Common Thresher Shark, Alopias vulpinus
Common Thresher Shark, Alopias vulpinus. Fish caught in coastal waters off Loreto, Baja California Sur, July 2015. Length: 2.7 meters (9 feet 0 inches). Catch, photograph and identification courtesy of Chris Wheaton, Fullerton, California.
The Common Thresher Shark, Alopias vulpinus, is member of the Thresher Shark or Alopiidae Family, that is also known as the Thintail Thresher and in Mexico as tiburón zorro común. 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 and Pacific Oceans.
The Common 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. Some fish have a white spot on the tips of their pectoral fins. They have a short rounded head with a long conical snout and a narrow rounded space between their medium-sized 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 narrow base without keels and is long, thin, and greater than 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 curved leading edge and pointed tips; and their pelvic fins are large. Their body is covered with rough scales and smooth denticles.
The Common Thresher Shark is a true pelagic species found in all tropical and subtropical open ocean waters from the surface to depths of 770 m (1,875 feet). They are highly migratory and found seasonally at higher latitudes following warm water masses. They are the largest Thresher Shark reaching a maximum of 5.75 m (18 feet 7 inches) in length and 510 kg (1,120 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 Killer 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 two to seven large pups per litter that are 1.2 m (4 feet inches) to 1.5 m (5 feet 0 inches) in length and 5.0 kg (11 lbs) to 5.9 kg (13 lbs) in length at birth. Their gestation period lasts nine months. They have lifespans of fifty years. The Common 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 Common Thresher Shark is a resident of all Mexican waters of both the Atlantic and the Pacific Oceans.
The Common Thresher Shark is exceedingly difficult to separate from, and is often confused with, the Pelagic Thresher Shark, Alopias pelagicus (dark patch over pectoral fin base; tail length equal to body length; tail with wide base; pectoral fins straight margins with rounded) and the Bigeye Thresher Shark, Alopias superciliosus (much larger eyes).
From a conservation perspective the Common Thresher Shark is currently considered to be 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. Contributing to the problem is the fact that they are often confused with both the Pelagic and Bigeye Thresher Sharks. Their population decline has been attributed to heavy overexploitation 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. In the United States regulations have recently been implemented setting commercial seasonal closures, limited permitted boats, operational ranges, and catch limits along with the banning of finning and recreational minimum sizes and retention limits. The Common 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.