Common Thresher Shark
The Common Thresher shark (Alopias vulpinus) is a subspecies of mackerel shark, which is amongst the fastest shark family in the ocean, including species such as the Great White shark, Mako shark and Porbeagle shark. Common Threshers are a large shark with a torpedo shaped body, but they are easily told apart from other shark species by their long whip-like tail. There are three species of Thresher sharks with the other two being the Pelagic Thresher shark (Alopias pelagicus) and the Bigeye Thresher shark (Alopias superciliosus).
Common Thresher shark anatomy and appearance
Common Thresher sharks are the largest of the Thresher shark subspecies and can grow up to 6 meters in length and weigh up to 350kg. They also have the longest lifespan of the three, living up to 50 years.
This subspecies is metallic purplish brown to grey above, becoming more bluish on the sides, with a white underside. Thresher sharks have a distinct skin texture with small, overlapping scales, also known as dermal denticles, that are rough to the touch. Unlike most sharks, which have large mouths they use to hunt, Thresher sharks have a small mouth set slightly below and backwards from the eyes. They have 24 to 29 rows of small, razor-sharp curved teeth. They regenerate two sets of new teeth every 10 days during their younger years. These sharks have no need of a large mouth due to their secret hunting weapon, their long whip-like tail, which can be as long as the body of fully grown adults. They have long, thin, slightly curved pectoral fins on either side of the body, which are used to help with direction changes.
Common Thresher shark habitat
Common Threshers are highly migratory, living in tropical and temperate seas around the world, including the United Kingdom. They usually live in deep water, up to 650m depth, although they usually stay in close proximity to land, and are also nocturnal, due to their sensitivity to UV light. They have evolved to be endothermic due to spending most of their time in deep waters in the open sea. This means they can make their body temperatures warmer than the surrounding waters by body shivering and other internal mechanisms.
Common Thresher shark diet
During the day Common Threshers are found in deeper waters, where they can feed on squid and octopus. They only tend to come up to coastal waters during the day where they hunt fish, primarily mackerel, herring, and sardines. Threshers charge shoals of fish, then abruptly slam to as stop and whip their tails over their body into the shoal. This stuns or injures the fish meaning the Threshers can take an easy meal of around seven fish in one go. Due to this successful hunting method, Threshers only need to eat around once every ten days.
Common Thresher shark population
Common Thresher sharks mature slowly, with females reaching maturity between 7-13 years and males at 8-14 years. Each female shark has two uteruses, and they tend to give birth to 2-4 shark pups each litter (with the time they are able to produce offspring reaching from 8- 14 years), ranging in size from 0.9 to 1.5 meters. There is no placenta to nourish the pups, instead they take part in a process called Oophagy and eat any unfertilized eggs, with the dominant pup in each embryo eating the smaller, weaker pups, which limits birthing numbers.
Common Thresher shark Conservation
The Common Thresher has most recently been assessed as vulnerable for The IUCN Red List of Threatened Species in 2018.
The Common Thresher is highly susceptible to both targeted and by-catch fishing exploitation because their habitat occurs within largely unregulated and under reported gill net (vertical panels of netting that hang from a line with regularly spaced floaters holding the line on the surface of the water) and longline (a commercial fishing angling technique that uses a long main line with baited hooks attached at intervals) fishing. Its low pup production rate, slow rate of maturing and limited time they can produce offspring, coupled with its high value in fishing, particularly for its meat, fins, and liver, make it vulnerable to rapid population depletion.
In the past, there have been serious declines in this shark species due to overfishing, for example, there was a 63-80% decline in northwest and western central Atlantic during 1986-2000. Subpopulations of the Common Thresher in parts of the world where data is less readily available are more seriously at risk of population depletion, as populations are not enabled to rebuild due to exploitation by unmanaged fisheries and its biological vulnerability.
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.
This adult common thresher shark was caught, tagged, and released alive during the 2009 Juvenile Shark Survey off of the Channel Islands. The hose in the sharks mouth pumps seawater across the gills so that the shark can breathe during the tagging operation.
Common Thresher shark and humans
Due to their nocturnal nature and propensity for deeper seas, human encounters with Common Thresher do not happen very often. The first footage of a thresher shark giving birth was captured in 2013.
Monad Shoal is a seamount located of the coast of Malapascua Island in the Philippines. It rises 200m from the seabed and forms a plateau 16-25m from the surface of the sea. In the early morning, this seamount is visited by mainly Pelagic Thresher sharks, who use it as a cleaning station. At present, there are five documented cleaning stations at the seamount. Monad Shoal is the only seamount in the world known to be visited almost every day by Thresher sharks. This location is one of the best places in the world for divers to see a Thresher Shark. Some sightings and video footage have been recorded of Common Thresher sharks breaching out of the water off the coast of Cardigan Bay in Wales, UK.
Other interesting facts
The Latin name for the thresher shark family (Alopiidae) comes from the Greek word Alopex, meaning fox. Some European countries refer to the Thresher shark as the Fox shark, possibly owing to their large eyes and long tails.