The Hammerhad Shark Family – Sphyrnidae
There are currently three members of the Hammerhead Shark or Sphyrnidae Family, two from the Pacific, and one found in the both the Atlantic and the Pacific presented in this website:
The Sphyrnidae Family includes the Bonnethead Sharks, the Hammerhead Sharks and the Scoophead Sharks. They are global travelers found in all tropical seas along coastlines and continental shelves. There are eight global members placed in two genera, of which six members are found in Mexican waters, three in the Pacific and three in both the Atlantic and the Pacific.
The Hammerhead Sharks are characterized by a strongly flattened front head with side extensions in the shape of an axe, mallet, or spade with eyes on the outer edges. Their distinctively-shaped head is known as a cephalofoil and is used for sensory reception, maneuvering, and prey manipulation. Their eyes, mounted on the extreme edges of the cephalofoil, afford these sharks with 360-degree vision in the vertical plane and allow them to see above and below at all times. They have elongated rounded bodies that are moderately slender, cylindrical or somewhat compressed. They are normally light gray with a greenish tint that transitions to white ventrally. They have a set of massive teeth with their front and rear teeth being blade-like with one point; their lower teeth are straight and their upper teeth are oblique and deeply notched on the rear side. Their teeth are distinctive and commonly found as fossils dating to the Miocene Epoch, twenty million years ago. They have five gill slits with the last one located over the front of their pectoral fins. Their heads also have electrosensory pores that allow them to locate electric signals emitted by potential prey. They have strongly asymmetrical crescent-shaped caudal fins with a notched base and a well-defined ventral lobe. They have two dorsal fins, the first being high, pointed, with a short base, and located behind the pelvic fins, and the second being much smaller and similar to the anal fin.
There are eight species of Hammerhead Sharks known globally, of which six frequent Mexican waters. They range from 90 cm (36 inches) to 6 meters (19.7 feet) in length and 3 kg (6.6 pounds) to 580 kg (1,279 pounds) in weight. They typically swim in large schools of as many as 100 individuals during the day and become solitary hunters at night. They consume a large range of prey including fish, crustaceans, octopus, and squid. Stingrays are one of their favorites. They normally patrol the bottom stalking prey and then utilize their head to pin down prey. Some Hammerheads are cannibalistic consuming other hammerheads including their own young. Hammerheads reproduce once a year with litter sizes of twelve to fifteen pups, which are born as miniature adults. Reproduction is viviparous with internal fertilization. Juveniles are left to fend for themselves and tend to form schools and travel together; they are known to inhabit shallow shoreline waters to avoid predation.
Of the eight known hammerheads, only three (the Great, the Scalloped, and the Smooth) are known to attack humans. Globally these attacks occur at a level of about fifty per year with fatalities exceedingly rare. They are generally very timid around divers and will depart the area.
From a conservation perspective these sharks are in serious trouble. Due to human demand for their fins, predominantly in southeast Asia, they have been significantly overfished. Historically fishermen cut off their fins (finning) and discarded the balance of the fish. Globally there are currently seven Sphyrna and one Eusphyra being monitored from a conservation perspective. The Great Hammerhead, Sphyrna mokarran and the Scalloped Hammerhead, Sphyrna lewini are considered Endangered; the Golden Hammerhead, Sphyrna tudes and the Smooth Hammerhead, Sphyrna zygaena are considered Vulnerable; the Scalloped Bonnethead, Sphyrna corona and the Winghead Shark, Eusphyra blochii are considered Near Threatened; the Bonnethead, Sphyrna tiburo is considered of Least Concern; and the Carolina Hammerhead, Sphyrna gilberti, the Scoophead, Sphyrna media, and the Whitefin Hammerhead, Sphyna couardi are presently being assessed. Globally catch rates of Hammerheads are in significant decline. As recently as ten years ago I was personally catching large Scalloped and Smooth Hammerheads on a fairly regular basis. Due to their overabundance of sharp teeth, they require strong wire leader to boat. The last time I caught one was at least five years ago. Both the Mexican and United States Governments have very recently placed a ban on the retention of all sharks by fishermen and this ban is actually being enforced and respected by local fishermen. As such they are no longer a targeted species.
The Hammerhead Sharks are important as food fish globally. They are also used to produce various sub-products, most significantly Vitamin A, which is obtained from their livers. Their fins are in high demand for use in Shark-Fin Soup predominantly in Asian markets and command high prices. In certain areas the Great Hammerhead and Scalloped Hammerhead are popular subjects for ecotouristic diving. Bonnetheads and Smooth Hammerheads can be found in many public aquariums, especially along the East and Gulf Coasts of the United States.