Scalloped Hammerhead, Sphyrna lewini
The Scalloped Hammerhead, Sphyrna lewini, whose Spanish common name is cornuda común, is a species in the Hammerhead Shark or Sphyrnidae Family, known collectively as tiburones martillo in Mexico. They are the most common of the Hammerhead Sharks. Globally, there are eight species in the genus Sphyrna, six of which are found in Mexican waters, three in the Pacific and three in both the Atlantic and the Pacific.
The Scalloped Hammerheads are one of the larger Hammerhead Sharks. They have moderately slender fusiform bodies with a tall first dorsal fin and low second dorsal and pelvic fins. They are brownish-gray to bronze or olive dorsally and pale yellow or white ventrally. The undersides of their pectoral fins have black tips. They have a strongly flattened head (the cephalofoil) that is 26 to 29% of total body length and has side extensions in the shape of an axe, mallet or spade. Their eyes are on the outer edges of the cephalofoil and their nostrils are set far apart. Their “blade” has five indentations; the three in the middle are modest and the two on the outer edges are deeper. Their mouth is broadly arched and equipped with small teeth with smooth or slightly serrated cusps set on large bases. Their upper jaw contains narrow triangular teeth, the first being nearly symmetrical and erect and the others being increasingly oblique toward the corner of the mouth. Their lower teeth are more erect and slender than their upper teeth. They have five gill slits with the last one being over the front of the pectoral fins and the third one being shorter than the first dorsal fin. Their anal fin is deeply notched and its base is longer than the base of the second dorsal fin. Their caudal fin is strongly asymmetrical; it is notched under the tip of the top lobe and has a large lower lobe. Their first dorsal fin is large and erect with a rounded tip; it is found slightly behind the pectoral fins and ends just before the pelvic fins. Their pelvic fins have a straight posterior margin.
The Scalloped Hammerheads are a coastal schooling pelagic species found over the continental shelf and in coastal waters at depths up to 980 meters (3,215 feet). They spend most of the day closer to shore and move offshore in search of prey at night. Their pups tend to stay in coastal zones near the bottom and are found in large numbers in estuaries and bays during the summer. They form large schools that migrate to cooler waters at higher latitudes in the summer. Horizontal migrations are observed from inshore bays to pelagic habitats in the third year of life. Females are larger than males reaching 4.3 meters (14.1 feet) in length versus males which have a maximum length of 3.7 meters (12.1 feet). The current IGFA world angling record is a fish measuring 2.29 meters (7.5 feet) and 353 pounds (152 kg) in weight caught off Key West, Florida in 2004 by a fly fisherman. They feed primarily on fish including anchovies, barracudas, conger eels, goatfish, grunts, halfbeaks, herrings, mackerels, mullets, parrotfish, sardines, silversides, and smaller elasmobranchs including angel sharks, blacktip reef sharks, and stingrays. Pups and juveniles feed mainly on benthic reef fish, demersal fish, and crustaceans. Juveniles are in turn preyed upon by larger sharks, however, adults have no predators. Reproduction is viviparous with eggs hatching inside the females with nourishment provided by a yolk sac placenta. Gestation periods are between nine to twelve months and culminate with the birth of 12 to 38 pups that are 38 cm (15 inches) to 45 cm (18 inches) in length. Females give birth in inshore waters during the summer. As predation of the pups and juveniles is high, they have a higher fecundity rate than most other sharks. They have life spans of just over thirty years.
In Mexican waters the Scalloped Hammerhead Sharks are found in all waters of the Atlantic and the Pacific.
The Scalloped Hammerhead can be differentiated from other hammerheads by the indentation located centrally on the front margin of their head. It is most likely confused with the Great Hammerhead, Sphyrna mokarran (first dorsal fin very tall, greater than third gill slit, and with a pointed tip; straight margin of head) and the Smooth Hammerhead, Sphyrna zygaena (broadly convex head profile with no central indentation).
The Scalloped Hammerheads are a focus fish for both commercial and recreational anglers and are of greater value than other sharks due to their high fin ray counts. They are caught in both inshore and offshore waters as targets and as by-catches via longlines, gill nets, drift nets, and trawls. Juveniles reside in shallow coastal waters making them very vulnerable to fishing pressures; their mortality rate is very high when caught by net. “Finning” is also practiced with fish of all ages; once caught, the fins are removed and the balance of the fish is returned to the ocean to die. Very seldom are fish caught and returned to the ocean unharmed; most are juveniles that have not matured and had an opportunity to breed. Principal locations in Mexico where such finning practices occur include Baja California, Baja California Sur, Sinaloa, and Sonora with the majority of fish being less than 100 cm (39 inches) in length. They are marketed fresh, dried, smoked or frozen. Their meat is deemed a poor choice for human consumption, although they are highly regarded for their fins and hides. Portions of the fish are also used as a source of Vitamin A and fishmeal. Hammerheads are considered potentially dangerous with total documented unprovoked attacks on humans numbering 21 with two fatalities. They have been reported to display mixed behavior toward divers taking either threatening postures or displaying non-aggressive behaviors.
From a conservation perspective, they are currently classified as an Endangered Species. Catch rates are not well monitored and their population trends are unknown, however, where catch data is available, declines are omnipresent and some locations in the world have documented a 50 to 98% reduction of the species over the last 30 years. As recently as 2006 it was estimated that the fins from 49,000 to 90,000 tons or 1.3 to 2.7 million Scalloped and Smooth Hammerheads were sold globally. Although their fecundity rate are high, they take 15 years to reach sexual maturity affording them relatively low resilience. Major threats include: 1. Gill net and trawl net fishing in inshore water, 2. Commercial gill net fishing offshore; 3. Major increasing global demand for shark fins driving raw fin prices up and increasing fishing pressure; 4. Targeted fishing at aggregating sites where they are easily caught with nets, for example at the Espiritu Santo sea mouth in the Sea of Cortez, the northern Sea of Cortez, and Bahia Almejas on the Pacific Coast of Baja; 5. The current lack of realistic regulations with strict enforcement. The solutions are not easy and are difficult to enforce but might include several of the following: 1. A restriction on the length of pelagic gills nets; 2. A ban on trawl fishing from waters that are less than three miles from shore; 3. The establishment of coastal protected areas in which fishing is banned; 4. A ban on the export of shark fins on a global basis. Without the implementation of strict regulations this species will become extinct fairly quickly.