The basking shark (Cetorhinus maximus) is the second-largest fish on Earth, exceeded in size only by the whale shark. Adults regularly reach 8–10 meters in length, with the largest confirmed specimen measuring 12.27 meters. Despite its enormous size and gaping mouth, the basking shark is a completely harmless filter feeder — it poses no threat to humans and feeds exclusively on tiny zooplankton, small fish eggs, and invertebrate larvae strained from the water as it swims.
Basking Shark Size: How Big Do They Get?
Basking sharks are among the largest animals alive today. Average adults measure 7–9 meters. Females tend to be larger than males. The record specimen measured 12.27 meters, and some historical reports — unverified — claimed individuals approaching 15 meters. Even at typical sizes, a basking shark is longer than a London double-decker bus and heavier than an elephant, with adults weighing up to 4,000 kg.
Their most striking feature is the enormous mouth — which can gape up to 1 meter wide and is held open continuously as the shark swims, filtering water through five pairs of gill slits that nearly encircle the head.
How Does a Basking Shark Feed?
Basking sharks are passive filter feeders. They swim forward with their mouths wide open, allowing water to flow through their gill arches which are lined with modified gill rakers — comb-like structures that trap zooplankton as water passes through. A basking shark processes up to 2,000 tons of water per hour during active feeding.
Their primary food source is copepods — tiny crustaceans that form dense concentrations near the ocean surface, particularly in spring and summer. Basking sharks are highly migratory, following these plankton blooms across ocean basins. The “basking” behavior that gives them their name — slow surface swimming with dorsal fin exposed — occurs during feeding, when they congregate at surface plankton patches.
Unlike whale sharks and manta rays, which actively pump water across their filter structures, basking sharks rely entirely on forward motion. If they stop swimming, feeding stops.
Where Do Basking Sharks Live?
Basking sharks are found in cool, temperate waters of both hemispheres — they prefer temperatures between 8°C and 14°C. Key locations for sightings and aggregations include:
- British Isles: The coastlines of Scotland, Ireland, Wales, and southwest England — particularly the Isle of Man and the Hebrides — are among the most reliable basking shark viewing locations in the world, especially May through August
- Northeastern United States and Canada: Gulf of Maine and Bay of Fundy during spring and summer
- California coast: Occasional aggregations in Monterey Bay and the Channel Islands
- Western Mediterranean
- Southern Australia and New Zealand
Despite their visible surface behavior in summer, basking sharks are highly migratory and spend much of the year in deeper water. Satellite tagging has revealed they cross the Atlantic and dive to depths exceeding 900 meters — their winter movements were largely unknown until recently.
Basking Shark Behavior
Basking sharks are not as solitary as once believed. Aggregations of dozens — occasionally hundreds — of individuals are documented at productive feeding grounds. Social behaviors including nose-to-tail circling, parallel swimming, and breaching (leaping entirely clear of the water) have been observed. These behaviors are thought to be related to courtship, though breeding in basking sharks has never been directly observed.
Despite their size, basking sharks are capable of spectacular breaches — launching their full body out of the water. The purpose is debated: parasite removal, communication, and courtship have all been proposed.
Basking Shark Conservation Status
The basking shark is listed as Endangered on the IUCN Red List. Their populations were decimated during the 20th century by targeted hunting for:
- Liver oil: Basking shark livers can weigh up to 600 kg and contain enormous quantities of squalene, a valuable oil used in industrial lubricants, cosmetics, and vitamin supplements
- Fins: Among the largest shark fins, highly valued in the fin trade
- Meat and cartilage
Major fisheries operated in Norway, Ireland, Scotland, Canada, and China through the mid-20th century. Norwegian fisheries alone killed tens of thousands of basking sharks annually at their peak. Most directed fisheries have now closed, and basking sharks are protected in many countries including the UK, US, Malta, and New Zealand. International trade is restricted under CITES Appendix II.
Recovery is extremely slow. Basking sharks have one of the lowest reproductive rates of any shark — gestation is estimated at 2–3 years, with small litter sizes. Populations are thought to be recovering in some regions but remain far below historical levels.
Key Facts
- Scientific name: Cetorhinus maximus
- Maximum confirmed length: 12.27 meters
- Maximum weight: ~4,000 kg
- Diet: Zooplankton (primarily copepods), fish eggs, invertebrate larvae
- Feeding method: Passive filter feeding — mouth open while swimming
- Water processed per hour: Up to 2,000 tons
- Conservation status: Endangered (IUCN)
- Lifespan: Estimated 50+ years
- Depth range: Surface to 900+ meters
Frequently Asked Questions
Are basking sharks dangerous?
No. Basking sharks are completely harmless to humans. They have no interest in large prey — their entire anatomy is adapted for filtering tiny plankton. The only risk is accidental collision with their large bodies, which has capsized small boats. They do not attack.
Can you swim with basking sharks?
In many locations, yes. Basking sharks in the UK are a major wildlife tourism attraction. Operators offer snorkeling and diving encounters. The sharks typically continue feeding without concern for nearby swimmers. Responsible guidelines ask people not to approach the gill area or touch the animals.
What is the difference between a basking shark and a whale shark?
Both are giant filter feeders, but the whale shark is significantly larger — up to 18 meters vs 12 for basking sharks. Whale sharks are tropical, while basking sharks prefer cold temperate water. Whale sharks have a distinctive spotted pattern; basking sharks are grey to dark brown without markings. Whale sharks actively pump water; basking sharks rely on ram filtration while swimming forward.
Where do basking sharks go in winter?
For decades this was unknown — they simply disappeared from surface waters in autumn. Satellite tagging has revealed they migrate to deeper water, possibly to the mid-Atlantic or tropics, and may feed on deep-water plankton or fast during winter months. Some tagged individuals crossed the Atlantic entirely.
Why do basking sharks breach?
The precise reason is unknown. Proposed explanations include dislodging sea lice and parasites, social communication, courtship display, and simply play. Breaching requires enormous energy expenditure for an animal their size, suggesting it serves an important purpose.