Basking shark - Cetorhinus maximus

General body structure

The basking shark Cetorhinus maximus is, after the whale shark, the second-largest fish on Earth. Adults usually reach 6 to 8 metres in length, in some cases over 10 metres. Body weight can exceed 4 tonnes.

The body is elongated and torpedo-shaped. The snout is conical and relatively short. Notable are the very large gill slits, which extend almost entirely around the head. When closed they form deep grooves along the sides behind the head.

The first dorsal fin is large and triangular. It is set far forward on the back. The second dorsal fin is noticeably smaller. The tail fin is crescent-shaped with a strongly developed upper lobe. This build supports a steady, energy-efficient swimming motion.

Head and mouthparts

The mouth of the basking shark can open wide and reach a width of over one metre. Unlike predatory shark species, it has very small, hook-shaped teeth. They are a few millimetres in size and play little role in feeding.

Central to the anatomy are the gill rakers. These specialised structures sit on the internal gill arches. They act like a sieve. When swimming with the mouth open, water flows in. Plankton and small organisms are trapped in the rakers. The filtered water exits the body through the five enormously enlarged gill slits.

The gill rakers are regularly shed and regenerated. This process is unique among large sharks and essential for their filter-feeding function.

Skeleton and musculature

Like all sharks, Cetorhinus maximus has no bony skeleton. The internal skeleton is composed entirely of cartilage. The cartilage is partially reinforced by calcification, especially around the spine and the bases of the fins.

The vertebral column is strongly developed and supports a massive trunk. Large red muscle masses along the sides of the body enable sustained swimming over long distances. The body is built for constant locomotion in open water.

Skin structure and dermal denticles

The skin of the basking shark is thick and rough. It is covered with placoid scales, also called dermal denticles. These microscopic structures resemble tiny teeth and are oriented backwards.

The dermal denticles reduce drag and protect against injury. In Cetorhinus maximus they are particularly large and give the skin a sandpaper-like texture.

Sensory organs

The eyes are relatively small in relation to body size. They are positioned on the sides of the head. Vision is adapted to detect strong contrasts in open water.

Like other shark species, the basking shark possesses ampullae of Lorenzini. These electroreceptors are located on the head and detect electric fields in the water. Because the basking shark feeds on plankton, they play a lesser role in prey detection than in predatory species.

The lateral line organ runs along the flanks of the body. It detects pressure differences and water movements. This structure supports orientation in open water.

Distinctive features compared to other large sharks

The body of Cetorhinus maximus is fully specialised for filter feeding in the open sea. Size, gill structure and fin shape form a functional system that clearly distinguishes it from large predatory sharks.

Global distribution of the basking shark

The distribution range of the basking shark includes temperate and boreal marine regions worldwide. It occurs in the Atlantic, Pacific and Indian Ocean. Tropical waters are only rarely used.

In the North Atlantic its occurrence ranges from Newfoundland and the Gulf of Maine to Norway and into the Barents Sea. In the eastern Atlantic it regularly occurs off the British Isles, Ireland, Iceland and along the French and Spanish coasts. In the Mediterranean there are scattered but regular records.

In the North Pacific the species ranges from Japan, across the Sea of Okhotsk, to Alaska. Along the North American west coast it is observed from California to British Columbia. The basking shark is also found in cooler waters off South Africa, Australia, New Zealand, Chile and Argentina.

Preferred water temperatures

The basking shark is adapted to cool to moderately warm waters. Most sightings occur at temperatures between 8 and 16 degrees Celsius. This temperature range strongly correlates with the occurrence of high plankton concentrations.

In warmer regions the species mostly remains in deeper, cooler layers of water. Near-surface occurrences are less frequently documented there.

Coastal habitats

During the summer months the basking shark is frequently observed close to shore. Typical habitats include continental shelves, bays and coastal frontal systems with high biological productivity.

Oceanographic structures such as current boundaries or upwelling areas play a central role. There, zooplankton accumulates at high densities, which facilitates feeding. Satellite data show that individuals concentrate in regions with elevated chlorophyll levels, an indicator of productive waters.

Open high seas and vertical utilisation

Outside the main sighting periods, the basking shark ranges widely across the open high seas. Telemetry studies demonstrate wide-ranging migrations over thousands of kilometres. Individuals from the northeast Atlantic have been recorded in the western Atlantic. Individuals from the North Pacific also cover interoceanic distances.

The species utilises a range of water depths. During the day it frequently occupies greater depths of several hundred metres. At night the animals ascend to near-surface waters. These vertical movements are associated with the diurnal migration of plankton.

Importance of marine structures

Front systems, ocean currents and upwelling zones determine the spatial distribution of the basking shark. Particularly relevant are:

• Continental shelves with high primary production
• Areas with seasonal plankton blooms
• Current fronts and temperature gradients
• Regions with stable food aggregations

Long-term observations from the northeast Atlantic show repeated use of certain coastal areas over several years. This site fidelity suggests stable ecological conditions.

Habitat in the Mediterranean

In the Mediterranean, Cetorhinus maximus occurs irregularly but is documented. Records include the Ligurian Sea and the western Mediterranean. The animals are typically found in productive basins with seasonal plankton blooms.

Observations suggest that the Mediterranean represents more of a marginal habitat. The main distribution remains tied to cooler Atlantic and Pacific regions.

Summary of habitat use

The distribution and habitat of the basking shark are closely linked to oceanographic processes. The species utilises large-scale marine areas, responds flexibly to seasonal changes and follows the dynamics of plankton-rich water masses.

Activity patterns and behaviour

The basking shark is a large, slow-swimming filter‑feeder. It moves with its mouth wide open through plankton-rich water. This behaviour gave rise to the English name ‘basking shark’. The apparent basking at the surface is, however, feeding.

Basking sharks are obligate ram ventilators. They must swim continuously so that water flows over the gill rakers. Active pumping, as in some other species, is not possible. If they stop swimming, their oxygen supply would be interrupted.

For most of the year adult animals live alone. However, during periods of high plankton density aggregations of several dozen up to around 100 individuals have been observed. Such groups form at productive fronts with concentrated zooplankton.

Vertical movements and diving behaviour

Modern telemetry studies show that basking sharks do not just swim at the surface. They undertake daily vertical migrations and regularly dive to greater depths. Individual tagged animals have been recorded at depths of around 1,000 metres.

These movements are related to the distribution of zooplankton in the water column. The shark follows its prey between near-surface layers and deeper areas. It is sensitive to temperature gradients and density stratification.

Social behaviour and torus formations

In summer, so-called torus formations are repeatedly documented. In these formations several animals swim in a tight circular pattern, spiralling from the surface into deeper layers. Males and females move close together, display head movements and touch one another with their fins.

Researchers interpret these circular formations as courtship behaviour. Drone footage from the north-east Atlantic documented that males swim close behind females and repeatedly seek body contact. Such behaviours are regarded as an indication of mate choice and preparation for mating.

Filtering mechanism

The basking shark feeds exclusively on plankton. It lacks functional grasping teeth for large prey. While swimming, water flows through the open mouth over specialised gill rakers. These act like a biological sieve.

Estimates suggest an adult can filter several million litres of water per hour. The gill rakers trap zooplankton particles and are regularly shed and renewed. This mechanism allows efficient energy intake at low swimming speeds.

Diet composition

The main components of the diet are small crustaceans, particularly copepods such as Calanus finmarchicus and Calanus helgolandicus. Also included are larval stages of crustaceans, fish eggs and fish larvae.

Stomach analyses show that the species feeds selectively in areas with high plankton concentration. Thermal fronts and convergence zones play a central role. Larger fish are not part of its regular prey.

Ecological specialisation

The strong dependence on zooplankton makes the basking shark vulnerable to changes in marine food webs. While filtering, it also ingests microplastics in addition to plankton. Studies indicate that both particles and chemical additives can enter the organism. Because of its very slow reproduction, additional pollutant loads can be significant at the population level.

The basking shark is one of the most widely migrating fish species of the temperate latitudes. Satellite tagging and genetic analyses show that individual animals traverse entire ocean basins. The migrations follow seasonal patterns and are closely linked to the availability of zooplankton.

Seasonal movements in the North Atlantic

In the Northeast Atlantic, basking sharks appear in spring and summer in coastal regions off Great Britain, Ireland and Norway. There plankton-rich water masses form. As temperatures fall in autumn, many animals move into deeper offshore areas.

Satellite data from a study by the Marine Biological Association show that tagged animals visit depths of over 800 metres in winter. These vertical migrations indicate an adaptation to seasonal changes in plankton layers.

Long-distance migrations across ocean basins

Individual basking sharks travel several thousand kilometres. Tagging projects in the Northwest Atlantic document migrations from the US East Coast to the Caribbean. Other animals moved between British waters and the Mediterranean.

A genetic study published in the journal Current Biology indicates a small genetic difference between populations in the North Atlantic and the South Pacific. This suggests repeated mixing over large distances.

Influence of ocean currents

Large-scale current systems such as the Gulf Stream influence migration corridors. Satellite telemetry from the North Atlantic shows a close coupling between the animals’ locations and frontal systems with high biological productivity. These fronts form where water masses of different temperatures meet.

Such areas concentrate plankton and create temporary feeding grounds. Basking sharks follow these features for weeks at a time. Changes due to climate variability can therefore have direct impacts on their migratory routes.

Conservation-relevant aspects of migrations

Their wide-ranging movements make protection difficult. Basking sharks traverse national territorial waters and international waters. Conservation measures by individual states are therefore of limited effectiveness.

The Convention on Migratory Species lists Cetorhinus maximus in Appendices I and II. This is based on documented cross-border migration. Effective protection requires international cooperation along the known migratory corridors.

Research programmes using satellite tags provide the basis for such measures. They show that basking sharks are not resident coastal animals, but large, mobile fish with ocean-wide ranges.

Reproductive mode

The basking shark is ovoviviparous. The fertilised eggs develop in the female’s uterus. The embryos hatch in the womb and are born as live young. There is no parental care.

Newborn animals measure about 1.5 to 2 metres. Evidence suggests that the embryos feed on unfertilised eggs in the uterus. This phenomenon is called oophagy.

Gestation period and litter size

The gestation is among the longest in the animal kingdom. Estimates range from two to over three years. Historical and modern reports indicate small litters, usually four to six young.

There are likely several years between births. The small litter size and long reproductive intervals result in a very low reproductive output per female.

Sexual maturity and life history

Males reach sexual maturity at a body length of about 4.6 to 6.1 metres. Females become sexually mature much later, reaching maturity only at around 8 to nearly 10 metres in length. The age at maturity can be more than a decade, in some reports up to 18 years.

The combination of late maturity, long gestation and small litter size makes Cetorhinus maximus one of the slowest-reproducing shark species. Lifespan is estimated at around 50 years.

Courtship and mating

Direct observations of mating are rare. The breeding season probably coincides with periods of high plankton density in spring and summer. Torus formations are considered a central element of courtship.

Drones and underwater cameras have documented males closely following females, repeatedly making body contact and showing colour changes. These behaviours are interpreted as a pre-copulatory phase. Actual mating probably takes place in deeper waters.

Birthing areas

The exact birth sites are not yet known. Juveniles are observed far less frequently than adult animals. This suggests that pregnant females withdraw to hard-to-reach or deeper areas to give birth. Identifying such birthing areas is a central research objective of current marine biology.

Despite its size, the basking shark is considered harmless to humans. It feeds exclusively on plankton. When swimming with its mouth wide open, it filters tiny organisms from the water. Teeth play no role in this.

Documented incidents involving humans are extremely rare. There are no confirmed cases of a basking shark deliberately attacking a person. Encounters with divers are generally calm. The animals do not show pronounced flight behaviour, but they do react sensitively to rapid approaches or touch.

Research projects in the United Kingdom and Canada repeatedly observed individuals approaching boats or snorkellers. The animals usually changed their swimming direction slowly, without showing aggressive behaviour. Authorities such as the UK Marine Management Organisation nevertheless recommend maintaining a minimum distance of several metres.

Historical use by humans

From the 18th to the 20th century, the basking shark was heavily fished. Seasonal catching stations existed especially in Ireland, Scotland, Norway and Canada. The main target was the large liver. It can make up to a quarter of the body weight and contains large quantities of oil.

The liver oil was used as fuel for lamps and as a lubricant. Later it was used in industry and as an ingredient in vitamin supplements. The skin was also processed. It was made into leather with a rough texture.

An estimated more than 12,000 animals were caught in Ireland between 1947 and 1975. Populations declined sharply in some regions. Because the basking shark grows slowly and produces only a few young, populations recovered only very slowly.

Modern threats from human activities

Targeted fishing for Cetorhinus maximus is now banned in many countries. Nevertheless, risks remain. Bycatch in drift nets and collisions with vessels are among the most frequently documented threats. Because of its slow swimming and frequent presence near the surface, the basking shark is particularly vulnerable to boat traffic.

Additionally, shifts in plankton distribution caused by climate change affect the species’ seasonal occurrence. Tourist activities such as uncontrolled approaches by speedboats can cause stress and alter natural behaviour.

Legal protection status

The basking shark is protected internationally. It is listed in Appendix II of the Washington Convention on International Trade in Endangered Species (CITES). International trade in products of this species is therefore strictly regulated. There is a complete ban on fishing in the European Union. The species is also protected under national law in the United Kingdom and in Canada.

The International Union for Conservation of Nature (IUCN) lists Cetorhinus maximus as Endangered. This is based on documented population declines in several ocean regions. Conservation measures focus on fishing bans, monitoring of vessel traffic in known aggregation areas and the regulation of ecotourism.

Basking shark tourism and research

In regions such as Scotland, the Isle of Man and off the coast of Canada, regulated wildlife tourism has developed. Observation tours bring economic revenue and promote support for conservation programmes. Operators are subject to codes of conduct that specify minimum distances and restricted boat speeds.

Scientific projects use satellite tags to document migratory movements. Such data show that individual animals travel long distances between national waters. International cooperation is therefore crucial for effective protection.

The relationship between the whale shark and humans has fundamentally changed over the past hundred years. What was once an economically exploited resource has become a protected species, whose conservation is increasingly a priority.