Basking Shark: The Second-Largest Fish

A 10-Meter Filter Feeder

A 10-meter-long shark swims slowly just below the surface of cold Atlantic water. Its mouth is open 1 meter wide. Its gill slits wrap nearly completely around its head. Its enormous dorsal fin breaks the water's surface like a traveling triangle.

This is a basking shark (Cetorhinus maximus) — the second-largest fish on Earth after the whale shark. Despite their massive size and fierce appearance, they eat only plankton. Their giant mouths are designed for filtering, not biting. They pose no threat to humans.

Yet they were hunted nearly to extinction — not for their meat, but for oil in their enormous livers.

Size and Appearance

Basking sharks are among the largest animals in temperate oceans.

Physical features:

• Length: up to 10 meters typically; 12.27 meters record
• Weight: up to 5 tons typical; 16 tons record
• Body shape: large, triangular
• Mouth: up to 1 meter wide when feeding
• Gill slits: extraordinarily long, nearly encircling head
• Color: gray-brown with paler underside
• Fins: massive dorsal fin very prominent

Size comparisons:

• Whale shark: 12 meters (largest fish)
• Basking shark: 10 meters (second largest)
• Great white: 6 meters (third largest shark)

Proportional features:

The most distinctive aspects:

• Mouth can be 1m wide (small relative to body but huge in absolute terms)
• Gill slits so long they nearly meet at the throat
• Dorsal fin rises about 1m above surface when feeding
• Tail is proportionally smaller than other sharks

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Filter Feeding

Basking sharks process enormous amounts of water.

Feeding mechanics:

• Mouth opens 1 meter wide
• Swim slowly with mouth open
• Water enters mouth, exits through gills
• Plankton filtered by gill rakers
• Food swallowed periodically

Volume processed:

• 2,000 cubic meters of water per hour
• Equivalent to filtering 10 swimming pools hourly
• Daily processing: 48,000 cubic meters

Food composition:

• Primary: copepods (small crustaceans)
• Secondary: fish eggs
• Tertiary: other zooplankton
• Rare: small fish

Filtering efficiency:

Their filtering system:

• Gill rakers: fine bony projections trap plankton
• Mucous coating: holds captured food
• Selective filter: separates food from water
• Continuous operation: hours of feeding per day

Plankton patches:

Basking sharks gather at plankton-rich locations:

• Upwelling currents: bring nutrient-rich deep water up
• Temperature fronts: plankton concentrates
• Seasonal blooms: spring/summer production
• Specific geographic regions: known feeding grounds

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Where They Live

Basking sharks inhabit temperate waters globally.

Main regions:

North Atlantic:

• British Isles (historical population)
• Norway and Scandinavia
• Eastern North America (New England to Florida)
• Mediterranean Sea
• Iceland

North Pacific:

• California coast
• Pacific Canada
• Japan (occasional)

Southern Hemisphere:

• Southern Australia
• New Zealand
• South Africa
• South America (Argentine waters)

Temperature preferences:

• Optimal: 8-14°C
• Tolerable: 6-18°C
• Avoid: tropical waters
• Follow: seasonal temperature gradients

Seasonal movements:

• Summer: move to cooler northern waters for feeding
• Winter: move to deeper southern waters
• Migration: can cross ocean basins

Depth range:

• Surface feeding: 0-50 meters
• Deeper migrations: up to 900 meters
• Cross-ocean travel: variable depths

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The Historical Hunting

Basking sharks were hunted intensively for centuries.

Why they were hunted:

Liver oil:

• Huge livers (up to 25% of body weight)
• Rich in squalene (valuable oil)
• Used for lamp oil historically
• Later: cosmetics, pharmaceuticals
• Still valuable today

Other uses:

• Meat for food
• Cartilage for traditional medicine
• Fins for shark fin soup
• Skin for leather

Historical scale:

• British Isles: hunted since medieval times
• Norway: major commercial operations
• Ireland: traditional basking shark hunting
• North America: targeted fishing
• Scale: thousands killed annually for decades

Commercial fishing peaks:

• 1700s: small-scale hunting
• 1800s: commercial operations expanded
• 1900-1950: industrial scale fishing
• Norway peak: 2,000-3,000 per year

Population collapse:

Centuries of hunting caused:

• Dramatic population declines
• Regional extinctions
• Nearly complete elimination from some seas
• Structural changes to ocean ecosystems

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Near Extinction Recovery

Modern conservation has helped some populations.

20th century changes:

• Post-WWII: demand for shark products decreased
• 1970s-1980s: environmental awareness grew
• 1990s: international protection began
• 2000s: most countries enacted protections

Current populations:

• Global: estimated 8,000-20,000 adults
• Severely depleted: compared to historical levels
• Recovery: slow but happening in some regions
• Still fragile: particularly vulnerable

Regional recoveries:

British Isles:

• Populations recovering
• Summer feeding aggregations returning
• Some research evidence of population growth
• Still below pre-hunting levels

Mediterranean:

• Slow recovery
• Some hopeful signs
• International cooperation continuing

Pacific:

• Less comprehensive data
• Some indicators of stable populations
• Climate change concerns growing

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Modern Threats

Basking sharks still face pressures.

Current threats:

Commercial fishing bycatch:

• Caught in tuna fisheries
• Longlining operations
• Gillnet entanglement
• Cumulative mortality significant

Illegal finning:

• High fin value in Asian markets
• Cross-border illegal trade
• Difficult to monitor
• Continued deaths

Ship strikes:

• Surface-feeding makes them vulnerable
• Growing ocean traffic
• Coastal shipping routes
• Fatal collisions regular

Climate change:

• Plankton distributions changing
• Temperature regimes shifting
• Migration patterns disrupted
• Feeding grounds altering

Pollution:

• Chemical accumulation in liver oil
• Microplastics entering food chain
• Shipping pollution
• Coastal development impacts

Slow recovery potential:

Their biology makes recovery slow:

• Late sexual maturity (20-30 years)
• Long gestation (6 years)
• Limited offspring per litter
• Infrequent reproduction
• Individual importance extreme

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Reproduction

Basking shark breeding is remarkably slow.

Reproduction basics:

• Sexual maturity: 20-30 years (among latest in fish)
• Gestation: 6 years (longest in any shark)
• Reproductive rate: approximately once every 4-6 years
• Litter size: 1-6 pups

Gestation:

6 years inside the mother makes basking shark the longest gestation of any vertebrate animal.

Young:

• Birth length: 1.5 meters
• Immediately independent: live birth
• Growth rate: slow
• Full size: 20-30 years to reach maturity

Implications:

Each adult represents:

• Decades of growth
• Rare breeding opportunities
• Irreplaceable lost reproduction if killed
• Long-term population investment

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Behavior

Basking sharks are generally passive.

Feeding behavior:

• Slow surface swimming
• Open-mouth filtering
• Solitary or in loose groups
• Follow plankton patches

Social behavior:

• Generally solitary
• Seasonal aggregations at food sources
• Mating gatherings (rare observations)
• Loose social structures

Dive behavior:

Modern tracking has revealed:

• Regular deep dives (up to 900m)
• Vertical movements for finding food
• Crossing different water masses
• Complex movement patterns

Interaction with humans:

• No documented attacks on humans
• Curious but non-aggressive
• Can be approached by swimmers
• Excellent subjects for eco-tourism

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Eco-Tourism

Basking sharks have become important for tourism.

Tourism regions:

Scotland (western coast):

• Summer aggregations
• Boat tours available
• Swimming with sharks possible
• Major tourism industry

Ireland (Cornwall, Devon):

• British summer populations
• Shark spotting trips
• Conservation awareness
• Economic significance

Cape Cod (Massachusetts):

• Occasional summer visitors
• Research and tourism
• Growing interest
• Limited aggregations

Tourism impact:

Well-managed tourism provides:

• Economic value for conservation
• Research opportunities
• Public education
• Political support for protection

Poorly-managed tourism can:

• Disturb feeding
• Modify behavior
• Stress animals
• Reduce reproductive success

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Scientific Research

Basking sharks are studied extensively.

Research areas:

Migration patterns:

• Satellite tracking studies
• Revealed transatlantic migrations
• Individual fidelity to feeding grounds
• Climate change impacts

Population genetics:

• DNA analysis of populations
• Recovery assessment
• Genetic diversity measurement
• Breeding success evaluation

Feeding ecology:

• Filter feeding dynamics
• Plankton preferences
• Efficiency measurements
• Ecosystem impacts

Conservation biology:

• Population estimates
• Threat assessment
• Management strategy development
• Protected area evaluation

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Conservation Programs

Multiple organizations protect basking sharks.

International:

CITES Appendix II:

• Trade regulation
• Monitoring system
• International cooperation
• Enforcement varies

IUCN:

• Endangered classification
• Red List assessment
• Conservation recommendations

Regional:

Marine Stewardship Council:

• Sustainable fishing certification
• Basking shark considerations

WWF:

• Global conservation programs
• Research funding
• Policy advocacy

National:

• UK/Ireland: protected species status
• US: protected under Marine Mammal Protection Act
• Canada: protected species
• EU: multiple protections
• Australia: protected under EPBC Act

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Cultural Legacy

Basking sharks have centuries of cultural presence.

Traditional knowledge:

British and Irish cultures:

• Centuries of folklore
• Fishing traditions
• Regional names (some prefer "sunfish")
• Cultural heritage around coastal communities

Historical hunting:

• Economic foundation for coastal communities
• Generations of specialized fishermen
• Traditional fishing methods
• Community traditions

Modern culture:

• Focus of conservation efforts
• Eco-tourism subject
• Research interest
• Educational value

Art and literature:

• Featured in maritime literature
• Traditional art subjects
• Educational materials
• Environmental advocacy

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Why They Matter

Basking sharks represent significant biology.

Ecological:

• Second-largest fish globally
• Filter-feeding on planktonic prey
• Important ocean food web role
• Nutrient cycling contributions
• Indicator of plankton populations

Scientific:

• Model for understanding filter feeding
• Longest gestation in any shark
• Climate change research subject
• Genetics of recovering populations

Conservation:

• Success story potential
• International cooperation model
• Economic value through tourism
• Cultural heritage preservation

Global significance:

• Among few surviving temperate filter feeders
• Replacement species (if lost): difficult
• Part of ocean health indicator system
• Climate sensitivity significant

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The Second Giant

Every basking shark swimming in temperate oceans represents the survival of a centuries-hunted species.

Their massive size, combined with gentle plankton-eating habits, initially made them conspicuous targets for commercial hunters. Their livers were especially valuable — producing oil that kept lamps burning in British homes for centuries. The scale of historical hunting was enormous.

Today, they represent both tragedy and hope. Tragedy for the populations lost through sustained hunting. Hope because they persist despite that history, because protections are now in place, because tourism has created economic reasons to preserve them, and because some populations are slowly recovering.

Each basking shark requires decades to reach reproductive maturity. Each is irreplaceable through short-term effort. Each represents the specific biology — filter feeding at massive scale — that makes them the second-largest fish.

The 10-meter sharks swimming off British coasts each summer continue what their ancestors have done for centuries. Surface cruising. Filter feeding. Migrating between feeding grounds. Occasionally surfacing in sunlit water, appearing to bask.

They deserve our protection. The shark that was once seen as a commercial product is now recognized as a magnificent living example of temperate ocean biology — the gentle giant of cold waters.

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Related Articles

• Whale Shark: The Largest Fish on Earth
• Great White Shark: The Ocean's Apex Predator
• Megalodon vs Great White Shark

Frequently Asked Questions

How big is a basking shark?

Basking sharks (Cetorhinus maximus) reach 10 meters (33 feet) in length and weigh up to 5 tons, making them the second-largest fish on Earth after whale sharks. The largest verified basking shark measured 12.27 meters (40 feet) and weighed approximately 16 tons, though specimens over 10 meters are rare. They are substantially larger than great whites (6-7 meters) but smaller than whale sharks (12 meters). They have large triangular bodies with enormous mouths -- up to 1 meter wide when feeding. Their gill slits are extraordinarily long, almost circling their heads entirely. Adults typically reach full size around 20-30 years of age. They live 50+ years in the wild, possibly reaching 100 years. Despite their massive size, they are completely harmless to humans, using their large mouths only to filter plankton from seawater. Their size and slow movements make them easy to approach, which unfortunately contributed to centuries of commercial hunting that nearly drove them to extinction.

Why are they called basking sharks?

Basking sharks earned their name from their habit of slowly cruising at the water's surface, appearing to 'bask' in sunlight. During feeding, they swim near the surface with their massive dorsal fins breaking the water. Historical fishermen observed them repeatedly surfacing in warm weather, suggesting they were sunbathing. Modern research has confirmed they do spend significant time near the surface, particularly in temperate waters during summer months. Their apparent sunbathing is actually feeding behavior -- they follow plankton concentrations which are most abundant in surface waters. Recent satellite tracking has shown they also make deep dives (up to 900 meters) and long-distance migrations, so they're not just surface dwellers. The historical name reflects how they appeared to observers before modern research methods. Their surface presence made them easy targets for historical whalers, who recognized them as valuable for liver oil and meat. The name remains cultural despite being biologically incomplete.

What do basking sharks eat?

Basking sharks eat only plankton, small fish eggs, and tiny crustaceans, similar to whale sharks. They are filter feeders that process enormous volumes of water to extract microscopic food. An adult basking shark can filter 2,000 cubic meters of water per hour -- roughly the water in 10 medium swimming pools. Their filtering system includes gill rakers (fine structures lined with mucus) that trap plankton while allowing water to pass through. They swim slowly with mouths wide open, collecting food continuously. Their food preference is specifically copepods (small crustaceans) and other zooplankton, with some fish eggs and smaller prey. They gather at plankton-rich locations seasonally, particularly where ocean currents create upwelling conditions. In British and Irish waters, they congregate each summer in specific feeding zones. During winter, they move to deeper waters where plankton concentrates at specific depths. They can extract sufficient nutrition from surprisingly small prey because they process such massive water volumes. Their specialized jaws don't need teeth for eating -- their tiny teeth are vestigial.

Where do basking sharks live?

Basking sharks inhabit temperate oceans worldwide, primarily in the North Atlantic, Mediterranean, North Pacific, and cooler Southern Hemisphere waters. They are found along coasts of North America (especially New England, California, and Gulf of Mexico), Europe (Ireland, Scotland, Scandinavia), Mediterranean, South Africa, New Zealand, Australia, and various Pacific locations. Their range is limited to temperate waters (typically 8-14°C), avoiding tropical regions where whale sharks dominate. They prefer productive coastal waters with abundant plankton. Summer brings them to northern waters including British Isles, where traditional hunting has occurred for centuries. Winter sends them south to deeper warmer waters. Some populations show strong seasonal patterns -- appearing at specific feeding grounds reliably year after year. Modern satellite tracking has revealed they migrate much further than previously thought, with individuals crossing ocean basins. They prefer water depths of 20-800 meters, alternating between surface feeding and deeper water. They're now rare in many historical regions due to centuries of overhunting for commercial products.

Are basking sharks endangered?

Yes, basking sharks are listed as Endangered by the IUCN, with populations severely depleted from historical hunting. They were commercially hunted for over 300 years in British, Norwegian, and Irish waters for their liver oil (rich in squalene), meat, cartilage, and fins. The scale of historical hunting was massive -- Norway alone killed 2,000-3,000 basking sharks annually for decades. Populations have declined dramatically from pre-hunting levels. Current global population is estimated at 8,000-20,000 mature individuals, with some regional populations nearly extinct. The British and Irish historical populations have largely recovered, but remain below pre-hunting levels. Modern threats include: commercial fishing bycatch (especially in tuna fisheries), illegal fishing for fins, climate change affecting plankton distributions, and ship strikes. Their slow reproduction (maturity at 20-30 years, 6-year gestation, limited offspring) means populations cannot quickly recover from losses. CITES Appendix II protection restricts international trade. Most countries have national protections. Conservation efforts include marine protected areas, research programs, and international cooperation. Their future requires sustained protection and habitat preservation.