Smooth Hammerhead, Common Hammerhead.
Anterior margin of hammer rounded with conspicuous lateral notches but no central notch. First dorsal rear margin straight. Free rear tip of first dorsal terminates well before level of pectoral fin origin. Second dorsal fin low with extremely long free rear tip. Pelvic fin posterior margins not noticeably falcate. Body greyish-brown to olive-brown above. Ventral surface pale.
Maximum length approximately 380cm. Maximum lengths sometimes listed as 4m/13ft are from C. R. Gilbert (1967) but he fails to cite any specimens so this is likely his ‘best guess’. Size at birth 50-61cm.
Inshore bays to offshore continental shelves. From the surface to at least 55m but based on its preference for cooler water when compared to other hammerheads, the smooth hammerhead probably ventures into deeper water in tropical regions.
In the western North Atlantic, tagged juveniles spent the summer in the New York Bight and then moved south to Cape Hatteras for the winter.
Circum-temperate/sub-tropical in most regions. Apparently absent from the Gulf of Mexico. Nursery grounds exist along both sides of the Gulf of California.
REGIONALLY CRITICALLY ENDANGERED
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 Globally 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.
In the Mediterranean Sea, a meta-analysis of time series of standardized indices of abundance showed that hammerhead sharks (mainly composed of the Smooth Hammerhead) have declined by >99% in abundance and biomass since the early 19th century. As the situation in the Mediterranean region has not changed in terms of fishing effort, the species is likely to still be threatened given this steep decline alongside possible local extinctions in various region of the basin. The Smooth Hammerhead is therefore assessed regionally as Critically Endangered under Criterion A2bd.
Citations and References
Ferretti, F., Soldo, A., Casper, B., Domingo, A., Gaibor, N., Heupel, M.R., Kotas, J., Lamónaca, A., Smith, W.D., Stevens, J., Vooren, C.M. & Pérez-Jiménez, J. 2016. Sphyrna zygaena. The IUCN Red List of Threatened Species 2016: e.T39388A16527905. Downloaded on 21 September 2020.
Seasonal Movements and Habitat Use of Juvenile Smooth Hammerhead Sharks in the Western North Atlantic Ocean and Significance for Management
Ryan K. Logan, Jeremy J. Vaudo, Lara L. Sousa, Mark Sampson, Bradley M. Wetherbee, Mahmood S. Shivji
A viviparous species with yolk-sac placenta. Litter size 20-50 pups.
Diet consists mainly of bony fishes, small sharks, skates, and stingrays.
Little is known about the habits of the smooth hammerhead. It is rarely encountered on the east coast of North America but more common in the eastern Pacific where it enters shallow bays to give birth e.g. La Paz Bay and protected coastal areas further north in the Gulf of California.
I have seen small juveniles (approx. 80cm) within 1km of the shore near Mboiki Bay and Waterfall Bluff in South Africa, indicating that this is another nursery area.
Reaction to divers
Shy and easily spooked by loud noises such as diver’s bubbles but relatively easy to approach on snorkel in baited situations.
Traditionally, a difficult species to encounter but the recent (circa 2016) introduction of organized shark feeds near Cabo San Lucas on the southern tip of Baja, has made photographing Smooth hammerheads relatively easy.
Officially, the trips are usually pitched as ‘mako encounters’ but towards the end of mako season, hammerheads are not uncommon.
Some animals tend to be characteristically shy but now and then a ‘player’ will arrive that allows extremely close encounters; essentially circling snorkelers in an effort to reach the bait.
If you are specifically hoping to see smooth hammerheads rather than blue and mako sharks, the best time to look for them in that region would be March to May.
Big Fish Expeditions offers Blue and Mako Snorkeling trips in March when all three species are likely to be encountered.
While chumming for larger sharks near Port Saint Johns in South Africa (during the annual Sardine Run) I have snorkeled with very small juvenile smooth hammerheads that approached the bait even though they were less than 1m long. I would not say this is a location worth visiting in order to see this species but it illustrates how bold smooth hammerheads become in the presence of bait.
Great hammerhead distinguished by straighter leading edge of hammer, proportionately taller dorsal fin, and often larger size.
Scalloped hammerhead distinguished by a leading edge of hammer which has a well-defined central notch.
Carolina hammerhead Outwardly identical to the scalloped hammerhead. Range restricted to the western Atlantic.