Pelagic Thresher Shark, Alopias pelagicus
Pelagic Thresher Shark, Fetus, Alopias pelagicus. Fish provided by the commercial fishermen of the greater Los Cabos area, Baja California Sur, October 2010. Length: 32 cm (13 inches).
Pelagic Thresher Shark, Immature, Alopias pelagicus. Fish caught from coastal waters off Loreto, Baja California Sur, February 2024. Length: ca. 2.4 m (8 feet). A catch and release. Tentative identification courtesy of Dr. Douglas Long. Dr. Long indicates that a dental exam would be require to confirm the identification.
Pelagic Thresher Shark, Alopias pelagicus. Fish caught from coastal waters of Cabo San Lucas, Baja California Sur, March 1984. Length: 2.7 meters (9 feet 0 inches). Catch, photograph and identification courtesy of George Brinkman, Guelph, Ontario, Canada.
The Pelagic Thresher Shark, Alopias pelagicus, is a member of the Thresher Shark or Alopiidae Family, and is known in Mexico as zorro pelágivo. Globally, there are three members of the Alopiidae Family that have been placed in one genera and all three members are found in Mexican waters, one in the Pacific Ocean and two in both the Atlantic Oceans and Pacific Ocean.
The Pelagic Thresher Shark is are easily recognizable by their enormously long caudal fin that comprises about half the total length of their stout cylindrical body. They are blue-gray dorsally and on their sides transitioning to white ventrally and have a dark patch above their pectoral fin base. They have a short rounded head with a long conical snout and a narrow space between their small eyes. They have a small mouth that opens ventrally and is equipped with small teeth. They have five gill openings with the last two being short and opening over their pectoral fin base. Their anal fin is small; their caudal fin has a broad base without keels and is approximately half the body length; their first dorsal fin is large and originates halfway between the pectoral and pelvic fins; their second dorsal fin is small; their pectoral fins are very long and narrow with a straight leading edge and broad tips; their pelvic fins are large. Their body is covered with rough scales and smooth denticles.
The Pelagic Thresher Sharks are a true pelagic highly migratory species found in all tropical and subtropical open ocean waters from the surface to depths of 700 m (2,300 feet). They are the smallest member of the Alopiidae Family reaching a maximum 3.65 m (12 feet 0 inches) in length and 88.4 kg (194 lbs) in weight with females being slightly larger than males. They have the ability to elevate their body temperature to above that of surrounding water which allows them to move into colder waters and swim faster. They are known to leap out of the water. They consume schooling fish such as flyingfish, herrings, mackerels, and tuna, as well as squid, all of which they manipulate, corral, and stun with their long whip-like tail. In turn they are preyed upon by other sharks and toothed whales. Reproduction is via aplacental viviparity with oophagy. Embryos are nourished by the yolk sac and ovulated eggs. They have very low fecundity rates with only two large pups per litter that are three to six feet in length at birth. The length of their reproductive cycle and gestation period is presently unknown. Females have a lifespan of up to twenty-eight years and males of around seventeen years. The Pelagic Thresher Shark is poorly studied with very limited information available about their lifestyle and behavioral patterns including specific details on age, growth, longevity, movement patterns, diet, habitat use, and reproduction.
The Pelagic Thresher Shark is a resident of Mexican waters of the Pacific Ocean but only present in El Niño years.
The Pelagic Thresher Shark is exceedingly difficult to separate from, and is often confused with, the Common Thresher Shark, Alopias vulpinus (lacks dark patch over pectoral fin base; tail length equal to half body length; tail with narrow base; pectoral fins curved with pointed tips) and the Bigeye Thresher Shark, Alopias superciliosus (much larger eyes).
From a conservation perspective the Pelagic Thresher Shark is currently classified as Vulnerable. With an unregulated global fishery and poorly monitored catch levels, population declines of greater than 80% over the last 30 years have been noted globally. This decline has been attributed to heavy over exploitation due to large demands for human consumption; the production of liver oil for cosmetics, health foods, and high-grade machine oil; hides for leather; fins for shark fin soup (an estimated 350,000 to 4 million thresher sharks are slaughtered annually just for their fins; after fin removal, carcasses are discarded at sea). In addition, there is significant by-catch via longline, drift net, and gill net fisheries for tuna and swordfish with high mortality rates and heavy fishing pressure by artisanal fishermen in well-known inshore nursery areas with aggregating females comprising 83% of the catch, 41% of which are pregnant. The Pelagic Thresher Sharks are popular global sport and big game targets for recreational anglers. They are considered good table-fare. They are viewed as “harmless” to humans and will avoid divers in the wild.