Sharks

Bigeye thresher

Alopias superciliosus

An open-ocean shark that rises from the deep at night. Few people ever see one, and no sighting is a promise.

VulnerableIn 13 Atlas countries

Vulnerable (IUCN) means it is at high risk of extinction in the wild, mainly because of fishing and slow breeding.

Length

Most are about 3.3 to 4.0 m (10.8 to 13.1 ft). The largest reported is about 4.9 m (16 ft). The long upper tail lobe is nearly as long as the body

Weight

Typical weight about 160 kg (353 lb). The largest reported is about 364 kg (802 lb)

Next to a person

About two adults head to toe in length, and as heavy as about two adults

What it eats

Schooling fish such as mackerel and herring, bottom fish such as hake, larger open-water fish such as lancetfish and small billfish, and squid. It uses its long tail to stun prey.

Around people

Rare, give space

How it lives

The bigeye thresher lives in the open ocean in tropical and temperate seas, from the surface to about 730 m. It has very large eyes, up to about 10 cm across. Females give birth to live young, usually 2 per litter and up to 4. Unborn pups feed on unfertilised eggs made by the mother. Pups are 64 to 106 cm (25 to 42 in) at birth. Females are reported to mature at about 12 to 14 years, and the oldest age reported is 20 years. Because of this slow breeding, a female may have only about 20 young in her whole life.

Where it goes

The bigeye thresher makes a daily vertical migration. It spends the day in deeper water between 300 and 500 m, beneath the thermocline where the water is about 6 to 12 degrees C, and moves up to less than 100 m at night. I did not find a full account of its long-distance movements.

We only draw a journey on the map when scientists have tracked it or clearly documented it. For this animal we have not found one yet.

Populations

Worldwide: Vulnerable (IUCN; FishBase dates the assessment to November 2018), and the population is described as declining. It is caught by commercial fisheries across its range, and it breeds slowly. It is listed on CITES Appendix II and on CMS Appendix II. I found no global population count, and I did not read any regional assessments.

Meet it respectfully

The bigeye thresher is an open-ocean species and is rarely seen by visitors. If you see one, keep your distance and let it go its own way. Never chase, touch, feed or bait it. Rules differ by place, so follow local rules and your guide.

Fun fact

Tap the bulb to light it up.

Sources for this page

Facts checked 5 October 2026. Photos: Unsplash licence, or Creative Commons as shown on each photo.

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