Basking Shark (Cetorhinus maximus): Facts, Size and Diet

What does a basking shark look like?

A basking shark has the classic streamlined shark shape, but with a few unmistakable features. Its snout is pointed and conical, and noticeably hooked in young sharks. The mouth is enormous, up to about 1 m (3.3 ft) wide, and white inside. Five pairs of very long gill slits almost circle the head, and when the mouth is open they gape wide.

The body is usually gray-brown to almost black or bluish on top, fading to a dull white below, often with paler mottling and scars. The tail is crescent-shaped, with a strong keel on each side of its base, and the tall, triangular dorsal fin may flop to one side in large sharks when it sticks out of the water.

Inside the gill slits are rows of dark, bristle-like gill rakers that trap food. The teeth are tiny, only about 5–6 mm (0.2 in) long and hooked, with often about 100 in each row. Like all sharks, the basking shark has a skeleton of cartilage and skin covered in small tooth-like scales.

It is the only living member of its family, Cetorhinidae, and belongs to the mackerel sharks (Lamniformes), the same order as the great white shark, makos and threshers. Its brain is the smallest, relative to body weight, of any shark, which fits its slow, passive lifestyle.

How big do basking sharks get?

Basking sharks are the largest fish in cool waters and second in size only to the whale shark. They regularly reach 7–8.5 m (23–28 ft), and some grow to 9–11 m (30–36 ft). An average adult of about 7.9 m (26 ft) weighs around 4.6 tonnes (5 short tons).

The largest one on record was trapped in a herring net in the Bay of Fundy, Canada, in 1851. It measured 12.27 m (40.3 ft) and was estimated to weigh about 16 tonnes. Older reports of sharks 12–14 m (39–46 ft) long were visual estimates without good evidence, and modern growth studies suggest that sharks over about 10 m (33 ft) are now very rare.

The liver is huge, making up as much as a quarter of the shark’s body weight. It is rich in oil, which helps the shark stay buoyant and stores energy. How it compares with other giants is shown in the ranking of the largest fish in the world.

How does a basking shark feed?

The basking shark is a ram filter feeder. It swims slowly forward, usually at about 3–4 km/h (2–2.5 mph), with its mouth wide open, so that water flows in through the mouth and out through the gill slits. The gill rakers, covered in mucus, catch the tiny animals in the water. Calculations suggest a medium-sized shark can filter hundreds of tonnes of seawater an hour.

Its main food is zooplankton, especially small crustaceans called copepods, such as Calanus species, along with fish eggs, larvae and other drifting animals. Basking sharks are choosy about where they feed. Studies off southwest England found that they gather in patches where zooplankton is much denser than in the surrounding water, often along fronts where different bodies of water meet.

Unlike the whale shark, which can suck or pump water into its mouth, the basking shark relies only on swimming to push water through its gills. Its gill rakers wear out and are replaced gradually all year, not shed all at once.

Habitat and range

Basking sharks live in cool and warm-temperate seas around the world, in both the Northern and Southern hemispheres. They prefer water of about 8–14.5 °C (46–58 °F) and are usually found over the continental shelf, often close to shore, in bays, around headlands and near islands. They range from the surface to depths of more than 1,000 m (3,300 ft).

In the northeast Atlantic they occur from Norway and Iceland south to the Mediterranean, with well-known summer hotspots off Cornwall, the Isle of Man, western Scotland and Ireland. In the northwest Atlantic they are seen from Newfoundland to New England and beyond, and in the Pacific from Alaska to Mexico, off Japan and China, and around southern Australia, New Zealand, South Africa and southern South America.

Migrations and winter

Basking sharks are seen at the surface mostly in spring and summer, so people once thought they spent the winter resting on the seabed after shedding their gill rakers. Satellite and archival tags have disproved this. A 2003 study of sharks tagged off Britain found that they stay active through winter, moving into deeper water where they probably feed on plankton that has sunk to depths of hundreds of meters.

Tags have also revealed long journeys. A female tagged off the Isle of Man in 2007 swam about 9,600 km (6,000 mi) to Newfoundland in 82 days, diving to 1,264 m (4,147 ft) on the way. Sharks tagged off Cape Cod, Massachusetts, swam south in winter, some crossing the equator at depths of 200–1,000 m (660–3,300 ft) to reach Brazil. Sharks tagged off western Scotland and the Isle of Man went as far as the Bay of Biscay, the Canary Islands and North Africa, and some returned to the same areas the next summer. The UK season is covered in When do basking sharks visit the UK?

Social life and breaching

Basking sharks are often seen alone, but in summer they gather where plankton is rich, sometimes in groups of dozens, and very occasionally hundreds have been counted in one area. Groups often sort themselves by size or sex.

In the Hebrides and the Bay of Fundy, small groups have been seen swimming nose to tail in slow circles, behavior that is thought to be part of courtship. Basking sharks also breach, launching their whole bodies clear of the water and crashing back down. Breaching seems most common in large groups and may be a social or courtship signal; another idea is that it helps remove parasites.

Reproduction and life cycle

Much of the basking shark’s breeding life is still a mystery. Like its relatives, it gives birth to live young that hatch from eggs inside the mother. The developing pups are thought to feed on extra unfertilized eggs produced by the mother, as other mackerel sharks do. Mating probably takes place in early summer.

Only one pregnant female has ever been reported, caught off Norway in 1943, and she carried six pups. Pups are born at about 1.5–2 m (5–6.5 ft) long, larger than most adult sharks of other species. Pregnancy is thought to last from one to three years, and females may rest for two years or more between litters.

Basking sharks grow slowly. Estimates of the age at which they mature range widely, from about 6 to 20 years, with females maturing later than males. They are thought to live about 50 years, although no one knows for sure.

Predators and threats

Adults have few natural predators. Orcas have been seen attacking and eating basking sharks off California and New Zealand, and great white sharks feed on their carcasses. Sea lampreys often cling to their skin and leave scars, but are unlikely to cut through it.

People have been the main threat. For centuries basking sharks were hunted for their liver oil, which was used for lamp fuel and industry, and later for fins, meat and fishmeal. Fisheries off Norway, Scotland, Ireland’s Achill Island, and Canada’s Pacific coast caught thousands. Some populations collapsed and have never recovered. Between 1945 and 1970, Canada even ran a program to kill basking sharks on its Pacific coast because they tangled salmon nets, and sightings there are now very rare.

Today the main dangers are accidental capture in fishing nets, entanglement in ropes and lines, and collisions with boats, especially where sharks feed at the surface near busy harbors.

Conservation status

The IUCN Red List has listed the basking shark as Endangered worldwide since a 2019 assessment, with a decreasing population, after earlier listing it as Vulnerable. No one knows how many exist. Their slow growth, late maturity and very few young mean that populations take decades to recover from losses.

The basking shark is now one of the most protected sharks. It has been fully protected in British waters since 1998 and in the Isle of Man since 1990, and in 2022 Ireland decided to protect it under its Wildlife Act. It has been illegal to fish for or keep basking sharks in European Union waters since 2008, and it is protected in US Atlantic and Gulf waters. International trade is controlled under CITES, and Canada lists its Pacific population as Endangered. In 2020 Scotland designated the Sea of the Hebrides Marine Protected Area partly to protect basking sharks.

Basking sharks and people

Despite their size, basking sharks are gentle and tolerant of boats and divers, which has made them a draw for wildlife tourism. They are not a danger to swimmers, but they are easily disturbed and can be struck by boats, so codes of conduct ask people to keep their distance. The real risks are explained in Are basking sharks dangerous?

Basking sharks have also fueled sea-monster stories. When a dead basking shark rots, the jaws and lower tail often drop away first, leaving a shape with a small “head”, a long neck and a thin tail. Several famous “sea serpent” carcasses, including the Stronsay Beast that washed ashore in Orkney in 1808, were later identified as decomposed basking sharks.