Bonnethead

Bonnethead, Sphyrna tiburo

The Bonnethead, Sphyrna tiburo, whose common Spanish name is cornuda cabeza de pala, is the smallest species in the Hammerhead Shark or Sphyrnidae Family, known collectively as tiburones martillo in Mexico. 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.

The Bonnetheads have moderately compact shark-like bodies that have unique compressed, wide, smooth, rounded, shovel shaped heads for which they are named. They range in color from gray to gray-brown dorsally that transitions to white ventrally. The anal fins are slightly concave; the caudal fin has a nearly straight upper margin and the lower lobe is approximartely 33% as long as the upper lobe and has a straight rear margin; the first dorsal fin originates just behind the base of the pectoral fins and is distinctly tall; the second dorsal fin is slightly less than half as long as the base of the first dorsal with a slender rear corner; the pectoral fins are short but well-developed with a straight rear margin. They have broad, smooth spade-like heads and are the only sharks that display sexual dimorphism in the head whereby females have more broadly rounded heads and males have a distinct bulge along the anterior margin of the cephalofoil. They have eyes that are located on the ends of the evenly rounded lobes of the flattened heads and arched mouths that are located ventrally that are equipped with small sharp teeth in the front and flat, broad molars in the back. They are covered with denticles in random patterns.

Bonnetheads are found on the continental and insular shelves, over reefs and within estuaries and shallow bays at depths between 30 and 260 feet. They reach a maximum length 1.50 meters (4 feet 11 inches) and 10.8 kg (24 pounds) in weight with females being larger than males. They must swim continuously to maintain buoyancy. They usually travel in small schools of up to fifteen individuals that follow changes in water temperatures (preferring waters that are in excess of 21oC (70oF)), salinity, depth, distance to tidal inlets, and water clarity. They are the only hammerhead that use their pectoral fins for swimming. During pupping season the females predominate in shallow waters. They make seasonal migrations toward the equator in schools that can number up to 1,000 individuals during the winter. The Bonnetheads have a well-developed sensory and nervous systems with strong vision and hearing capabilities. They have the ability to communicate with fellow sharks via the release of a cerebrospinal fluid. The Bonnetheads feed primarily on crustaceans (blue crabs, mollusks and shrimp) and small fish locating prey via electroreception. In turn they are preyed upon by larger sharks. Reproduction in the Bonnethead is viviparous with females capable of storing sperm for up to four months, with a short four to five month gestations period following by the delivery of between four and sixteen live pups that are 30 cm (12 inches) to 35 cm (14 inches) in length. Reproduction if viviparous with short gestation periods with birthing occurring in shallow sea grass habitats. . The females are able to store sperm for up to four months that is utilized in the spring to ensure that the pups are born during optimal conditions for their survival. The females stop eating during this period and the males move to different locations, an adaptation to avoid feeding upon their own young. The Bonnetheads are scientifically interesting as they are believed to be the current form of twenty-five million years of evolution. They have life spans of up to eighteen years.

In Mexican waters the Bonnetheads are found in all oceanic waters of the Atlantic and the Pacific but only on a seasonal basis.

The Bonnetheads are an easy identification that cannot be confused with any other species due to the smooth, rounded, shovel-shaped head.

The Bonnetheads are a common catch of inshore artisanal, commercial and recreational fishermen. They are caught via shrimp trawlers, nets, longlines and hook and line. They are heavily targeted in Mexican waters of the Gulf of Mexico by artisanal fishermen. Population levels are estimated to be 5 million individuals with annual catch levels are in excess of 500,000 individuals of which 15% are made by recreational anglers. They provide great sport on light tackle for recreational anglers. As a food they are not in high demand and marketed fresh, frozen or dried salted. They are also processed into fish meal, used as a crab bait, and are a component of the aquarium trade. From a conservation perspective the Bonnetheads are classified to be of Least Concern being common over a wide range and stable populations. They benefit, unlike most sharks, from short generation times and high population growth rates. Their pupping areas are subject to human development and related pollution and I believe that very soon they will become Vulnerable. They are viewed as being harmless to humans.

Bonnethead Shark, Sphyrna tiburo. Fish caught from coastal waters off Botany Bay, Edisto Beach, SC, December 2012. Length: 1.07 m (3 feet 6 inches). Catch, photo and identification courtesy of Josh Leisen (lifelistfishing.com), Gaylord, MI.

Bonnethead Shark, Sphyrna tiburo. Fish caught off the Channel 5 Bridge (MM 71.4), Florida Keys, FL, January 2017. Length: 1.11 cm (3 feet 8 inches). Catch, photo and identification courtesy of Josh Leisen (lifelistfishing.com), Gaylord, MI.

Bonnethead Shark, Sphyrna tiburo. Fish caught off the No Name Key Bridge (MM 30), Florida Keys, FL, January 2017. Length: 1.01 m (3 feet 4 inches). Catch, photo and identification courtesy of Josh Leisen (lifelistfishing.com), Gaylord, MI.