Smooth Hammerhead

About the Smooth Hammerhead

The smooth hammerhead is a large species of hammerhead shark, the second largest after the great hammerhead (Sphyrna mokarran). They’re usually about 3 meters/9 ft 10 in long. The largest specimen ever found was 5 meters/16 ft 5 in long.

It’s found in most of the world’s temperate and tropical seas, but unlike other hammerheads, the smooth hammerhead prefers slightly cooler waters. In the summer large schools migrate towards the poles to find more temperate waters.

Their name comes from the shape of their cephalofoil (their hammer). Most hammerheads have indentations at the front part of their hammer, but Sphyrna zygaena doesn’t, hence their name “smooth” hammerhead.

Biology:

Sphyrna Zygaena is viviparous, meaning giving birth to live young that develop inside the mother’s body, rather than hatching from eggs.. Females become sexually mature when they’re about 250-300 cm/8 ft 2.4 in to 9 ft 10 in long. Babies are born no longer than 60 cm/1 ft 11.6 in after a gestation period of 11 months. The average litter size is 35

They can weigh up to 400 kg/881.8 lbs and their lifespan is estimated to be about 20 years.

Behaviour:

Smooth hammerheads are highly migratory and can be found both inshore and offshore. They’re mostly found between 1-140 meters/3.3 to 459.3 ft deep. Their main preys are smaller sharks, rays, and skates, but will eat anything they can catch. Stingrays seem to be a common prey item, as stingray barbs are often found inside smooth hammerheads’ mouths. In one case, 95 barbs were found in a single individual. Their predators are larger sharks, especially when still young, and killer whales.

Their migrations are notorious for their school size. Schools of over a hundred juveniles have been observed.

Due to their size, they’re potentially dangerous to humans, but there are very few confirmed cases of attacks on people as they’re often mistaken for other hammerhead species.

Conservation:

Often caught as bycatch in tuna fisheries, their meat is consumed and their fins are highly-valued in the shark fin market due to their high number of fin rays. They’re also used in traditional Chinese medicine and as a source of cartilage and fishmeal. The main product from the species that is traded is the fins (CITES 2013). Hammerhead fins are among the main shark species in the fin trade and one of the preferred species for shark fin soup (Clarke et al. 2006a, Dent and Clarke 2015, Fields et al. 2018). Smooth Hammerhead accounted for 1.7% of the fin imported and was the fourth most abundant traded fin species in Hong Kong in 2014 (Fields et al. 2018). The meat, liver oil, skin, cartilage, and jaws may also be used (White et al. 2006, Miller 2016, Almerón-Souza et al. 2018).

They are a vulnerable species according to the IUCN and are listed in CITES Appendix II, which makes it illegal to export it without a permit. They’re protected by some nations such as the United States and Brazil. They have declined in abundance by 89% since 1986. The main threats to their survival are their high by-catch mortality and targeted fisheries for their valuable fins.

The Smooth Hammerhead is caught globally as target and bycatch in coastal and pelagic commercial and small-scale longline, purse seine, and gillnet fisheries, and is often retained for the fins, and sometimes the meat. It has undergone steep historic declines in the Atlantic but the introduction of management measures may be allowing slow recovery, lesser declines in the South Pacific, and increases in the Indian Ocean. The weighted global population trends estimated a median reduction of 21.8–64.8%, with the highest probability of <20% and >80% reduction over three generation lengths (72.3 years). The Northwest Atlantic data that includes the period after management changes led to the lower estimated global reduction. However, there is uncertainty in some of the catch data, limited regional representation of some time-series, and intensive-fisheries in data-poor regions that are suspected to have driven declines, balanced with the relatively lower level of artisanal fisheries threat to this species compared to that of the Scalloped Hammerhead (S. lewini) and Great Hammerhead (S. mokarran), due to its generally more temperate distribution. Expert judgement elicitation thus inferred a global population reduction of 30–49% and the Smooth Hammerhead is therefore assessed as Vulnerable A2bd. More robust species-specific data and monitoring of catches is required to improve certainty of catch estimates for a future assessment of this species.

The success of actions agreed through international wildlife and fisheries treaties depends on implementation at the domestic level; for sharks, such follow up actions have to date been seriously lacking.

In 2010, the International Commission for the Conservation of Atlantic Tunas (ICCAT) prohibited retention, transshipment, landing, and sale of Great Hammerheads (and other hammerhead species) for ICCAT fisheries operating in the Convention Area. There are exceptions for local consumption in developing countries, provided they cap catches, meet catch data reporting requirements, and ensure fins are not traded internationally. The Western and Central Pacific Fisheries Commission (WCPFC) designated the Great Hammerhead as a ‘key shark species’ in 2010, but has yet to adopt hammerhead catch limits. Several proposals to ban hammerhead landings and/or set regional hammerhead fishing limits through the Inter-American Tropical Tuna Commission (IATTC) have been defeated.

In 2013, the Smooth Hammerhead was added to Appendix II of the Convention on International Trade in Endangered Species (CITES) based on its similarity to the Scalloped Hammerhead. The listing requires CITES Parties to ensure that exports be accompanied by permits based on findings that parts are sourced from legal and sustainable fisheries. In 2018, the Smooth Hammerhead was added to Annex 1 of the Convention on Migratory Species (CMS) Memorandum of Understanding (MoU) for Migratory Sharks, even though the species has not (yet) been included on the CMS Appendices. The MoU is aimed at facilitating regional conservation of listed shark and ray species. Great and Scalloped Hammerhead Sharks were listed on CMS Appendix II in 2014. Initiatives to prevent capture, minimize bycatch mortality, promote safe release, and improve catch (including discard) reporting are needed, as is full implementation of additional commitments agreed through international treaties.

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Smooth Hammerhead Gallery