Silky Shark, Carcharhinus falciformis
The Silky Shark, Carcharhinus falciformis, whose common Spanish name is tiburón piloto, is a member of the Requiem Shark or Carcharhinidae Family, known collectively as tiburones gambuso in Mexico. Their common name is derived from the texture of their skin. In the Eastern Pacific they are called “net-eater sharks” for their tendency to ravage tuna seine nets. Globally, there are thirty-five species in the genus Carcharhinus, seventeen of which are found in Mexican waters, seven in the Atlantic, four in the Pacific, and six in both oceans.
The Silky Sharks have slender, streamlined, and fairly long bodies. They are deep metallic brown-gray dorsally and transition to white ventrally. Their fins have dark tips except for the first dorsal fin. Their head has a moderately long and rounded snout with very small flaps in front of the nostrils, medium-sized circular eyes, and a mouth equipped with massive teeth set in 13 to 17 rows. Their upper teeth are triangular and strongly serrated with a notch on the posterior edge. Their lower teeth are narrow, erect, and with smooth edges. They have five sets of gill slits. Their anal fin originates slightly ahead of the second dorsal fin and has a deep notch in the posterior margin. Their caudal fin is high with a well-developed lower lobe. Their first dorsal fin is relatively small with a rounded apex and an “S” shaped rear margin; it originates behind the tips of the pectoral fins and has a free rear tip that is about half as long as the fin is tall. Their second dorsal fin is very small and low, and smaller than the anal fin, with a drawn-out rear tip that is three times longer than the fin is tall. They have a low narrow ridge on their back between the two dorsal fins. Their pectoral fins are narrow, sickle-shaped, and extraordinarily long. Their skin is densely covered with very small overlapping dermal denticles and is smooth to the touch.
The Silky Sharks are one of the most abundant sharks in the ocean with populations in the tens of millions. They inhabit all global tropical waters with temperatures in excess of 23oC (73oF). They are normally found in the open ocean near the edges of the continental shelf and over deep water reefs but will visit coastal waters on occasion. They reach a maximum of 3.5 meters (11 feet 5 inches) in length 346 kg (763 pounds) in weight with females being larger than males. They are highly mobile being capable of moving up to 60 km in one day as well as migratory being known to cover distances of up to 1,300 km. They can be found from the surface (where they spend the majority of their time) to depths of 12,400 feet. Juveniles are found in coastal nurseries and adults move to deeper waters further offshore. Younger fish tend to form schools, which include tuna, and can migrate in groups of up to 1,000 individuals. Small pilotfish and various jacks are known to accompany Silky Sharks. They have an acute sense of hearing that allows them to locate other feeding animals and other food sources. They are aggressive predators feeding on cephalopods and a wide variety of fish, including tuna, by first compacting in schools and then launching mouth-open in slashing attacks. In turn they are preyed upon by larger sharks and killer whales. Reproduction is viviparous with long reproduction cycles as they require at least five years to reach sexual maturity; females give birth to up to 16 live 70 cm (28 inches) to 85 cm (33 inches) miniature adults on annual or biannual cycles. They have a lifespan of up to twenty-two years. They date to the Miocene Period, twenty-three to five million years ago. They are currently the subject of various scientific studies to investigate the sensory biology of sharks.
In Mexican waters the Silky Sharks are found in all waters of the Atlantic and the Pacific.
The Silky Shark is not straightforward to identify but can be distinguished from other large requiem sharks by its relatively small first dorsal fin with a concave rear margin, its very small second dorsal fin with a long free rear tip, and its large sickle-shaped pectoral fins.
The Silky Sharks are caught in large numbers by commercial and artisanal fishermen. They are second to the Blue Shark, Prionace glauca, as the most caught shark species globally. Even greater numbers are caught as a by-catch of international longline and purse seines, and gill net swordfish and tuna fishermen with total annual catches estimated to approach 115,000 fish or 12,000 tons in 2000. Estimates of Silky Shark catches by Mexican fishermen vary between 23,000 and 32,000 tons per annum. They are also targeted by recreational fishermen but at a low level. Approximately 50% of the fish caught by commercial fishermen are returned to the ocean, however, many are already dead or unable to survive. Commercially they are valued for their fins (many fish are finned at sea before their bodies are discarded) and to a lesser extent for their meat, hide, liver oil, and jaws. Normally their meat is retained for local subsistence and their fins are exported to Asian markets. Annual sales of Silky Shark fins are estimated to vary between 500,000 and 1.5 million fish. Historically they have been abundant and a mainstay of commercial and artisanal shark fishermen. Their numbers have been in significant global decline and from a conservation perspective they have recently been downgraded from Least Concern to Near Threatened. Monitoring has been hampered by poor catch data, significant underreporting, and problematic identifications. Some studies have concluded that the Silky Shark populations in the Gulf of Mexico, for example, have been reduced by 91% from the 1950s to the 1990s, with the majority of caught fish now being smaller and immature. In some global regions they are currently considered Vulnerable. The management of the Silky Shark including protective regulations are ongoing but made difficult by the limited data available on global stock status and migration patterns. The Silky Sharks are considered dangerous to humans due to their size and cutting teeth and can behave aggressively toward divers, although documented attacks on humans are rare.
Silky Shark, Carcharhinus falciformis. Fish provided by the commercial fishermen of the greater Los Cabos area, Baja California Sur. Length: 1.07 meters (3 feet 6 inches).
Silky Shark, Carcharhinus falciformis. Underwater photo taken in coastal waters off Kailua-Kona, HI, October 2015. Length: 1.27 meters (5 feet 0 inches). Photo courtesy of Bob Hillis, Ivins, UT.