Pelagic Thresher sharks (Alopias pelagicus) are a type of mackerel shark, like Great White sharks, Mako sharks and Porbeagle sharks. Unlike great whites, which are known for their sharp triangular teeth, the Pelagic Thresher shark is known for its long whip-like tail, which can instantly immobilise multiple prey. There are three species of Thresher sharks with the other two being the Common Thresher shark (Alopias vulpinus) and the Bigeye Thresher shark (Alopias superciliosus).
Pelagic Thresher shark anatomy and appearance
Pelagic Thresher sharks are rather cartoonish looking, with torpedo-shaped bodies, big eyes, long tail and a small, downturned mouth. They are smaller and lighter in color than the other two Thresher species. Pelagic Threshers have a blue-grey back, light blue-grey sides, and a white underbelly. Their flexible tails can be longer than their bodies as adults. To offer them more flexibility the pelagic thresher has a cartilaginous skeleton as with all other sharks and rays.
The shark has two dorsal fins on its back: a triangular upper fin near the base of its caudal fin that helps with steering while swimming and a crescent-shaped lower fin around its midsection that helps in stabilizing it when turning quickly or moving suddenly, as when it uses its tail to stun prey. Four pectoral fins on each side near its head also assist in moving more powerfully and changing direction quickly.
Pelagic Threshers grow between 3-4 meters in length depending on the sex, weigh 150-160 kg and have an average lifespan of 29 years. As a slow-growing shark, males take around 8-9 years to reach maturity, and females around 13-14 years, with females having a longer lifespan.
Pelagic Thresher shark habitat
They are only found in the Indian and Pacific oceans, living in the upper zone of the ocean (0-300m) and have been observed around coral reefs and seamounts, although they can reach depths of up to 750m. Pelagic thresher sharks are primarily active and feed during the night, seeking cover in deeper waters during the day. This is likely due to their sensitivity to ultraviolet light, which can damage their skin.
Scientists believe that they are highly migratory, but there are insufficient tagging studies to confirm this theory. Little is known about the ecology of pelagic thresher sharks, as they are difficult to study in their natural habitat (open ocean).
Pelagic Thresher sharks attract divers from around the world to the island of Malapascua in the Philippines. Thresher sharks congregate around a shallow shoal near the island. This is the location for Shark Guardian Thresher shark expeditions for 2024
Pelagic Thresher shark diet
The pelagic thresher shark’s secret weapon is its distinctive tail when hunting. It has a variety of prey including sardines, mackerel, herring, bluefish, shrimp, and squid. Pelagic threshers have been observed eating tuna in the wild, indicating they may target larger prey when they have the opportunity. The majority of shark species hunt by separating an individual fish from the safety of the group and chasing it. In a bid to protect themselves from this style of attack, fish often pack together in a spherical formation known as a bait ball. However, the thresher shark has found a way to use this bait ball to its advantage.
Video: Thresher Shark hunting by slapping tail at fish
Pelagic threshers swim fast toward the tight shoal of fish then use its pectoral fins like an emergency brake. As it slams to a stop, the shark whips its tail over its head toward the bait ball, sometimes following this with a sideways slap of its tail. They can reach speeds of up to 45 miles per hour, meaning the force of the tail-slap sends a shockwave through the water so powerful that it forms bubbles. The shockwave stuns the prey and allows the shark to easily collect its reward. Using this method of hunting means the shark can eat between two and seven sardines after each whip from its tail. This is more energy efficient and has a higher chance of success than chasing prey one by one, and because of this success rate, they only need to consume food about once every 10 days. This hunting method was not confirmed until 2010, when the first footage was captured of it using its whip-like tail to hunt.
Pelagic Thresher shark population
Researchers know these mysterious sharks have two pups in each litter, after a gestation period of around 9 months. In the womb, pups are not connected to the mother via a placenta. Instead, pups are cannibalistic and feed on her unfertilized eggs, a process known as oophagy. The mother then gives birth to uncommonly large live young, which can vary in size from around 0.9-1.5m— almost half as long as their mother. Researchers theorize that their unusual size could be to minimise their chances of being preyed upon by other sharks. Females give birth to around 40 pups during their lifetime.
The Pelagic Thresher is estimated to be declining in both the Pacific and Indian Oceans. To estimate a global population trend, three generation population trends for each region were estimated according to the relative size of each region. The overall estimated reduction was 50–79% reduction over three generation lengths (55.5 years), based on abundance data and levels of exploitation. However, trends are uncertain with data being estimates.
Pelagic Thresher shark Conservation
Pelagic Thresher has most recently been assessed as endangered for The IUCN Red List of Threatened Species in 2018.
Despite being harmless to humans, this species is threatened due to high demand from global and local fisheries for their meat. Their livers are also used for vitamins and cosmetic products, their skins in leather production, and their fins prized for shark fin soup. The liver is especially sought after because it can comprise up to 10% of the shark's weight. Sport fishers are also a threat to pelagic thresher sharks; they often catch this species by the tail when it becomes trapped on the bait hook and is retrieved in reverse. Because pelagic thresher sharks need to swim forward to pass oxygenated water over their gills, they often die after being captured, even if they are released quickly. Thresher sharks are also often caught as by-catch because their habitats are heavily fished.
Because of their low reproduction rate, it is difficult for thresher shark populations to recover from exploitation and threats. However, some conservation efforts are underway. Markedly, in 2017, all three thresher shark species became protected under the Convention on International Trade in Endangered Species (CITES), the international treaty which aims to protect threatened animals from the wildlife trade.
Pelagic Thresher shark and humans
As pelagic thresher sharks mainly live offshore, there are very few places they can potentially interact with humans, making them hard to study. The most well-known location for scuba divers to encounter the pelagic thresher is Monad Shoal off the shores of Malapascua, an island in the Philippines. There are no known attacks by thresher sharks on humans. They have small teeth and jaws for its size and they tend to flee from divers.
Other interesting facts
Threshers are one of the few shark species with the ability to fully breach the water, an ability shared with dolphins and whales. Some researchers believe it is an attempt to remove parasites from their body.