The basking shark, Cetorhinus maximus, is the second-largest fish in the world’s oceans, surpassed only by the whale shark. It is often observed swimming slowly near the surface, a behavior that led to its common name, as it appears to be soaking up the sun’s warmth. Despite its imposing size, the basking shark is a gentle filter feeder and poses no threat to humans.
Physical Traits and Distinctive Behavior
The basking shark has a large, grey-brown or blackish body, typically lighter on its underside. It can reach lengths of about 11 meters (36 feet) and weigh up to 3.9 metric tons (4.3 tons), though some reports suggest even larger specimens exist. A distinctive feature is its wide, white-lined mouth, which can open to over 1 meter, coupled with five greatly enlarged gill slits that nearly encircle its head.
Its conical snout and a large, dark, triangular dorsal fin are often visible when it feeds near the surface. The shark’s feeding mechanism involves swimming slowly with its mouth wide open, filtering vast quantities of water through its gills. Specialized black, bristle-like gill rakers trap zooplankton, such as copepods, small crustaceans, and fish eggs, which are its primary food source.
This passive feeding method allows the basking shark to process an average of 6,000 liters (nearly 2,000 gallons) of water per hour. Unlike some other filter-feeding sharks, the basking shark relies solely on ram feeding, pushing water through its gills by swimming forward. Despite having hundreds of tiny, hooked teeth, they are not used for feeding but may serve a grip function during mating.
Basking sharks are occasionally observed breaching, where they launch themselves out of the water. This behavior may be related to communication with other sharks or potentially to dislodge parasites. While usually solitary, they can form single-sex shoals, especially in areas with dense plankton blooms, where they may engage in courtship behaviors.
Habitat and Conservation
Basking sharks are a migratory species, found in all temperate oceans globally in coastal and open ocean environments. They prefer water temperatures ranging from 8 to 14.5 °C (46.4 to 58.1 °F) and migrate across ocean basins in search of abundant plankton. While often seen near the surface, they can dive to great depths and have been confirmed to cross warmer equatorial waters.
As filter feeders, basking sharks play a role in regulating zooplankton populations within marine ecosystems. They contribute to the overall health and stability of the marine ecosystem, and their movements can indicate areas of high productivity.
The basking shark is classified as endangered by the International Union for Conservation of Nature (IUCN), with global populations estimated to have declined by 50–79% over the past century. Historically, they were heavily hunted for their liver oil, meat, and fins. While targeted fishing has largely ceased in many regions, current threats include accidental bycatch in fishing gear and boat strikes due to their surface feeding habits.
Conservation efforts include their listing under Appendix II of CITES and on the OSPAR List of Threatened and/or Declining Species. However, their slow growth rate, with females reaching sexual maturity possibly between 12-20 years of age, and low fecundity, make their populations particularly vulnerable to these pressures.