Smooth Hammerhead, Sphyrna zygaena
The Smooth Hammerhead, Sphyrna zygaena, whose Spanish common name is cornuda prieta, is a species in the Hammerhead Shark or Sphyrnidae Family, known collectively as tiburones martillo in Mexico. 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 Smooth Hammerheads are one of the larger and wider-ranging 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 two indentations on the outer edges and no indentations in the middle. Their mouth is ventral, broadly arched, and equipped with small smooth teeth or slightly serrated cusps set on large bases. Their upper jaw contains narrow triangular teeth, the first being nearly symmetrical and erect and others being increasing 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. Their anal fin is deeply notched and its base is equal in length to 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 moderately tall with a rounded apex that is falcate in shape; it has a free rear tip located over the pectoral fins and does not reach the pelvic fin origin. Their second dorsal fin is low and ends before the caudal fin base. Their pectoral and pelvic fins have a straight or slightly concave posterior margin.
The Smooth Hammerheads are a coastal schooling pelagic species found over the continental shelf and in coastal waters at depths up to 200 meters (655 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 over sandy bottoms at depths of 10 meters (35 feet) 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. Known pupping and nursery areas include the northern Sea of Cortez. Adults are found either singularly or in small schools and are commonly located over deep reefs on the edge of the continental shelf. Females are larger than males achieving a maximum length of 4.0 meters (13.1 feet) versus males which have a maximum length of 3.7 meters (12.1 feet) and can weigh up to 363 kg (800 pounds). They feed primarily on fish including hakes, mackerels, and ribbonfish, and smaller elasmobranchs including various sharks, skates, and stingrays. They also consume squid and other crustaceans. 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 ten and eleven months and culminate with the birth of 29 to 49 pups measuring 50 cm (20 inches) and having a sex ratio of one to one. 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 a lifespan of just over twenty years. They are a poorly studied species and very little is known about their behavioral patterns.
In Mexican waters the Smooth Hammerheads are found in all waters of the Pacific.
The Smooth Hammerhead is most likely confused with the Great Hammerhead, Sphyrna mokarran (first dorsal fin very tall with a pointed tip and straight rear edge; straight margin of head) and the Scalloped Hammerhead, Sphyrna lewini (broadly convex head profile with central indentation).
The Smooth 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, handlines, gill nets, purse-seines (targeting tuna), drift nets, and pelagic and bottom trawls. The majority of fish are immature and have a high mortality rate. Juveniles are especially vulnerable as they reside in shallow coastal waters; 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. They are marketed fresh, dried, smoked or frozen. Their meat is generally deemed of poor quality for human consumption, although they are highly regarded for their fins, which command the highest prices of any shark fin. Their hides are also utilized for leather products. 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 a Vulnerable Species. Catch rates are not well monitored and their population trends are unknown as they are easily confused with the Scalloped Hammerhead. However, where catch data is available, declines are omnipresent and some locations in the world have documented a reduction of up to 80% 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 rates are high, they take many 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; 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.