Scalloped Hammerhead, Sphyrna lewini
Scalloped Hammerhead, Sphyrna lewini. Fish caught from coastal waters off Point Palmilla, Baja California Sur, April 2010. Length: 1.2 m (3 feet 11 inches). Weight: 4.5 kg (10 lbs 0 oz).
Scalloped Hammerhead, Sphyrna lewini. Fish caught from coastal waters off Cabo San Lucas, Baja California Sur, April 2010. Length: 1.2 m (3 feet 11 inches). Weight: 4.5 kg (10 lbs 0 oz). Catch, photograph and identification courtesy of Eli (obsessive-angling.wordpress.com).
The Scalloped Hammerhead, Sphyrna lewini, is a member of the Hammerhead Shark or Sphyrnidae Family, and is known in Mexico as cornuda común. They are the most common of the Hammerhead Sharks. Globally, there are eight species in the genus Sphyrna, of which six are found in Mexican waters, three in the Pacific and three in both the Atlantic and the Pacific Oceans.
The Scalloped Hammerhead is one of the larger Hammerhead Sharks. They have a moderately slender fusiform body 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 5 indentations; the 3 in the middle are modest and the 2 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 5 gill slits with the last 1 being over the front of the pectoral fins and the third 1 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 and 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 and is found slightly behind the pectoral fins and ends just before the pelvic fins; and, their pelvic fins have a straight posterior margin.
The Scalloped Hammerhead is a coastal schooling pelagic species found over the continental shelf and in coastal waters at depths up to 980 m (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 m (14 feet 1 inches) in length versus males which have a maximum length of 3.7 m (12 feet 2 inches). As of January 1, 2023, the International Game Fish Association world record stood at 161 kg (353 lbs) with the fish caught in coastal waters off Key West, Florida in April 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 crustaceans, benthic reef fish, and demersal fish. Juveniles are in turn preyed upon by larger sharks; 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 9 to 12 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.
The Scalloped Hammerhead Shark is a resident of all Mexican waters of the Atlantic and the Pacific Ocean.
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, with a pointed tip; straight margin of head) and the Smooth Hammerhead, Sphyrna zygaena (broadly convex head profile with no central indentation).
From a conservation perspective the Scalloped Hammerhead is currently considered to be Endangered. They 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 1.0 m (3 feet 3 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. 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 1,300,000 to 2,700,000 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.
Hammerheads are considered potentially dangerous with total documented unprovoked attacks on humans numbering twenty-one with two fatalities. They have been reported to display mixed behavior toward divers taking either threatening postures or displaying non-aggressive behaviors.