Pelagic Thresher Shark

Pelagic Thresher Shark, Alopias pelagicus

The Pelagic Thresher Shark, Alopias pelagicus, whose common Spanish name is zorro pelágivo is a species in the Thresher Shark or Alopiidae Family, known collectively as tiburones zorro in Mexico. Globally, there are three members of the family placed in one genera and all three members are found in Mexican waters, two in the Atlantic and the Pacific, and this species which is found only in the Pacific.

The Pelagic Thresher Sharks 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 and second dorsal fin are 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 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 2,300 feet. They are the smallest member of the Alopiidae Family reaching a maximum 3.65 meters (12 feet) in length and 88.4 kg (194 pounds) 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 28 years and males of around 17 years. They are a poorly studied species and very little is known about their behavioral patterns.

In Mexican waters the Pelagic Thresher Sharks and found in all waters of the Pacific but only 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).

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. From a conservation perspective, they are 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.

Pelagic Thresher Shark (2)

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, Alopias pelagicus. Fish caught from coastal waters of Cabo San Lucas, Baja California Sur, March 1984. Length: 2.7 meters (9 feet). Catch, photo and identification courtesy of George Brinkman, Guelph, Ontario, Canada.