammerhead, or simply hammerhead.
|Phylogenetic tree of hammerhead sharks.|
Studies based on morphology have generally regarded the smooth hammerhead as one of the more derived members of its family, grouped together with the scalloped hammerhead (S. lewini) and the great hammerhead (S. mokarran). Phylogenetic analyses based on nuclear and mitochondrial DNA have concluded differently: while the smooth and great hammerheads are closely related, they are not as closely related to the scalloped hammerhead as the other Sphyrna species. Furthermore, the smooth hammerhead is among the more basal hammerhead species, indicating that the first hammerheads to evolve had large cephalofoils.
The second-largest hammerhead next to the great hammerhead, the smooth hammerhead typically measures 2.5–3.5 m (8.2–11.5 ft) long, with a maximum recorded length and weight of 5 m (16 ft) and 400 kg (880 lb) respectively. The smooth hammerhead differs from other large hammerheads in the shape of its cephalofoil, which has a curved front margin without an indentation in the center. The cephalofoil is wide but short, measuring 26–29% of the body length across. The nostrils are located near the ends of the cephalofoil, with long grooves running towards the center. There are 26–32 tooth rows in the upper jaw and 25–30 tooth rows in the lower jaw. Each tooth is triangular in shape, with smooth to weakly serrated edges.
The body is streamlined, without a dorsal ridge between the two dorsal fins. The first dorsal fin is moderately tall and falcate (sickle-like) in shape, with a rounded tip. The pectoral and pelvic fins are not falcate, rather having nearly straight rear margins. The anal fin is larger than the second dorsal fin, with long free rear tip and a strong notch in the rear margin. The dermal denticles are densely packed, each with 5–7 horizontal ridges (3 in juveniles) leading to a W-shaped rear margin. The back is dark brownish gray to olive in color, in contrast to the simple brown of most other hammerheads, becoming lighter on the flanks. The belly is white, and sometimes the pectoral fins have dark edges underneath.
Distribution and habitat
Of the hammerhead sharks, the smooth hammerhead is the species most tolerant of temperate water, and occurs worldwide to higher latitudes than any other species. In the Atlantic, it occurs from Nova Scotia to the Virgin Islands and from Brazil to southern Argentina in the west, and from the British Isles to Côte d'Ivoire, including the Mediterranean Sea, in the east. In the Indian Ocean, it is found off South Africa, India, and Sri Lanka. In the western Pacific, it occurs from the Gulf of Tonkin to southern Japan and Siberia, as well as off Australia and New Zealand. In the central and eastern Pacific, it occurs off the Hawaiian Islands, California, Panama, the Galapagos Islands, Ecuador, and Chile. This species is usually considered to be amphitemperate (absent from the tropics) in distribution, though there are rare reports from tropical waters such as in the Gulf of Mannar off India, and off southern Mozambique. Its presence in the tropics is difficult to determine due to confusion with other hammerhead species.
Compared to the scalloped and great hammerheads, the smooth hammerhead stays closer to the surface, in water less than 20 m (66 ft) deep. However, it has been recorded diving to a depth of 200 m (660 ft). It prefers inshore waters such as bays and estuaries, but is sometimes found in the open ocean over the continental shelf, and around oceanic islands. This shark has also been reported entering freshwater habitats, such as the Indian River in Florida. In the summer, smooth hammerheads migrate poleward to stay in cooler water, heading back towards the equator in winter.
Biology and ecology
Adult smooth hammerheads are either solitary or form small groups. They may come together in great numbers during their annual migrations; schools of over a hundred juveniles under 1.5 m (4.9 ft) long have been observed off the eastern Cape of South Africa, and schools thousands strong have been reported off California. During hot summer weather, they can be seen swimming just below the surface with their dorsal fins exposed. Young smooth hammerheads are preyed upon by larger sharks such as the dusky shark (Carcharhinus obscurus); adults have been observed being consumed by killer whales (Orcinus orca) off New Zealand. Known parasites of the smooth hammerhead include the nematodes Parascarophis sphyrnae and Contracaecum spp.
The smooth hammerhead is an active-swimming predator that feeds on bony fishes, rays, sharks (including of its own species), cephalopods, and to a lesser extent crustaceans such as shrimp, crabs, and barnacles. They readily scavenge from fishing lines. In some areas, stingrays are a favored prey and comprise a majority of its diet. The venomous barbs of stingrays are often found lodged in and around the mouths of these sharks; one specimen examined contained 95 such spines. In northern Europe, the smooth hammerhead feeds on herring and seabass, while in North America it takes Spanish mackerel and menhaden. Off South Africa, smooth hammerheads feed on squid such as Loligo vulgaris and small schooling fish such as pilchard over the deep coral reefs at the edge of the continental shelf, with individuals over 2 m (6.6 ft) long taking increasing numbers of smaller sharks and rays. Off Australia, squid are the most important prey, followed by bony fish.
Like other hammerheads, the smooth hammerhead is viviparous: once the young exhaust their supply of yolk, the empty yolk sac is converted into a placental connection through which the mother delivers nourishment. Females bear relatively large litters of 20–50 pups after a gestation period of 10–11 months. Birthing occurs in shallow coastal nurseries, such as Bulls Bay in North Carolina. The pups measure 50–61 cm (20–24 in) long at birth; females reach maturity at 2.7 m (8.9 ft) long and males at 2.1–2.5 m (6.9–8.2 ft) long, depending upon locality. Off South Africa, newly mated females have been caught in February and females with full-term embryos in November; off the east coast of Australia, birthing takes place between January and March, with ovulation taking place around the same time. This shark is thought to live for 20 years or more.
The smooth hammerhead is potentially dangerous to humans. As of 2008, the International Shark Attack File lists 34 attacks attributable to large hammerhead sharks, 17 of them unprovoked (1 fatal). However, due to the smooth hammerhead's occurrence in temperate regions where people are less likely to enter the water, it was likely responsible for a minority of these attacks. Off southern California, this species has been reported to steal catches from sport fishers and divers.
Smooth hammerheads are caught by commercial fisheries throughout the world, including those off the United States (East and West Coasts), Brazil, Spain, Taiwan, the Philippines, southwestern Australia, and western Africa, primarily using gillnets and longlines. Fishery catches of smooth hammerheads are difficult to quantify due to a frequent lack of distinction between them and other large hammerheads. The meat is sold fresh, dried and salted, or smoked, though in most markets it is considered undesirable and there are reports of poisoning. Much more valuable are the fins, which have the highest rating for use in shark fin soup and often leads to captured sharks being finned at sea. Additionally, the liver oil is used for vitamins, the skin for leather, and the offal for fishmeal. This shark is also used in Chinese medicine.
Many other fisheries from every ocean also take smooth hammerheads as bycatch, and they are caught in some numbers by recreational anglers. Smooth hammerheads are also killed by entanglement in shark nets used to protect beaches. Fewer than 10 smooth hammerheads were caught annually in the nets off KwaZulu-Natal, South Africa, from 1978–1990. In contrast, in the nets off New South Wales, Australia, smooth hammerheads comprised 50% of the 4,715 sharks captured from 1972–1990. At present, this species remains relatively common and has been assessed as "Vulnerable (VU)" by the World Conservation Union. Off New Zealand, it is a prohibited target species and is the most abundant shark along the northwest coast. It also does not appear to have been negatively impacted by fishing off southern Australia. Off the eastern United States, catches of this species are regulated by the National Marine Fisheries Service (NMFS) Atlantic shark Fishery Management Plan (FMP), under which it is classified as a Large Coastal Shark (LCS). In 2013 smooth hammerhead and other great elasmobranchs were added to Appendix 2 of CITES, bringing shark fishing and commerce of these species under licensing and regulation.
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