Northern Ireland's Priority Species

Cetorhinus maximus – basking shark

Cetorhinus maximus
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Cetorhinus maximus (Gunnerus, 1765)
Family: Cetorhinidae

The basking shark can grow to 12m long and is the largest fish found in Northern Ireland waters. It is completely harmless to humans, feeding only on tiny plankton. Its slow rate of breeding makes the population very vulnerable to disturbance of any kind, particularly over-fishing. In the past, basking sharks were hunted mainly for liver oil – many populations have not recovered from this exploitation even 50 years later. Nowadays they are killed for their fins which are highly valued in the Far East and are the main ingredient in shark fin soup.

In brief

  • Found in the seas around Northern Ireland

  • Most likely to be seen from April to September

  • Listed as a UK Priority Species

  • Main threats to the population are the shark fin trade and accidental capture in fishing nets and traps.

Species description
The body is streamlined and grey/brown in colour – the underside is paler. Fully grown animals may grow to over 10m in length. The first dorsal fin is triangular, very large (up to 1 to 2m tall) and often droops over to one side when out of the water. The pectoral fins are also large and can reach 2m in length. The tail is crescent moon shaped.

The head is dominated by the large conical snout and five enormous gill slits. The gill rakers are found just inside the gill slits and are used to sieve plankton from the water. The mouth is huge, up to 1m across when open and contains numerous tiny hook-shaped teeth. The eyes are tiny and are located to the front of the head just behind the snout which is noticeably hooked in juvenile animals.

Life cycle
Basking sharks are a migratory species and travel thousands of kilometers searching for plankton on which to feed. During the summer months they are found in coastal waters and are usually seen ‘basking’ – swimming slowly, often with their huge mouths open, in the surface waters. The tip of the nose, first dorsal fin and top of the tail can all be visible above the water at the same time when the animal is feeding near the surface. It is believed that they spend the winter months feeding in deep water offshore. Basking sharks do not breed until they are between 12 and 18 years old; females mature later than males. Around six pups may be produced every 2 to 3 years – the young sharks hatch inside their mother’s body and remain there for 1 to 3 years feeding on small infertile eggs that the female produces for them. They measure between 1.5 and 2m long when they are born and may live for up to 50 years. Many aspects of the life cycle of the basking shark are poorly understood and further research is needed to help understand this gentle giant.

Similar species
It is unlikely to be confused with other shark species due to their enormous size. Basking sharks are easily distinguished from whales and dolphins by the way they swim – sharks move their tails from side to side in a snake-like movement when swimming, whales and dolphins move their tails up and down in the water.

How to see this species
Basking sharks may be seen in coastal waters from May to September. The sea around Rathlin Island, County Antrim and the waters off Strangford Lough, County Down are areas where basking sharks are regularly seen.

Current status
Basking sharks are found in Northern Ireland coastal waters where there is a good supply of plankton. The actual population size is not known. It is listed as Vulnerable on the IUCN Red List of threatened species 2004, on Appendix 11 of CITES, Appendix 1 and 11 of the Bonn Convention on Migratory Species, and Schedule 5 of the UK Wildlife and Countryside Act 1981.

Why is this species a priority in Northern Ireland?

  • Listed as a UK Priority Species.

Basking sharks are very vulnerable to overfishing due to their slow growth rate, late maturity and small numbers of young produced. Local populations can decline rapidly and are very slow to recover.

Threats/Causes of decline

  • Hunting for body parts – fins, meat and cartilage

  • Accidental capture in fishing nets

  • Collision with boats

  • Disturbance by public.

Conservation of this species

Current action
There is a UK Species Action Plan for this species which was published in 1999.

  • Protection of the basking shark in Northern Ireland waters is currently under consideration as part of the current review of the Wildlife (Northern Ireland) Order 1985.

Proposed objectives/actions

  • Extend protection provided in GB waters to Northern Ireland

  • Maintain the current basking shark population

  • Develop and implement a code of conduct to reduce levels of harassment. Disseminate this to maritime users

  • Commission research to elucidate the life cycle of this poorly understood species

  • Quantify and monitor population size, structure, dynamics and movement patterns and range of individuals occurring in UK waters

  • Improve long-term studies to: assess scientifically the population trends; elucidate migration and over-wintering areas which may identify locations where basking sharks mate and the pregnant females reside; minimise unnatural mortality in these areas.

What you can do
To report basking shark sightings contact CEDaR, National Museums Northern Ireland, 153 Bangor Road, Cultra, Co. Down, BT18 0EU, Tel: 028 9039 5264, [at]

Further information

Species Action Plan for the Basking Shark (Cetorhinus maximus)

Manx Basking Shark Watch

The Wildlife Trusts Basking Shark Survey

Basking Shark Watch Project

Basking Shark - ARKive

European Basking Shark Identification Project


Text written by:
Angela Ross, Curator of Vertebrates, Ulster Museum