Bigeye Thresher Shark

## About

An adult bigeye thresher shark nearing a vessel.

The bigeye thresher shark ***(Alopias superciliosus)*** is a relatively small species of shark commonly known for their distinctly large tails used as their signature weapons during hunting their prey. This specie of thresher shark enjoys occupying warm and temperate water withing the oceans they occupy. The bigeye thresher poses a metabolic brownish, to purplish grey and a creamy white underbelly. This specie is one of three species of thresher shark and is specifically distinguishable for its larger eyes in comparison to the other thresher species. According to Marine.bio bigeye thresher sharks can grow to 3.5-4.88 m (11.4-16 ft) long and weigh in at 363.8 kg (802 lb), making them the largest species of thresher shark. The body is covered in dermal denticles that are assorted in a similar fashion to scales. These sharks are known as lamniform or mackerel sharks, meaning that they can raise their body temperature above that of the surrounding climate using a special type of circulatory system known through as rete mirabile. This is likely most evident around their head where most censor receptors are, working when hunting their prey in colder waters. With a life expectancy of 20 years, males mature very late, reaching sexual maturity at around age 10 and females around age 12. Mating is not seasonal and bigeye threshers posses distant partners that will produce 2 to 4 pups each year, which measure around 1-1.4 m (3.2-4.5ft) at birth.

## Diet

Like most other shark species and specifically thresher species, the bigeye thresher often indulges in a variety of fish, ranging from species of tuna, hake, herring, lancetfish, mackerel, small billfish, and larger fish like bullet tuna to marine life like crabs and squids. Hunting often operates like other sharks as they identify prey using their tuned senses. Though what makes bigeye -and all thresher species unique-, is their unusual style of killing and eating their prey. These sharks are widely renowned for their long tails in which they use to stun their prey by striking or whipping them in critical areas, before consuming their kill. Sources like sharksider have also reported bigeye threshers propelling themselves out of the water and striking birds as prey with their tails.

## Conservation status

The bigeye thresher shark is listed as Vulnerable according to IUCN, with a currently decreasing population. The bigeye thresher is targeted globally for by-catch and commercially small-scale pelagic longline, purse seine, and gillnet fisheries. Most by-catch is attained in high seas waters around the globe though, they have also been seen to been captured by coastal longlines, gillnets, trammel nets, and sometimes trawls. Because the bigeye thresher operates very close to the surface, (typically at night) they are often revealed by fisheries, which can lead to unwanted attention that may lead to their capture. Though during the day, bigeye threshers dive to much deeper depths where many commercial tuna fishers do not operate. This shark in particular is also hunted because of the quality meat thought to be located in their fins. The species is also one of the most common shark species fished in big-game recreational fishers and even though they will often use the catch and release method, if a bigeye is hooked anywhere on its body, it can lead to a lowered chance of survival, 78% chance of survival for tail-hooked sharks and 0% for mouth-hooked sharks. Bigeye threshers that particularly occupy the Atlantic ocean have been put under heavier protection from local fisheries according to federal government laws.

## Range and Population

The bigeye thresher's current population trend is seen as decreasing though, the global population is currently unknown because of their extensive range. Fishing across the Atlantic ocean has been a growing issue and heavily impacted the bigeye thresher's Atlantic population for a solid two decades prior to law enforcement placed in 1992. The decades prior to this saw the bigeye thresher population begin to decrease by 3.1%, consistently with an estimated median reduction of 83.1% over three generation lengths. This was very detrimental the the shark's population in the Atlantic region. These numbers have been thought to have established in the next few years following the new enforcement. In the southern Atlantic, long-line fleet data shows that; there was a decreasing population trend occurring with bigeye threshers that was prominent from 1971 to 2001. However, this data was not considered to represent a huge threat due to the overall low catch rates. Also, a fishing risk assessment on the bigeye threshers showed that the fishing mortality rates of the sharks had gone down significantly, as since 2000, the species has a generally low presence in Pacific pelagic long-line fisheries. A data analysis on Hawaii long-line fisheries also observed that the average mortality rates of the thresher sharks was going up immensely, revealing that annual rates of increase of 0.4%, consistent with a median increase of 24.0%. In the Indian ocean, due to hook mortality rates, the bigeye threshers experience an annual rate of reduction of 4.4% with 0.35% of sharks caught per 1,000 hooks. Though due to many various threats, over the past 50 years the bigeye thresher shark's population has decreased by around 30-49%, causing shorter generation lengths. Throughout the world, Atlantic and Indian populations are seen as decreasing whilst populations around Hawaii are observed as increasing.

## Behaviour

As said before, the bigeye thresher has a very unique hunting technique compared to other sharks, as instead of using its teeth and jaws as its main hunting mechanism, the bigeye thresher uses its very large tail to stun or strike and potentially paralyze its prey before consuming it. It is stated by sharksider that thresher sharks live by a diel vertical migration, meaning that they predominately lurk at 300-500 m (980–1,640 ft) during the day and surface during the night to hunt for their potential prey. Bigeye threshers are ovoviviparous meaning that they will give birth to two pups though, these numbers may vary between one to four. Males begin to become sexually mature around the age of 9-10 whereas females range from 12-14 years. The average lifespan of a bigeye thresher will be around 19 years, however females will reach approximately 20 years of age. Their name '*bigeye thresher'* comes from the fact that they have very large eyes that posses acute vision, which allows them to dive to extensive depths where they still manage to visually adapt.

### Relationship with Humans

As said earlier bigeye thresher sharks have had extensive issues with over-fishing in the past. Though aside from the fishing factor that affects their mortality rate, human-wildlife conflict is not considered common and sharks are often noticeably docile nature. Especially because of their low reproductive rate, the shark's population is at very high risk of longline fisheries and trawls. Even this shark's meat is traded and cooked up as shark-fin soup or processed for oil in some countries.

## In Captivity

According to ZOOINSTITUTES there are no current records of bigeye threshers found in captivity.

## Sources

* Bigeye thresher shark - ZOOINSTITUTES | https://zooinstitutes.com/?#
* Bigeye thresher shark - sharksider | https://www.sharksider.com/bigeye-thresher-shark/
* Bigeye thresher shark - IUCN | https://www.iucnredlist.org/species/161696/894216
* Bigeye thresher shark - Marine.bio | https://www.marinebio.org/species/bigeye-thresher-sharks/alopias-superciliosus/