Smooth hammerhead shark (Sphyrna zygaena)

TAXONOMY AND NAMING

Class : Chondrichthyes
Subclass : Elasmobranchii
Order : Carcharhiniformes
Clade : Neoselachii
Family : Sphyrnidae
Genus/species : Sphyrna zygaena
Common names :
English : Smooth hammerhead shark
Spanish : Tiburón martillo liso
French : Requin marteau commun

ANATOMY AND MORPHOLOGY

Sphyrna zygaena is a large species of hammerhead shark, growing to a maximum reported size of 420 cm. Age at maturity is estimated to be 9 years (Cortés et al., 2015). Like other hammerhead shark species, S. zygaena are viviparous (i.e. live-bearing) (Compagno, 1984 ; Ebert et al., 2013). Juveniles of this species have been observed to form large aggregations (Smale, 1991). The reproductive cycle of smooth hammerheads is estimated to be biennial with a 10-11 month gestation, and litter size ranges from 20-50 young ranging in size from 50-65 cm (Ebert and Stehman 2013). Longevity is unknown, but the species has been aged to at least 18 years for males and 21 years for females (Coelho et al., 2011). Like many large-bodied shark species, S. zygaena is among the top predators (feeding at trophic level 4.2) in the marine food web (Cortés, 1999). The species feeds on a large variety of teleosts (i.e. bony fish), elasmobranchs, crustaceans and cephalopod species (Smale and Cliff, 1998 ; Cortés, 1999).

BEHAVIOUR AND LIFE HISTORY

Although scientific studies on the movements and migrations of this species are limited (and more research is needed), the data available are indicative of S. zygaena making inshore-offshore migrations. Such migrations would lead to S. zygaena moving from national to international waters and across jurisdictional boundaries. There is also evidence of north-south movements, which may be seasonal migrations.

DISTRIBUTION AND ABUNDANCE

Sphyrna zygaena has a circumglobal distribution in tropical to warm temperate waters, generally between the 59°N and 55°S latitude (FAO, 2010). Accurate data on the global range of S. zygaena is limited. It is a pelagic species that occurs in both coastal and oceanic waters, thus occurring along the continental shelves (at depths of 20–200 m) and also making excursions into more oceanic habitats (Smale, 1991 ; Ebert, 2003). According to Clarke et al. (2015), this is the most oceanic of all hammerhead sharks. Young individuals occur in coastal habitats in the first years of their life, with their habitat range extending out to oceanic zones as they grow (Smale, 1991 ; Diemer et al., 2011 ; Clarke et al., 2015).

THREATS

Hammerhead sharks are taken as direct catch or incidental catch in domestic and artisanal fisheries, as well as industrial pelagic fisheries on the high seas. Furthermore, smooth hammerhead fins they are the most common hammerhead fished off western South America for export to Asia. The high value of the fins on the international market is the main driver for hammerhead fisheries (directed and bycatch).

Like many other shark species smooth hammerhead sharks rely on inshore areas for pupping and nursery grounds. Coastal developments may have resulted in habitat degradation and destruction of potential nursery areas (Knip et al., 2010), although there is no direct evidence that such habitat degradation has negatively impacted on the abundance or range of this species (Miller, 2016).

ABUNDANCE AND CONSERVATION STATUS

Species-specific data on hammerhead sharks are lacking for many regions but it is likely that populations of hammerhead sharks, as a group, have declined. The magnitude of any decline in S. zygaena, however, is unknown. The IUCN defines the Smooth hammerhead’s conservation status as ‘Vulnerable’ and its trend ‘decreasing’ (Rigby, 2019).

PROTECTION STATUS

Several range states have developed national plans of action, as well as regional plans of action for Pacific Island States, the Central American Isthmus (OSPESCA) and the European Union. One of the main priorities in shark management and conservation in the past two decades has been the prohibition of shark finning. Many countries have already adopted finning bans in their waters and/or in their fisheries.

SPAW Protocol : yes since 2017 _ Annex III

CITES : yes_ Annex II

CMS : yes

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

Bibliography

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