Common Thresher Shark, Alopias vulpinus
The Common Thresher Shark, Alopias vulpinus, whose common Spanish name is tiburón zorro común is member of the Thresher Shark or Alopiidae Family, known collectively as tiburones zorro in Mexico. This fish is also known as the Thintail Thresher. 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 one in the Pacific.
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 and second dorsal fin are 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 pectoral fins are very long and narrow with a curved leading edge and pointed tips. Their pelvic fins are large. Their body is covered with rough scales and smooth denticles.
The Common Thresher Sharks are a true pelagic species found in all tropical and subtropical open ocean waters from the surface to depths of 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 length of 5.75 meters (18.9 feet) and weight of 510 kg (1,120 pounds) 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 four to five feet in length and weigh 11 to 13 pounds at birth. Their gestation period lasts nine months. Their lifespan is estimated to be between 45 and 50 years. They are a poorly studied species and very little is known about their behavioral patterns.
In Mexican waters the Common Thresher Sharks are found in all waters of both the Atlantic and the Pacific.
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).
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. 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. 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.
Common Thresher Shark, Alopias vulpinus. Fish caught in coastal waters off Loreto, Baja California Sur, July 2015. Length: 2.7 meters (9 feet). Catch, photo and identification courtesy of Chris Wheaton, Loreto.