Basking Shark

The basking shark regularly reaches 7–8.5 m (23–28 ft) in length with some individuals reaching 9–11 m (30–36 ft). The average length of an adult is around 7.9 m (26 ft) weighing about 4.65 t (4.58 long tons; 5.13 short tons). Historical sightings suggest basking sharks around 12 m (39 ft) in length, including three basking sharks estimated at ~40 fod (12.5 metres or 41 ft) and ~45 fod (14 metres or 46 ft) were reported between 1884 and 1905, but these visual estimates lack good evidence. 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. Its weight has been estimated at 16 t (16 long tons; 18 short tons). A study looking at the growth and longevity of the basking shark suggested that individuals larger than ~10 m (33 ft) are unlikely. It is the second-largest extant fish species, after the whale shark.

The basking shark is a coastal-pelagic shark found worldwide in boreal to warm-temperate waters. It lives around the continental shelf and occasionally enters brackish waters. It is found from the surface down to at least 910 m (2,990 ft). It prefers temperatures of 8 to 14.5 °C (46.4 to 58.1 °F) but has been confirmed to cross the much warmer waters at the equator. It is often seen close to land, including in bays with narrow openings. The shark follows plankton concentrations in the water column, so it is often visible at the surface. It characteristically migrates with the seasons.

Basking sharks have few predators. White sharks have been reported to scavenge on the remains of these sharks. Killer whales have been observed feeding on basking sharks off California in the US and New Zealand. Lampreys are often seen attached to them, although they are unlikely to be able to cut through the shark's thick skin.

Basking sharks do not hibernate and are active year-round. 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.

Basking sharks are ovoviviparous: the developing embryos first rely on a yolk sac, with no placental connection. 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 females, only the right ovary appears to function, and it is currently unknown why only one of the organs seems to function.

Gestation is thought to span over a year (perhaps two to three years), with a small, though unknown, number of young born fully developed at 1.5–2 m (4 ft 11 in – 6 ft 7 in). Only one pregnant female is known to have been caught; she was carrying six unborn young.

Mating is thought to occur in early summer, and birthing in late summer, following the female's movement into shallow waters.

Aside from direct catches, by-catches in trawl nets have been one of several threats to basking sharks. In New Zealand, basking sharks had been abundant historically; however, after the mass by-catches recorded in the 1990s and 2000s, confirmations of the species became very scarce. Management plans have been declared to promote effective conservation. In June 2018, the Department of Conservation classified the basking shark as "Threatened - Nationally Vulnerable" under the New Zealand Threat Classification System.