Bluntnose Sixgill Shark, Hexanchus griseus
The Bluntnose Sixgill Shark, Hexanchus griseus, whose common Spanish name is tiburón de seis branquias, is a species in the Cow Shark or Hexanchidae Family, known collectively as tiburones cañabotas in Mexico. Globally, there are only three species in the genus Hexanchus, two of which are found in Mexican waters, one in the Atlantic and one in the Pacific.
The Bluntnose Sixgill Sharks have heavy fusiform bodies that taper at both ends. They are either gray, olive-green, or brown dorsally fading to a paler coloration ventrally and have a distinct light-colored stripe along each flank close to their lateral line. Their fins have pale edges. They have a broad head, a broadly rounded snout, large fluorescent green teardrop-shaped eyes, and a wide broadly rounded ventral mouth with six rows of low, blade-like, comb-shaped teeth on each side and nine smaller single serrated cusped teeth on each side of their upper jaw. They have six very long gill slits making them somewhat unique and giving rise to their common name. Their anal fin is smaller than their single dorsal fin which is set far back on the body just behind their pelvic fins. Their pectoral fins have broad bases and rounded tips and their pelvic fins have straight margins and rounded tips.
The Bluntnose Sixgill Sharks are primarily a deep water species living over the outer continental and insular shelves as well as the upper slopes and are found at depths up to 9,000 feet. They reach a maximum length of 4.8 meters (15.8 feet) and weight of 590 kg (1,300 pounds); their lifespan can be up to 80 years. They are diurnal being inactive and resting on the bottom during daylight hours and migrating vertically toward the surface and shallow waters to feed at night. They are strong slow swimmers and normally solitary. They feed on a fairly wide variety of fish and invertebrates and are preyed on by sea lions, white sharks, and orca whales. Reproduction is ovoviviparous with eggs hatching inside the females with litter sizes of 22 to 108 individuals that are approximately 70 cm (28 inches) in length. The pups reside on the upper slopes and outer continental shelves before moving to coastal inshore waters. As they mature they move into deeper waters with adults seldom found above 300 feet.
The Bluntnose Sixgill Shark is a highly migratory wide-ranging circumglobal species. In Mexican waters they have a limited distribution being found along the entire west coast of Baja and in the southern half of the Sea of Cortez.
The Bluntnose Sixgill Shark cannot be confused with any other species due to its gill count.
The Bluntnose Sixgill Sharks are often caught as a by-catch of other fisheries by handline, longline, gill nets, traps, and trawls. They are smoked and exported to European markets. They are also used for meat, liver oil, salted and dried food products, fish meal, and pet foods. They have been heavily targeted by sport fisheries and commercial fishermen along the northwest coast of the United States. These fisheries have collapsed due to excessive fishing pressures with a significant decline in the populations for the last twenty years. They are not known to attack humans. From a conservation perspective they are considered Near Threatened and/or Vulnerable.
Bluntnose Sixgill Shark, Hexanchus griseus, juvenile. Fish provided by the commercial fishermen of the greater Los Cabos area, Baja California Sur, March 2011. Length: 75 cm (30 inches). Identification courtesy of H.J. Walker, Jr., Scripps Institute of Oceanography, La Jolla, CA.