Bigeye Thresher Shark
Named after its, you guessed it, big eyes, the Bigeye Thresher shark (Alopias superciliosus) is one of three subspecies of Thresher shark. The other two subspecies are the Common Thresher shark (Alopias vulpinus) and the Pelagic Thresher shark (Alopias pelagicus). Along with its large eyes, the Bigeye Thresher also has the characteristic long, whip-like tail of the Thresher species. All three Thresher Sharks are part of the Mackerel shark family, which includes the Great White and Mako sharks. Mackerel sharks are known as some of the greatest hunters in the ocean and also included the extinct Megalodon and Ginsu sharks.
Bigeye Thresher shark anatomy and appearance
The most obvious feature of the Bigeye, along with their long caudal fin, which can be as long as the sharks body, is their large eyes which are placed in keyhole-shaped sockets that allow them to be rotated upward. These can be up to 10cm long and are longer than they are wide. Their large eyes give them excellent vision, even in low light conditions. This species can also be distinguished by a pair of deep grooves on the top of its head, which make it look helmet shaped. This is where its scientific name is derived. Their snout is long and bulbous, dermal denticles (small scales) are all over their bodies, and they have moderately large, razor-sharp teeth set in their small jaws. There are 19–24 rows of teeth in the upper jaw and 20–24 tooth rows in the lower one. Its coloration is a deep, metallic violet to purplish brown above and creamy white below.
Bigeye Thresher shark habitat
Bigeye Threshers are found in tropical and temperate waters around the world, although they do not tend to venture as far north as the Common Thresher and are therefore not found in the UK. Their range encompasses the Galapagos Islands, the Caribbean, the Mediterranean Sea, the Pacific Ocean, South Africa, the Indian Ocean, and the Atlantic Ocean. They live in deeper waters during the day at depths of up to 955m, coming up to shallower waters at night to hunt, a process called diel vertical migration. All three Thresher species undergo this diel vertical migration; however, it is not common in other sharks.
Bigeye Thresher shark diet
As with the Common Thresher and Pelagic Thresher, the Bigeye uses its whip-like tail to hunt prey. It swims towards the shoal of fish, slamming to a stop, while whipping its tail over its body and stunning its prey, the shark then effortlessly collects its reward. It can easily eat up to seven fish in each whip of its tail, making it a highly effective predator. The Bigeye’s diet mainly comprises fish such as tuna, hake, herring, lancetfish, mackerel, billfish and whiting, as well as crabs, squid, and octopus. Unconfirmed reports, however, suggest that the Bigeye may use their tails to stun birds out of the air.
Video: Pelagic Thresher shark hunting using the tail to stun fish
Bigeye Thresher shark population
Bigeye Threshers typically give birth to 2 pups in each litter, but this can vary from 1-4 pups. Males become sexually mature between the ages of 9-10 and can live up to 19 years, while females become sexually mature between 12-14 and have a lifespan of up to 20 years.
There are no estimates of global population size of the Bigeye Thresher. The Bigeye Thresher is estimated to be declining in the Atlantic and Indian Oceans and increasing around Hawaii. To estimate a global population trend, the estimated three generation population (over 55 years) trends for each region were weighted according to the relative size of each region. The overall estimated median population reduction was 36.5%, however the Hawaii region may not correctly represent the Pacific where the trends are uncertain.
This species is especially vulnerable to fisheries exploitation both by target and bycatch, as its epipelagic habitat occurs within the range of many largely unregulated gillnet (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, in which it is readily caught. It is fished throughout its habitat range.
Bigeye Thresher shark Conservation
As the Bigeye Thresher has a low reproductive rate and is at risk from fishing activities, IUCN Red List of Threatened Species has most recently assessed this species as vulnerable in 2018.
The success of conservation actions agreed through international wildlife and fisheries treaties depends on implementation at a local level, and for sharks, this has been seriously lacking. Some conservation efforts involving Bigeye Threshers have been implemented, such as the International Commission for the Conservation of Atlantic Tunas (ICCAT) adopting the requirement for the prompt release and minimal harm to any Bigeyes retrieved alive, in 2008. In 2009, ICCAT banned retention, transshipment, landing, storage, and sale of Bigeye Threshers, except in Mexico. In 2009, the Indian Ocean Tuna Commission (IOTC) adopted similar bans for all three Thresher species.
All Thresher species were listed on Appendix II of the Convention on Migratory Species (CMS), which commits to work regionally towards conservation. The CMS also covers the species on the Memorandum of Understanding for Migratory Sharks, aimed at enabling conservation.
In 2016, all three Thresher sharks were added to Appendix II of the Convention on International Trade in Endangered Species (CITES), which requires exports from CITES Parties to be accompanied by permits based on findings that parts are sourced from legal and sustainable fisheries.
Bigeye Thresher shark and humans
This shark infrequently interacts with humans and is not considered a threat with its small mouth. It is often caught as bycatch worldwide by both longline fisheries and trawls. Its liver is processed for its oil, the fins for shark fin soup, and the skin is used in leather products. Sport fishers also target it, and it can often die during this process, even if the intention is to catch and release. This species if often caught by the tail when it becomes trapped on the bait hook and is retrieved in reverse. Thresher sharks need to swim forward to pass oxygenated water over their gills.
Other interesting facts
Thresher sharks are named after their exceptionally long thresher-shaped tail. A ‘thresher’ is more commonly known as a scythe, which historically was used as an agricultural hand tool for harvesting crops. It is sometimes depicted being held by the grim reaper in illustrations. This is very fitting as their tails mark impending doom for their prey.
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