A slow-moving filter feeder, its common name derives from its habit of feeding at the surface, appearing to be basking in the warmer water there.

[6] Basking sharks have been shown from satellite tracking to overwinter in both continental shelf (less than 200 m or 660 ft) and deeper waters.

[15][20][19][21] A 12.27 m (40.3 ft) specimen trapped in a herring net in the Bay of Fundy, Canada, in 1851 has been credited as the largest recorded.

[25] The two species can be easily distinguished by the basking shark's cavernous jaw, up to 1 m (3 ft 3 in) in width, longer and more obvious gill slits that nearly encircle the head and are accompanied by well-developed gill rakers, smaller eyes, much larger overall size and smaller average girth.

The basking shark's liver, which may account for 25% of its body weight, runs the entire length of the abdominal cavity and is thought to play a role in buoyancy regulation and long-term energy storage.

[7] In winter, basking sharks often move to deeper depths, even down to 900 m (3,000 ft) and have been tracked making vertical movements consistent with feeding on overwintering zooplankton.

[14] Such interpretations are speculative, however, and difficult to verify; breaching in large marine animals such as whales and sharks might equally well be intraspecific threat displays of size and strength.

Remaining at depths between 200 and 1,000 metres (660 and 3,280 ft) for many weeks, the tagged sharks crossed the equator to reach Brazil.

Basking sharks are usually solitary, but during summer months in particular, they aggregate in dense patches of zooplankton, where they engage in social behaviour.

[14] Small schools in the Bay of Fundy and the Hebrides have been seen swimming nose to tail in circles; their social behaviour in summer months has been studied and is thought to represent courtship.

The basking shark is a ram feeder, filtering zooplankton, very small fish, and invertebrates from the water with its gill rakers by swimming forwards with its mouth open.

A 5-metre-long (16 ft) basking shark has been calculated to filter up to 500 short tons (450 t) of water per hour swimming at an observed speed of 0.85 metres per second (3.1 km/h; 1.9 mph).

Samples taken in the presence of feeding individuals recorded zooplankton densities 75% higher than adjacent non-feeding areas.

[41] Basking sharks feed preferentially in zooplankton patches dominated by small planktonic crustaceans called calanoid copepods (on average 1,700 individuals per cubic metre of water).

Their seemingly useless teeth may play a role before birth in helping them feed on the mother's unfertilized ova (a behaviour known as oophagy).

In New Zealand, basking sharks had been abundant historically; however, after the mass by-catches recorded in the 1990s and 2000s,[49] confirmations of the species became very scarce.

[50][51] In June 2018 the Department of Conservation classified the basking shark as "Threatened - Nationally Vulnerable" under the New Zealand Threat Classification System.

Historically, the basking shark has been a staple of fisheries because of its slow swimming speed, placid nature, and previously abundant numbers.

Commercially, it was put to many uses: the flesh for food and fishmeal, the hide for leather, and its large liver (which has a high squalene content) for oil.

As a result of rapidly declining numbers, the basking shark has been protected in some territorial waters and trade in its products is restricted in many countries under CITES.

[55] Once considered a nuisance along the Canadian Pacific coast, basking sharks were the target of a government eradication programme from 1945 to 1970.

As of 2008[update], efforts were underway to determine whether any sharks still lived in the area and monitor their potential recovery.