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{{Short description|Species of shark}}
{{Speciesbox
| name = Scalloped hammerhead
| image = Scalloped Hammerhead Shark Sphyrna Lewini (226845659).jpeg
| image_caption = Off [[Cocos Island]], [[Costa Rica]]
| image2 = Sphyrna lewini Gervais.jpg
| status = CR
| status_system = IUCN3.1
| status_ref = <ref name=IUCN>{{Cite iucn |author1=Rigby, C.L.|author2=Dulvy, N.K.|author3=Barreto, R.|author4=Carlson, J.|author5=Fernando, D.|author6=Fordham, S.|author7=Francis, M.P.|author8=Herman, K.|author9=Jabado, R.W.|author10=Liu, K.M.|author11=Marshall, A.|author12=Pacoureau, N.|author13=Romanov, E.|author14=Sherley, R.B.|author15=Winker, H. |title=''Sphyrna lewini'' |year=2019 |page=e.T39385A2918526|access-date=20 May 2023}}</ref>
| status2 = CITES_A2 
| status2_system = CITES 
| status2_ref = <ref>{{Cite web|title=Appendices {{!}} CITES|url=https://cites.org/eng/app/appendices.php|access-date=2022-01-14|website=cites.org}}</ref>
| genus = Sphyrna
| species = lewini
| authority = ([[Edward Griffith (zoologist)|E. Griffith]] & [[Charles Hamilton Smith|C. H. Smith]], 1834)
| synonyms = ''Sphyrna couardi'' {{small|Cadenat, 1951}}
| range_map = Sphyrna lewini distribution map.svg
| range_map_caption = Range of the scalloped hammerhead
}}

The '''scalloped hammerhead''' ('''''Sphyrna lewini''''') is a [[species]] of [[hammerhead shark]] in the [[family (biology)|family]] [[Hammerhead shark|Sphyrnidae]]. It was originally known as ''Zygaena lewini''. The [[Greek language|Greek]] word ''sphyrna'' translates into "hammer" in English, referring to the shape of this shark's head, which is its most distinguishing characteristic. The shark's eyes and [[nostrils]] are at the tips of the extensions. It is a fairly large hammerhead, but is still smaller than both the [[Great hammerhead|great]] and [[smooth hammerhead]]s.

This shark is also known as the '''bronze''', '''kinky-headed''', or '''southern hammerhead'''. It primarily lives in warm, temperate, and tropical coastal waters all around the globe between [[latitude]]s [[46th parallel north|46°N]] and [[36th parallel south|36°S]], down to a depth of {{convert|500|m|abbr=on}}. It is the most common of all hammerheads.

==Taxonomy==
The scalloped hammerhead was first named ''Zygaena lewini'' and then renamed ''Sphyrna lewini'' by [[Edward Griffith (zoologist)|Edward Griffith]] and [[Charles Hamilton Smith|Hamilton Smith]] in 1834. It has also been named ''Cestracion leeuwenii'' by Day in 1865, ''Zygaena erythraea'' by Klunzinger in 1871, ''Cestracion oceanica''  by Garman in 1913, and ''Sphyrna diplana'' by Springer in 1941. ''Sphyrna'' comes from the Greek and translates to hammer.<ref name="FLMNH">{{cite web |last=Bester |first=Cathleen |title=Greek Translation |url=https://www.floridamuseum.ufl.edu/discover-fish/species-profiles/sphyrna-zygaena/#:~:text=The%20name%20Sphyrna%20translates%20from,is%20still%20in%20use%20today. |publisher=Florida Museum of Natural History}}</ref>

It is a sister species to ''[[Sphyrna gilberti]]'', differing by the number of vertebrae.<ref name=Quattro>{{Cite journal | last1 = Quattro | first1 = J. M. | last2 = Driggers | first2 = W. B. I. I. | last3 = Grady | first3 = J. M. | last4 = Ulrich | first4 = G. F. | last5 = Roberts | first5 = M. A. | title = ''Sphyrna gilberti'' sp. Nov., a new hammerhead shark (Carcharhiniformes, Sphyrnidae) from the western Atlantic Ocean | doi = 10.11646/zootaxa.3702.2.5 | journal = Zootaxa | volume = 3702 | issue = 2 | pages = 159–78 | year = 2013 | pmid =  26146715| doi-access = free }}</ref>  Though once considered a distinct species, McEachran and Serret synonymized ''Sphyrna couardi'' with ''Sphyrna lewini'' in 1986.<ref>Martin, R. Aidan. (February 24, 1998). [http://www.elasmo-research.org/education/topics/ng_hh_taxonomy.htm Recent Changes in Hammerhead Taxonomy]. ''ReefQuest Centre for Shark Research''. Retrieved on October 18, 2008.</ref> These sharks are classified as ground sharks in the order Carcharhiniformes.<ref name=":2">{{Cite report|last1=Miller |first1=Margaret H.|last2=Carlson|first2=John|last3=Cooper|first3=Peter|last4=Kobayashi|first4=Donald|last5=Nammack|first5=Marta|last6=Wilson|first6=Jackie |date=March 2013 |title=Status Review Report: Scalloped Hammerhead Shark (Sphyrna lewini)|publisher=National Marine Fisheries Service, Office of Protected Resources|url=https://repository.library.noaa.gov/view/noaa/17697/noaa_17697_DS1.pdf|access-date=20 May 2023}}</ref>

==Description==
The scalloped hammerhead is easily distinguished from other hammerhead sharks by the central indentation on the anterior margin of the head.<ref name=":2" /> There are also two indentations on either side of the central indentation, which gives the "scalloped" look.<ref name=":2" /> They have a very large first dorsal fin that is slightly hooked and a smaller second dorsal fin.<ref name=":2" /> Scalloped hammerhead sharks are typically uniform grey, grayish brown, bronze, or olive with a white underside for countershading.<ref name=":2" />

Typically, males measure {{convert|1.5|to|1.8|m|ft|abbr=on}} and weigh about {{convert|29|kg|lb|abbr=on}} when they attain sexual maturity, whereas the larger females measure {{convert|2.5|m|ft|abbr=on}} and weigh about {{convert|36.2|kg|lb|abbr=on}}  at sexual maturity.<ref>{{cite web | url=https://marinesanctuary.org/blog/scalloped-hammerhead-shark/#:~:text=Male%20scalloped%20hammerhead%20sharks%20can,to%2014%20feet%20in%20length. | title=Scalloped Hammerhead Sharks - Facts and Conservation &#124; Endangered }}</ref> The maximum length of the scalloped hammerhead is {{convert|4.3|m|ft|abbr=on}} and the maximum weight is {{convert|152.4|kg|lb|abbr=on}}, per [[FishBase]].<ref>[http://www.fishbase.org/summary/Sphyrna-lewini.html ''Sphyrna lewini'', Scalloped hammerhead : fisheries, gamefish]. Fishbase.org (2012-07-03). Retrieved on 2013-05-23.</ref> A female caught off of [[Miami]] was found to have measured {{convert|3.26|m|ft|abbr=on}} and reportedly weighed {{convert|200|kg|lb|abbr=on}}, though was in a [[Pregnancy|gravid]] state then.<ref name="Castro">Castro, José I. (2011) ''The Sharks of North America''. Oxford University Press. {{ISBN|978-0-19-539294-4}}</ref>

These sharks have a very high metabolic rate, which governs behavior in acquiring food. They occupy tertiary trophic levels.<ref name="link.springer_c">{{cite journal|title=Estimation of daily energetic requirements in young scalloped hammerhead sharks, ''Sphyrna lewini'' |date=1 August 2006  |doi=10.1007/s10641-006-9016-5 |volume=76 |issue=2–4  |journal=Environmental Biology of Fishes |pages=139–149|last1=Duncan  |first1=Kanesa May  |s2cid=10155904 }}</ref> The scalloped hammerhead shark, like many other species, uses the shore as a breeding ground.<ref name="link.springer_c" /> Due to high metabolic rates, young scalloped hammerhead sharks need a lot of food, or they will starve.

''S. lewini'' is also noted for its large and complex brain, with high levels of cognitive capabilities, social intelligence, sensorimotor functions, intricate migrations, complex habitat relationships, and athletic capture of prey. Their social intelligence is especially important for their aggregative behavior, allowing the species to reproduce with the fittest members and more easily proliferate the species.<ref>{{Cite journal|last1=Gallagher|first1=Austin J.|last2=Hammerschlag|first2=Neil|last3=Shiffman|first3=David S.|last4=Giery|first4=Sean T.|date=2014-05-27|title=Evolved for Extinction: The Cost and Conservation Implications of Specialization in Hammerhead Sharks|journal=BioScience|volume=64|issue=7|pages=619–624|doi=10.1093/biosci/biu071|issn=0006-3568|doi-access=free}}</ref>

The dentition of these sharks consists of small, narrow, triangular teeth that have smooth edges, except in larger fish, which may have somewhat serrated teeth.<ref name=":2" /> The teeth in the front of the mouth have erect cusps, and the teeth in the bottom jaw have cusps that are more erect than those of the upper jaw.<ref name=":2" />
<gallery mode="packed">
File:Sphyrna lewini upper teeth.jpg|Upper teeth
File:Sphyrna lewini lower teeth.jpg|Lower teeth
File:Sphyrna lewini juvenile.jpg|Juvenile
File:Sphyrna lewini young.jpg|Juvenile
</gallery>

==Distribution and habitat==
[[File:Scalloped hammerhead-tokyosealifepark-2019-1-8.webm|thumb|250px|Scalloped hammerhead swimming (video)]]

The scalloped hammerhead is a coastal [[Pelagic zone|pelagic]] species; it occurs over continental and insular shelves and in nearby deeper water. It is found in warm temperate and [[Tropics|tropical]] waters,<ref>{{cite web |title=Scalloped Hammerhead Sharks- Facts and Conservation |url=https://marinesanctuary.org/blog/scalloped-hammerhead-shark/ |website=National Marine Sanctuary Foundation |access-date=21 March 2023}}</ref> worldwide from 46°N to 36°S. It can be found down to depths over {{convert|500|m|abbr=on}}, but is most often found above {{convert|25|m|abbr=on}}.<ref name="fishbase">{{cite book|editor1=Froese, Ranier|editor2=Pauly, Daniel |title=''Sphyrna lewini''|publisher=FishBase|access-date=10 December 2008|url=http://www.fishbase.org/Summary/SpeciesSummary.php?id=912}}</ref> During the day, they are more often found close to shore, and at night, they hunt further offshore. Adults are found alone, in pairs, or in small [[Shoaling and schooling|schools]], while young sharks occur in larger schools.<ref name="FLMNH"/>

Juveniles and pups thrive in shallow coastal waters, such as bays and mangroves, which provide shelter from predators and waters high in nutrients from deposited sediments. Research carried out by the nongovernmental organization [[Misión Tiburón]], using conventional and acoustic shark tagging methods, found that adult scalloped hammerheads migrate from the pelagic waters surrounding Cocos Island to the mangroves in the tropical fjord of [[Golfo Dulce, Costa Rica|Golfo Dulce]] - a tropical fjord on the Pacific Coast of Costa Rica.<ref>{{cite web |url=http://misiontiburon.org/en/hammerhead-shark/ |title=Hammerhead Shark – Mision Tiburon |website=misiontiburon.org |url-status=dead |archive-url=https://web.archive.org/web/20160703060440/http://misiontiburon.org/en/hammerhead-shark/ |archive-date=2016-07-03}} </ref> Here, female sharks give birth to live young; juveniles remain in the shallow root system of the mangroves for around three years. After this time they leave Golfo Dulce and migrate back to Cocos Island, to feed in pelagic waters.

==Behavior==

===Schooling===
[[File:Sphyrna lewini school.jpg|thumb|School of scalloped hammerheads]]

These sharks are often seen during the night, day, and morning in big schools, sometimes numbering hundreds, most likely because large groups can obtain food more easily than singles or small groups, especially larger and trickier prey, as commonly seen.  The younger the sharks, the closer to the surface they tend to be, while the adults are found much deeper in the ocean.  They are not considered dangerous and are normally not aggressive towards humans.

===Sexual dimorphism===
The female scalloped hammerheads migrate offshore at a smaller size than males<ref name="link.springer">{{cite journal |title=The determinants of sexual segregation in the scalloped hammerhead shark, ''Sphyrna lewini'' |date=1 January 1987 |doi=10.1007/BF00002325 |volume=18 |journal=Environmental Biology of Fishes |pages=27–40|last1=Klimley |first1=A. Pete |s2cid=29448517 }}</ref> because the larger classes of the hammerhead, such as those from {{convert|100|to|140|cm|in}} long, travel deeper down.<ref name="link.springer" /> Males and females differ in that males are observed to stay deeper than female sharks in general.

Sexual maturity generally occurs once the scalloped hammerhead attains {{convert|240|cm|ft}} in total or longer. Physically, the mature females have considerably wider uteri than their maturing counterparts. A lack of mating scars has been found on mature females.<ref name="link.springer_a">{{cite journal |title=Aspects of Reproductive Biology of the Scalloped Hammerhead Shark, ''Sphyrna lewini'', off Northeastern Brazil |date=1 June 2001 |doi=10.1023/A:1011040716421 |volume=61 |issue=2  |journal=Environmental Biology of Fishes |pages=151–159|last1=Hazin  |first1=Fabio  |last2=Fischer  |first2=Alessandra  |last3=Broadhurst  |first3=Matt  |s2cid=44367667 }}</ref> Unlike females, males reach sexual maturity at a much smaller size.

The male-to-female ratio of the scalloped hammerhead is 1:1.29.<ref name="link.springer_a" />  Females probably are capable of giving birth annually,<ref name="link.springer_a" /> usually in the summer.{{citation needed|date=May 2023}}
===Navigation===
Scalloped hammerhead sharks have a homing behavior to navigate in the ocean.<ref name="link.springer_b">{{cite journal|title=Highly directional swimming by scalloped hammerhead sharks, ''Sphyrna lewini'', and subsurface irradiance, temperature, bathymetry, and geomagnetic field |date=1993 |doi=10.1007/BF00346421 |volume=117 |journal=Marine Biology |pages=1–22|last1=Klimley |first1=A. P. |s2cid=86223091 }}</ref> They move in the night and use the environment as a map, similar to a human reading a topographical map.<ref name="link.springer_b" /> By experimentation in tagging these sharks, one could test for any guidance in a shark's movement.<ref name="link.springer_b" /> These sharks use a point-to-point type of school swimming, and do not favor going too deep, where temperature changes hitchhike with current speed and directional change.
[[File:Scalloped_hammerhead_cocos.jpg|right|frameless]]
The scalloped hammerhead uses deep water to survive as safety and feeding.<ref>{{cite journal |title=Scalloped hammerhead shark ''Sphyrna lewini'', uses deep-water, hypoxic zone in the Gulf of California |doi=10.1111/j.1095-8649.2009.02230.x |volume=74 |issue=7 |journal=Journal of Fish Biology |pages=1682–1687|year=2009 |last1=Jorgensen |first1=S. J. |last2=Klimley |first2=A. P. |last3=Muhlia-Melo |first3=A. F. |pmid=20735666 }}</ref>  Although they have high metabolic rates, they have a tendency to be sedentary and allow currents to carry them as they swim. As a result, this causes scalloped hammerheads to be selective where they swim and the depth at which they tend to stay. They also make use of Earth's magnetic field.<ref name="sharkinfo.ch">{{Cite web|url=https://www.sharkinfo.ch/SI2_00e/slewini.html|title=Fact Sheet: Scalloped Hammerhead Shark|website=www.sharkinfo.ch|access-date=30 March 2020}}</ref>

=== Predation ===
The scalloped hammerhead has several advantages to capture its prey. The shape of its head allows it to bury into the seafloor and pin stingrays down. The wide head and special sensory cells allow the scalloped hammerhead to successfully detect fish.<ref>{{Cite web|url=https://oceana.org/marine-life/sharks-rays/smooth-hammerhead-shark|title=Smooth Hammerhead Shark|website=Oceana|language=en|access-date=18 February 2020}}</ref>
==Reproduction==
The gestation period is reported to be around 12 months.<ref name="link.springer_d">{{cite journal|title=Age, growth and reproductive biology of the silky shark, ''Carcharhinus falciformis'', and the scalloped hammerhead, ''Sphyrna lewini'', from the northwestern Gulf of Mexico |date=1 July 1987 |doi=10.1007/BF00005346 |volume=19 |issue=3 |journal=Environmental Biology of Fishes |pages=161–173|last1=Branstetter |first1=Steven |s2cid=41048341 }}</ref> Scalloped hammerheads give live birth. Compared to other species, the scalloped hammerhead produces large litters (12–41 pups),<ref name="link.springer_d" />  and this is most likely due to high infant mortality. Like most sharks, parental care is not seen.<ref name="onlinelibrary.wiley">{{cite journal |title=Global phylogeography of the scalloped hammerhead shark (''Sphyrna lewini'') |doi=10.1111/j.1365-294X.2006.02933.x |pmid=16780437 |volume=15 |issue=8 |journal=Molecular Ecology |pages=2239–2251|year=2006 |last1=Duncan |first1=K. M. |last2=Martin |first2=A. P. |last3=Bowen |first3=B. W. |last4=De Couet |first4=H. G. |s2cid=25219770 }}</ref>  Nursery grounds for this species are predictable and repeated over the years, and they are faithful to their natal sites.<ref name="onlinelibrary.wiley" /> Their natal sites still cause high infant mortality; a lack of resources prevents all the young from surviving.  As a result, only the fittest grow to maturity. Also, should a population get depleted, it recovers through reproduction and not immigration.<ref name="onlinelibrary.wiley" />  This species does not seem to attack each other even in periods of starvation. In addition, scalloped hammerheads have migratory behaviors. As a result, deprivation results from migration and young growth. While the Taiwan scalloped hammerhead seems to have an earlier maturity rate, it is still reported to be slow to mature.<ref name="scholarspace.manoa.hawaii">{{cite journal |url=http://scholarspace.manoa.hawaii.edu/handle/10125/1267| last1 = Chen | first1 = CT | last2 = Leu | first2 = TC | last3 = Joung | first3 = SJ | last4 = Lo | first4 = NCH | year = 1990 | title = Age and growth of the scalloped hammerhead, ''Sphyrna lewini'', in northeastern Taiwan waters | journal = [[Pacific Science]] | volume = 44 | issue = 2| pages = 156–170 | hdl = 10125/1267 }}</ref>

==Diet==
Scalloped hammerheads are generalists and opportunistic predators, eating whatever is available and abundant in their area. There is no difference in what male and female sharks eat, but larger sharks are noted to eat larger prey than smaller, juvenile hammerheads. Juveniles typically feed on inshore fish, while adults feed on inshore fish as well as larger organisms that live in deeper waters.<ref>{{Cite journal|last1=Torres-Rojas|first1=Yassir|last2=Hernandez-Herrera|first2=Agustin|last3=Galvan-Magaña|first3=Felipe|title=Feeding habits of the scalloped hammerhead shark, Sphyrna lewini, in Mazatlán waters, southern Gulf of California, Mexico|journal=Cybium|volume=30|pages=85–90}}</ref>

Specifically, this shark feeds primarily on fish such as [[sardine]]s, [[mackerel]], and [[herring]], and occasionally they feed on cephalopods such as [[squid]] and [[octopus]]. Larger specimens may also feed on smaller species of shark such as the [[blacktip reef shark]], ''Carcharhinus melanopterus''.

==Conservation==
The scalloped hammerhead was the first shark species to be protected under the U.S. [[Endangered Species Act]].<ref name=":4">{{Cite journal |last1=Wells |first1=R. J. David |last2=TinHan |first2=Thomas C. |last3=Dance |first3=Michael A. |last4=Drymon |first4=J. Marcus |last5=Falterman |first5=Brett |last6=Ajemian |first6=Matthew J. |last7=Stunz |first7=Gregory W. |last8=Mohan |first8=John A. |last9=Hoffmayer |first9=Eric R. |last10=Driggers |first10=William B. |last11=McKinney |first11=Jennifer A. |date=2018 |title=Movement, Behavior, and Habitat Use of a Marine Apex Predator, the Scalloped Hammerhead |journal=Frontiers in Marine Science |volume=5 |doi=10.3389/fmars.2018.00321 |issn=2296-7745 |doi-access=free |url=https://repository.library.noaa.gov/view/noaa/46664/noaa_46664_DS1.pdf }}</ref> As of 2021, the scalloped hammerhead has been categorized as Critically Endangered by the IUCN Red List.<ref name=":0">{{Cite web|title=The IUCN Red List of Threatened Species|url=https://www.iucnredlist.org/search?searchType=species|access-date=2021-04-30|website=IUCN Red List of Threatened Species}}</ref> The IUCN cites overfishing as the main cause for the drop in population numbers.<ref name=":0" /> These sharks are slow growing, mature late, and low fecundity, which makes them very vulnerable to fishing.<ref name=":3">{{Cite journal |last1=Santos |first1=Catarina C. |last2=Coelho |first2=Rui |date=2018-06-06 |editor-last=Tserpes |editor-first=George |title=Migrations and habitat use of the smooth hammerhead shark (Sphyrna zygaena) in the Atlantic Ocean |journal=PLOS ONE |language=en |volume=13 |issue=6 |pages=e0198664 |doi=10.1371/journal.pone.0198664 |issn=1932-6203 |pmc=5991395 |pmid=29874297 |bibcode=2018PLoSO..1398664S |doi-access=free }}</ref> Scalloped hammerheads are over harvested for their large sizes and fins that have 'high fin needle content;' their fins are sold for US $100–120 per kilogram.<ref name=":1">{{Cite journal|last1=Chapman|first1=Demian D.|last2=Pinhal|first2=Danillo|last3=Shivji|first3=Mahmood S.|date=2009-12-01|title=Tracking the fin trade: genetic stock identification in western Atlantic scalloped hammerhead sharks Sphyrna lewini|url=https://www.int-res.com/abstracts/esr/v9/n3/p221-228/|journal=Endangered Species Research|language=en|volume=9|issue=3|pages=221–228|doi=10.3354/esr00241|issn=1863-5407|doi-access=free|hdl=11449/71350|hdl-access=free}}</ref> Many conservation efforts have been taken to protect this species, such as using genetic information from fins purchased from a Hong Kong market to pinpoint where scalloped hammerheads are being caught and track their exploitation levels.<ref name=":1" /> Another important effort has been protecting ''Sphyrna lewini'' nurseries. Scalloped hammerheads return to their birthplace to have their own pups; the pups will then spend their first years of life in these coastal nursery areas where they are safe from predators and have less competition for feeding before they venture into the open ocean.<ref>{{Cite journal|last1=Kinney|first1=Michael John|last2=Simpfendorfer|first2=Colin Ashley|date=2009|title=Reassessing the value of nursery areas to shark conservation and management|journal=Conservation Letters|language=en|volume=2|issue=2|pages=53–60|doi=10.1111/j.1755-263X.2008.00046.x|issn=1755-263X|doi-access=free}}</ref> Fishing bans have been placed on some of these areas, such as in the Western Yucatan Peninsula, during breeding seasons to protect the young and juvenile scalloped hammerheads.<ref>{{Cite journal|last1=Cuevas-Gómez|first1=Gabriela Alejandra|last2=Pérez-Jiménez|first2=Juan Carlos|last3=Méndez-Loeza|first3=Iván|last4=Carrera-Fernández|first4=Maribel|last5=Castillo-Géniz|first5=José Leonardo|date=2020|title=Identification of a nursery area for the critically endangered hammerhead shark (Sphyrna lewini) amid intense fisheries in the southern Gulf of Mexico|url=https://onlinelibrary.wiley.com/doi/abs/10.1111/jfb.14471|journal=Journal of Fish Biology|language=en|volume=97|issue=4|pages=1087–1096|doi=10.1111/jfb.14471|pmid=32691418|s2cid=220671717|issn=1095-8649|url-access=subscription}}</ref>

In many areas, officials have implemented management regulations on fishing vessels. They have made a regulation that prohibits taking the sharks onboard to transship, sell, or store for future selling. <ref name=":3" /> This not only protects the scalloped hammerhead shark, but also the entire Order of Carcharhiniformes. Hammerhead sharks are not only being intentionally caught, but also being caught non-intentionally on longlines and nets. Bycatch has also greatly reduced these species because they often migrate in large groups. <ref name=":3" /> This allows the fisheries to catch many sharks at once. The mortality rate of the sharks that are caught is almost always 90% of the individuals. <ref name=":4" /> Even though it is hard to prevent these sharks from biting the bait or getting in the net, there has been devices put in place for bycatch reduction. Along the Eastern Atlantic, regulations have been implemented to control the over exploitation of the scalloped hammerhead.<ref name=":2" /> The team in this area has concluded this species is at high risk of extinction now and for the future.<ref name=":2" /> It is essential to understand the movement patterns of these sharks to be able to place effective conservation and management efforts. <ref name=":4" />

Marine reserves have been implemented in many areas to allow the sharks to recover. It is very important to place these reserves over areas that sharks use as breeding grounds and nurseries, so they have somewhere to raise their pups. It is also beneficial to place the marine reserves in areas where they are prized as a non consumptive resource through shark diving tourism. <ref>{{Cite journal |last1=Gallagher |first1=Austin J. |last2=Klimley |first2=A. Peter |date=2018-07-21 |title=The biology and conservation status of the large hammerhead shark complex: the great, scalloped, and smooth hammerheads |url=http://dx.doi.org/10.1007/s11160-018-9530-5 |journal=Reviews in Fish Biology and Fisheries |volume=28 |issue=4 |pages=777–794 |doi=10.1007/s11160-018-9530-5 |s2cid=254982548 |issn=0960-3166}}</ref>

In parts of the Atlantic Ocean, their populations had declined by over 95% in the past 30 years.  Among the reasons for this drop are overfishing and the rise in demand for shark fins. Researchers attribute this growth in demand to the increase in shark fins as an expensive delicacy (such as in [[shark fin soup]]) and in 2008 called for a ban on [[shark finning]], a practice in which the shark's fins are cut off and the rest of the animal is thrown back in the water to die. Hammerheads are among the most commonly caught sharks for finning.<ref name="End">{{cite web | title=Hammerhead Shark Makes Endangered Species List | publisher= redorbit.com | url=http://www.redorbit.com/news/science/1259813/hammerhead_shark_makes_endangered_species_list/ |date=19 February 2008|author=Savage, Sam }}</ref> "This species tendency to aggregate in large groups making capture in large numbers on long lines, bottom nets and trawls even easier."<ref>{{Cite web|url=https://www.floridamuseum.ufl.edu/discover-fish/species-profiles/sphyrna-lewini/|title=''Sphyrna lewini''|date=12 May 2017|website=Florida Museum|language=en-US|access-date=25 April 2020}}</ref>

Hammerhead sharks are overfished all around the world for their fins and liver oil. {{as of|2020}} an estimated 1.3 to 2.7 million fins are collected each year from smooth and scalloped hammerhead sharks for the shark-fin trade.<ref>{{Cite web|url=https://oceana.org/marine-life/sharks-rays/smooth-hammerhead-shark|title=Smooth Hammerhead Shark|website=Oceana|language=en|access-date=2 March 2020}}</ref>  [[DNA barcoding]] can assist in the identification of scalloped hammerhead remains to aid conservation efforts.<ref>{{Cite journal|title=HACSim: an R package to estimate intraspecific sample sizes for genetic diversity assessment using haplotype accumulation curves|last1=Phillips|first1=Jarrett|last2=French|first2=Steven H.|last3=Hanner|first3=Robert H.|last4=Gillis|first4=Daniel J.|year=2020|journal=PeerJ Computer Science|volume=6|pages=e243 |doi=10.7717/peerj-cs.243|pmid=33816897 |pmc=7924493 |id=e243|doi-access=free}}</ref>

According to a January 2021 study in ''[[Nature (journal)|Nature]]'' which studied 31 species of  sharks and [[ray fish|rays]], the number of these species found in open oceans had dropped by 71 per cent in around 50 years. The scalloped hammerhead was included in the study.<ref name="Briggs 2021">{{cite web | last=Briggs | first=Helen | title=Extinction: 'Time is running out' to save sharks and rays | website=[[BBC News]] | date=28 January 2021 | url=https://www.bbc.com/news/science-environment-55830732 | access-date=29 January 2021}}</ref><ref name="Richardson 2021">{{cite news | last=Richardson | first=Holly | title=Shark, ray populations have declined by 'alarming' 70 per cent since 1970s, study finds | website=ABC News|publisher=[[Australian Broadcasting Corporation]] | date=27 January 2021 | url=https://www.abc.net.au/news/2021-01-28/alarming-70pc-decline-in-shark-and-ray-numbers-study-says/13096442 | access-date=29 January 2021}}</ref>

==See also==
{{Portal|Sharks}}
* [[List of sharks]]

==References==
{{Reflist}}

==External links==
{{Commons category|Sphyrna lewini}}
* https://oceana.org/marine-life/sharks-rays/smooth-hammerhead-shark
<references group="https://oceana.org/marine-life/sharks-rays/smooth-hammerhead-shark" />
* {{ITIS |id=160508 |taxon=Sphyrna lewini |access-date=23 January 2006}}
* {{FishBase|genus=Sphyrna|species=lewini|year=2005|month=09}}
* ARKive – [https://web.archive.org/web/20060422091202/http://www.arkive.org/species/GES/fish/Sphyrna_lewini/ images and movies of the scalloped hammerhead (''Sphyrna lewini'')]
* [http://www.sharkinfo.ch/SI2_00e/slewini.html Shark Info page about the scalloped hammerhead]
* [http://shark-references.com/species/view/Sphyrna-lewini Species Description of ''Sphyrna lewini'' at www.shark-references.com]
* {{SealifePhotos|105816}}

{{Carcharhiniformes}}
{{Taxonbar|from=Q873701}}

[[Category:Sphyrna|scalloped hammerhead]]
[[Category:Pantropical fish]]
[[Category:Fish described in 1834|scalloped hammerhead]]
[[Category:Taxa named by Edward Griffith (zoologist)|scalloped hammerhead]]
[[Category:Taxa named by Charles Hamilton Smith]]
[[Category:Species endangered by use as food]]
[[Category:Fish of the Dominican Republic]]
[[Category:ESA endangered species]]

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