Basking Shark

Basking Shark
  • Scientific name: Cetorhinus maximus
  • Size: Typically 22 – 28ft and 9000lb, although can be larger.
  • UK shore caught record: N/a
  • IUCN Status: EN (Endangered)
  • Distribution: Widespread distribution throughout most of the world’s temperate oceans.
  • Feeds on: Plankton
  • Description: Huge shark species which is usually black or dark grey, occasionally with brownish tinge. Massive mouth appears toothless and gill slits are very large for filter feeding. Large pectoral, dorsal and tail fins. Snout is pointed and eyes are small.

The basking shark is the largest fish in UK waters, and the second biggest extant fish species in the world (after the whale shark). It can grow to over thirty feet in length and can weigh in excess of five and a half tons. The mouth can be over a metre wide, and does contain very small (but sharp) teeth. Basking sharks are found in the southern and northern hemisphere, but avoid the warm waters around the equator. Basking shark are migratory and are usually spotted in British waters in the summer, travelling to deeper waters in the winter. This species can be found around the whole of the UK, but it is more likley to be found along the west coast of the British Isles and is rare in the North Sea.

Basking Shark Feeding

Left, a basking shark feeding at the surface and, right, a basking shark continues to feed as divers observe.

Basking shark are not aggressive and often make no attempt to move away from boats or divers, meaning that people can easily observe them. In some areas this can provide a boost to tourism people can view basking sharks from a boat and divers can swim alongside this species. In July 2020 a four and a half metre (15ft) basking shark had to be euthanised after beaching itself on a shallow beach in Filey Bay, Yorkshire. Lifeboat crews had tried to get the shark back out to sea but were unsuccessful.

Behaviour and Feeding

Basking Shark Distribution

Basking shark distribution around the world, although populations are now patchy due to overfishing and hunting of this species.

Basking sharks are one of the few shark species which filter feed on plankton. They cruise slowly through the sea, often at the surface, with their mouth wide open to filter plankton from the water. A large basking shark can process over one million litres of sea water per hour through the thousands of rakers in its gills. This behavior gave the shark its name as people originally thought they were basking in the sun. Basking sharks are known to come close to land during the summer, and can sometimes be spotted from the shore, and have been known to come into estuaries and large harbours during calm weather. Many basking sharks will be solitary, but they have been spotted moving in pairs and occasionally form into groups made up of tens or even hundreds of basking sharks. Basking sharks come into shallower, inshore waters during warmer weather as plankton density in much higher in these areas during summer, as this footage taken in late May 2016 off the coast of Cornwall shows. It is thought that basking sharks take some form of hibernation in much deeper water in winter although this any many other aspects of the basking sharks life cycle, such as migration patterns, are poorly understood. Basking sharks are usually slow moving but they have occasionally been spotted breaching (jumping in such as way that their entire body, apart from the tail, is out of the water). The reason for this is unknown but one theory is that the basking sharks are trying to rid themselves of parasites such as lampreys.

Reproduction

Little is known about the breeding cycle of basking shark. Basking sharks are thought to need to be around ten years old before they can reproduce. Breeding takes place in summer with eggs develop inside of the female, with gestation taking up to three years. Young basking shark are between four and six feet in length when they are born. In 2018 new research was produced by an innovative tagging system for basking sharks which suggested that basking sharks may gather in Scottish waters to mate.

Conservation Status

Basking were classed as Vulnerable by the IUCN (International Union for the Conservation of Nature) but in a reassessment in 2018 this was changed to Endangered (EN) and this species has a decreasing population trend. They were previously hunted for their flesh which, although not highly rated as food, is edible and their liver was used to produce fish oil. The skin can also be made into leather and its bones are used in traditional Chinese medicine, although their is no independently verified medical evidence that basking shark bones have any medicinal qualities. Although there are no longer legitimate fisheries targeting this species they are caught illegally for their fins with the IUCN stating that a single large fin from a basking shark can sell for as much as $57,000 on the black market in China. Strikes by vessels are also believed to be a major threat to this species. Legislation to protect basking sharks, including protected areas for this species, is now in place in many parts of the world.

Basking Shark – Mistaken for a Sea Monster

Harper's Weekly 1868 "Wonderful Fish"

The “wonderful fish” featured in Harper’s Weekly, 1868.

The washed up and decomposed carcasses of basking sharks have long been mistaken for sea serpents or used as evidence for the existence of sea creatures which are either extinct or have never existed. The picture above is from Harper’s Weekly from 24th October, 1868 and shows the carcass of a massive fish washed up near New York. Originally thought to be a sea monster or serpent it is now believed to have been a basking shark. The scale between the creature and people is accurate, although it is unknown why the artist added reptile-like rear legs. One theory is that the carcass was partially decomposed when it was washed up, and the artist used creative licence to add a huge dorsal fin, larger eye and simply guessed that the remains of the rear fins were once a pair of legs.

Zuiyo Maru Carcass

Top one of the five photos of the Zuiyo Maru carcass, and below, an artists impression of what a plesiosaur would have looked like when alive. Although their is an undoubted resemblance the Zuiyo Maru carcass is now thought to be the remains of a basking shark.

Another famous case of a discovered sea monster, now thought to be basking shark carcass is the Zuiyo-Maru case from 1977. In April of that year the Japanese fishing vessel the Zuiyo-Maru was trawling off the coast of New Zealand. On hauling the nets on board the fishermen were shocked to find a 33ft long, 4000lb carcass of what appeared to be a prehistoric sea monster. The remains of the creature were hoisted into the air for examination, but the captain refused to allow it to be taken back to shore, due to its size, smell and the fact that it would spoil the rest of the catch if it was put in the hold. However, one crew member took five photographs and some tissue samples before the carcass was dumped back into the sea. Once the photographs were made public a great deal of interest was generated, with many people believing the corpse was that of a plesiosaur – an aquatic dinosaur believed to have died out at the end of the Cretaceous period, sixty-five million years ago. Several eminent Japanese scientists backed up this theory. Realising the potential importance of the carcass the trawler company instructed their crews to re-trawl the area where it had been dumped into the sea, but they were unable to recover it. However, the following year a team of Japanese scientists from the Tokyo University of Marine Science and and Technology carried out a report which cast serious doubt on the plesiosaur theory. They pointed out that the body was the same size and proportions of a basking shark, and that basking shark were known to decompose in such as way that the main body rots away leaving the pectoral and pelvic fins and upper head area, leaving a carcass which superficially looks like the remains of a plesiosaur. Today the Zuiyo-Maru case is widely thought to be explained by being the remains of a basking shark.

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