Alopias vulpinus

Common thresher sharks, Alopias pelagicus, are found in tropical and temperate temperate waters in almost every major ocean. They are found along the coast of North America from Oregon to Mexico in the Pacific Ocean and from Maine to Florida in the Atlantic Ocean. Common thresher sharks are also commonly found around Asia and occasionally in the central and western Pacific Ocean. Although little is known about the migration of this species, fishing records suggest that they move north, away from the equator, during summer months and that they move south, toward the equator, during winter months. (Cartamil, 2009; Springer, 1943)

Common thresher sharks primarily live in temperate waters beyond the continental shelf and do not stray much more than 30 km from the coast. During the day, they stay near the edge of the continental shelf at an average depth of 110 m. Common thresher sharks have been documented diving to depths of 217 m below sea level, though this is uncommon. At night, members of this species spend most of their time at a mid-range depths, remaining near or on the continental shelf. ("Preliminary investigations into the age and growth of the shortfin mako, Isurus oxyrinchus, white shark, Carcharodon carcharias, and thresher shark, Alopias vulpinus, in the western North Atlantic Ocean", 2002; Cartamil, 2009)

Common thresher sharks weigh 348 kg on average and can reach up to 500 kg. They range from 1.6 to 6 m in length, averaging 2.74 m. Up to 50 % of a thresher's length is due to the characteristic enlarged upper lobe of its caudal fin. Alopias pelagicus is the largest of the thresher species, and, unlike other threshers, they have "erect and narrow cusps" (Springer, 1943) on their teeth. Like other species of threshers, common threshers have relatively small eyes near the front of the head. Common thresher shaks can be identified by their dark green dorsal fin; in other similar species, dorsal fins are blue to purple. ("Preliminary investigations into the age and growth of the shortfin mako, Isurus oxyrinchus, white shark, Carcharodon carcharias, and thresher shark, Alopias vulpinus, in the western North Atlantic Ocean", 2002; Bernal and Sepulveda, 2005; Eitner, 1995; Springer, 1943)

Common thresher sharks are oviparous. Immediately after birth, young are independent and albe to survive on their own. However, because newborn sharks are 69 to 92 cm in length, they are easy targets for larger sharks. Consequently, thresher sharks stay in a nursery area for roughly 3 years until they are large enough to avoid predation. This species grows fairly slowly, taking 9 to 13 years to reach sexual maturity. Males usually mature earlier than females, at around 9 to 10 years age, while females mature at 12 to 13 years. Common thresher sharks are indeterminate growers. ("Age and growth estimates of the bigeye thresher shark, Alopias superciliosus, in Northeastern Taiwan waters", 1998; "Preliminary investigations into the age and growth of the shortfin mako, Isurus oxyrinchus, white shark, Carcharodon carcharias, and thresher shark, Alopias vulpinus, in the western North Atlantic Ocean", 2002; Bodson, 1983; Cartamil, 2009)

In some parts of the world, common thresher sharks breed all year long. The migratory patterns of common thresher sharks near North America suggest they breed in northern waters during the spring and sumer and release their pups into nurseries along the coast as they travel south for the winter months. Common thresher sharks are polygynous (the male impregnates multiple females), but little is known about their mating behavior. ("Preliminary investigations into the age and growth of the shortfin mako, Isurus oxyrinchus, white shark, Carcharodon carcharias, and thresher shark, Alopias vulpinus, in the western North Atlantic Ocean", 2002; Cartamil, 2009)

In some parts of the world, common thresher sharks are thought to breed all year long. The migratory patterns of common thresher sharks near North America suggest they breed in northern waters during the spring and sumer and release their pups into nurseries along the coast as they travel south for the winter months. Females are oviviparous and can only carry two pups at a time. Pups are born independent, but remain in a nursery area for approximately 3 years for safety. Male common thresher sharks reach maturity at 9 to 10 years of age, and females at 12.3 to 13.4 years of age. ("Age and growth estimates of the bigeye thresher shark, Alopias superciliosus, in Northeastern Taiwan waters", 1998; Bodson, 1983; Cartamil, 2009; Fujita, 1981)

There is little to no post-birth parental investment among common thresher sharks. Once born, pups are fully independent and remain in a shallow nursery area for protection. (Cartamil, 2009)

The lifespan of common thresher sharks in the wild is poorly documented, and threshers are not held in captivity. Other species of thresher sharks, such as Alopias superciliosus and Alopias pelagicus, can live 20 to 30 years in the wild. The largest common thresher shark ever recorded was 4.75 m long and 510 kg. Using the growth coefficient of common thresher sharks, this shark was determined to be 43 years old. Thus Alopias pelagicus may have a greater lifespan than other members of its genus. ("Age and growth estimates of the bigeye thresher shark, Alopias superciliosus, in Northeastern Taiwan waters", 1998; Cartamil, 2009)

Common thresher sharks are solitary. Because they are migratory animals, common threshers are considered to be free-range predators. Little is known about their exact migratory path, but evidence suggests they travel south (toward the equator) to warmer waters during the winter and north (away from the equator) to cooler waters during the summer. ("Preliminary investigations into the age and growth of the shortfin mako, Isurus oxyrinchus, white shark, Carcharodon carcharias, and thresher shark, Alopias vulpinus, in the western North Atlantic Ocean", 2002; Cartamil, 2009)

On a daily basis, thresher sharks exhibit vertical migration, moving to shallower waters at night and to deeper waters during the day. However, they do not claim or defend these areas as territories. (Cartamil, 2009)

As solitary animals, very little is known about communication between common thresher sharks. Because this species has poor vision, they often rely on other senses to detect prey. Like most sharks, common threshers perceive their environment in many ways. The lateral line in all sharks detects vibrations in the surrounding waters. This aides in locating prey from great distances, as vibrations travel well in water. Common threshers also have a strong sense of smell, and chemicals can be detected in low concentrations. Upon finding a potential meal, most threshers will bump the object with their nose or take a small test bite to determine if the object is edible before committing to a full strike. Common thresher sharks also use electromagnetic senses to perceive their environment and hunt prey. They use sensory organs clustered in their nose and head to sense impulses in the water from injured and dying fish. ("Feeding habits of the common thresher shark (Alopias vulpinus) sampled from the California-based drift gill net fishery, 1998-1 999", 2001; Cartamil, 2009)

Like most sharks, common thresher sharks are carnivores and feed mainly on small fish that travel in schools. Thresher sharks use their enlarged caudal fin as a means to herd schools of fish into tightly packed balls to maximize strike success. Common threshers eat a variety of fish, including sardines, and different species of anchovies, mackerel, hake, squid and red crab from deep waters. In warmer waters, members of this species feed primarily on anchovies, but in cooler waters they feed mostly on squid and sardines. ("Diet differences in the thresher shark (Alopias vulpinus) during transition from a warm-water regime to a cool-water regime off California-Oregon, 1998–2000", 2004; "Feeding habits of the common thresher shark (Alopias vulpinus) sampled from the California-based drift gill net fishery, 1998-1 999", 2001; Cartamil, 2009)

There are few known predators of common thresher sharks. Other sharks, like makos, reef sharks and even members of the same species, eat juvenile common threshers. Pups usually keep to shallow nursery areas that are separate from adults as a defense from predation. (Cartamil, 2009)

Common thresher sharks are often used as bio-indicators of pollutants due to their diet and near-shore habitat. Common threshers act as host for many parasitic copepods such as Nemesis robusta and Bariaka alopiae. Nemesis robusta usually infects the gills of common thresher sharks causing gill erosion and inflammation, resulting in reduced gas exchange. Another common copepod parasite of this speices is Gangliopus pyriformis, although exactly how this copepod affects common threshers is unknown. Like most sharks and rays, common threshers have a mutualistic relationship with pilot fish. Pilot fish eat copepods and other parasites from the shark while the shark provides for the pilot fish. (Baughman and Springer, 1950; Benz and Adamson, 1990; Borucinska, et al., 2009; Izawa, 2010)

Common thresher sharks are an integral part of global commercial fishing. Although they are not target speecies for fishing in the Americas, they are often incidentally caught in commercial gill nets. In other countries, including China, they are the third most targeted catch of fisheries, valued less only to the game fish Xiphias gladius and sailfish Istiophorus albicans. Common thresher sharks make up a large part of Chinese fish markets. In many areas, the demand for thresher shark meat has led to overfishing and a major decrease in population size. Also, the livers of common thresher sharks contain a small amount of oil that is considered valuable and sold in high dollar amounts. (Baughman and Springer, 1950; Baum, et al., 2003)

Because common thresher sharks stay beyond areas where humans swim, they do not pose any physical threat to humans. They do, however, cause damage to commercial fisheries by destroying nets and other equipment when caught in drift nets. (Cartamil, 2009)

Because common thresher sharks are sold in many foreign fish markets, the demand for their meat is high. In many areas of the Atlantic ocean, populations of common thresher sharks have been reduced up to 67 % in the last ten years. However, stricter catch-and-release policies in the Pacific have led to more stable populations. The IUCN lists Alopias pelagicus as threatened. Many conservation attempts have developed "no-fishing" marine reserves to reduce the number of common thresher sharks that are caught in nets. However, forcing commercial fisheries to move fishing zone to establish marine reserves has consequently put other species in danger. (Baum, et al., 2003; "Alopias vulpinus, Thresher Shark", 2010)