The pelagic thresher (alopias pelagicus) is one of three types of thresher sharks. Their most notable feature is their incredibly long caudal (tail) fin, which measures approximately half of their total body length. On average, when fully grown, these sharks are about 10 feet in length and weigh about 153 pounds. Pelagic thresher sharks are often confused with the common thresher (alopias vulpinus). The common thresher does not have dark skin patches above the base of the pectoral fin while the pelagic thresher does. Pelagic thresher sharks have smaller (yet still very large) eyes than the common thresher, and they lack deep horizontal grooves along the anterior dorsal fin that are present in the common thresher. Their teeth are small, curved, and razor-sharp. Lighter in color than other thresher species, the pelagic thresher has a blue-gray dorsal fin, lighter blue-gray sides, a white ventral (front/belly) surface, and in some cases a silvery gill and flank region.
Habitat & Location
Very little is known about where pelagic threshers reside due to the fact that they are commonly confused with the common thresher. It is believed they inhabit mainly the surface waters of the open ocean. They are found in warm and temperate waters in the Pacific and Indian oceans and are abundant off the northeastern shore of Taiwan and can be found off the North American coasts of California and Mexico.
Pelagic threshers are no threat to humans. In fact, they tend to steer clear of swimmers and divers and there have been no reportings of a pelagic thresher attack. Fish, primarily herrings, flyingfishes, and mackerels, as well as squid, are their main source of food. They herd schools of fish or squid closer to each other and swing their tail and stun their target(s). Stunning their prey makes it easier for the shark to capture and consume.
Reproduction & Growth
Thresher sharks are ovoviviparous (“eggs are retained within the body of the female in a brood chamber where the embryo develops, receiving nourishment from a yolk sac. This is the method of reproduction for the ‘live-bearing’ fishes where pups hatch from egg capsules inside the mother’s uterus and are born soon afterward” aka, aplacental viviparity). Typically these sharks live about 16 years and reach sexual maturity at around 7 to 8 years for males (or 8 feet) and 8 to 9 years (or 9 feet) for females. Pups are very large at birth, ranging between 5 and 6 feet; this gives them less of a chance of becoming prey to other sharks. Typically only one to two pups are born per litter giving the species a very low annual growth rate of between 2 and 4%.
Like all shark species, the pelagic thresher is susceptible to getting caught in longlines and large fishing nets (bycatch). But not every human-caused fatality to these sharks is an accident. Pelagic threshers are very sought after for their meat, liver, skin, and fins. Their skin is often used to make leather and their liver is used for oil to make squalene which is used for cosmetics, health foods, and sometimes high-grade machine oil. In the Atlantic ocean, thresher shark abundance has decreased by as much as 67%. Their fins(used for shark fin soup) are highly abundant in the Hong Kong fin market, however, pelagic thresher catches are widely under-reported making it difficult to make conservation efforts in their favor.