The basking shark, Cetorhinus maximus, is a magnificent and often misunderstood inhabitant of the ocean. It is the second-largest fish globally, a true gentle giant of the marine world. Despite its immense size, it is known for its peaceful demeanor, contrasting with the predatory image often associated with sharks. It is found in temperate waters worldwide.
Physical Characteristics and Distribution
Basking sharks typically reach 26 to 29 feet (7.9 to 8.8 meters) in length, though some exceed 33 feet (10 meters). The largest recorded specimen, caught in Canada’s Bay of Fundy in 1851, measured approximately 40.3 feet (12.27 meters) and was estimated to weigh 16 metric tons. It features a conical snout and immense gill slits that nearly encircle its head.
Its skin is dark grey to brownish-black, often mottled, with lighter patches on its underside and snout. Its caudal fin is crescent-shaped with a strong lateral keel, and its skin is covered in rough, teeth-like structures called dermal denticles. This migratory species is found in all temperate oceans globally, inhabiting coastal and offshore waters down to at least 2,990 feet (910 meters). They are known to follow the seasonal blooms of plankton, migrating thousands of kilometers in search of these rich food sources.
Feeding and Behavior
The basking shark is a filter feeder, meaning it consumes microscopic organisms from the water rather than hunting larger prey. It employs a unique feeding mechanism known as passive ram filtration, swimming with its enormous mouth wide open to filter vast quantities of water. Its mouth can expand significantly, resembling a large net, and is lined with specialized, dark, bristle-like gill rakers that trap plankton and small invertebrates as water flows through.
A single basking shark, measuring approximately 16 feet (5 meters), can filter up to 500 short tons (450 metric tons) of water per hour while swimming at a speed of about 1.9 miles per hour (3.1 kilometers per hour). They are not indiscriminate feeders; studies have shown that feeding basking sharks are found in areas with zooplankton densities 75% higher than adjacent non-feeding areas, indicating a selective feeding strategy. This slow, deliberate swimming near the surface, often with its dorsal fin visible, gives the shark its common name, as it appears to be “basking” in the sun.
Reproduction and Conservation Status
Basking sharks are ovoviviparous, meaning eggs hatch inside the female’s body, and live young are born. Embryos are nourished by their yolk sacs, and after hatching internally, the young may feed on unfertilized eggs within the womb. Gestation is long, around 2 to 3 years, which is among the longest for any vertebrate. Females typically give birth to a relatively small number of large pups, each measuring about 5 to 6 feet (1.5 to 1.8 meters) at birth.
They mature slowly, with males reaching sexual maturity at about 12 to 16 years of age and females between 16 to 20 years. This slow reproductive rate, combined with historical and ongoing threats, has led to their current conservation status. The International Union for Conservation of Nature (IUCN) lists the basking shark as Endangered globally. Primary threats include accidental bycatch in fishing gear, such as gillnets and trawls, as well as finning for traditional markets. Climate change, which can affect the distribution of their plankton food source, also poses a potential threat.
Interactions with Humans
Basking sharks are considered harmless to humans due to their filter-feeding diet and docile temperament. They are not predatory towards large animals and generally do not evade approaching boats. This gentle nature allows for human interaction, particularly through ecotourism activities like shark-watching tours.
These experiences offer people a chance to observe basking sharks in their natural habitat, fostering appreciation for the species and marine conservation efforts. Historically, however, interactions were less benign. Basking sharks were heavily hunted for centuries, primarily for their oil-rich livers, fins, and meat. This overexploitation led to significant population declines, with some regional populations disappearing entirely. Modern conservation efforts now prohibit or restrict targeted fishing for basking sharks through international agreements.