Bigeye Thresher Shark

About the Bigeye Thresher Shark

The bigeye thresher is a large shark of the Alopiidae family. Its most distinctive characteristics are its extremely large eyes, the grooves on its head, and its long tail whose upper lobe can reach about half its total length.

Biology and Behaviour

The average adult length of the bigeye thresher is between 3.35 and 4 m (10.9–13.1 ft), and the maximum reported length is 4.84 m (15.9 ft). Its eyes are adapted to see in very low light. The bigeye thresher spends the day in deep waters and moves up toward the surface at night to feed. It has been recorded from the surface down to about 955 m (3,133 ft). Its main prey are squid and fish, and it also feeds on crustaceans. It stuns its prey by using its tail as a whip. It is known to swim in circles around schools of fish to make them more compact before striking them to maximize its success ratio.

Reproduction and Lifespan

Bigeye Threshers are ovoviviparous, meaning embryos develop in eggs that hatch inside the mother’s body. They give birth to litters of 2 to 4 pups, usually two, one for each uterus. The pups are born very large, usually measuring about 1.3 to 1.4 m (4.3–4.6 ft). The gestation period lasts about 12 months, and the reproductive cycle is likely annual.

Males become sexually mature at around 9–10 years and females at around 12–14 years. Their life expectancy is about 19 years for males and 20 years for females.

Conservation

Because of their slow growth rates and relatively low fecundity, it is extremely difficult to restore populations if they are overfished. This species is often caught in gillnet and longline fisheries and its numbers are rapidly decreasing. The global population is estimated to have declined by 30–49% over the last three generations. The IUCN considers them a vulnerable species. Regional trends differ, with declines reported in the Atlantic and Indian Oceans.

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