Sharks off our Shores
PUBLISHED: 11:37 20 June 2008 | UPDATED: 15:17 20 February 2013
In the July issue we look at the various species of shark that inhabit our waters and where and how to spot them in Cornwall.
Sharks off our Shores
Sharks are some of the most evolved creatures on our planet. Approximately 21 known species of sharks live or visit our coastal waters, not to be feared but to be appreciated. This summer you may catch a glimpse of one of them...
Sharks are intelligent, sophisticated animals that have been around on this Earth since long before dinosaurs existed; an evolutionary success story. There are over 450 known species of shark, living at all depths in waters all over the world, and each is specially adapted to its particular way of life, some being predators, others being parasites, grazers or scavengers. Britain's coastal waters are home to at least 21 species of shark.
Sharks have no bones; their skeleton is made of cartilage (or swim bladder) unlike bony fish. Their skin possesses small dermal denticles, which give the shark its rough, sometimes sandpaper-like texture.
They grow and mature slowly with long pregnancies, producing few young. This makes them susceptible to over-fishing. There is a demand for shark products for food, medicine and cosmetics, and millions of sharks are killed each year for these purposes. This has led to a huge decline in the numbers of sharks found in our seas, so although they inhabit our waters, you are unlikely to encounter them during a swim in the sea. Here in the UK, over 50% of our shark species are considered to be under threat, so any encounter should be considered a privilege.
Lesser Spotted Dogfish (Scyliorhinus caniculus) (aka Rough Hound or Cat Shark)
These are small sharks, growing up to a maximum of 75cm, and the most common encountered by divers around the coast of Britain on sand or mud in shallow water. You will often see these pretty creatures dozing on the seabed during the day, sometimes in pairs and even small groups.
Characterised by a blunt head with a rounded snout, its slender body, often dark brown or grey in colour and covered in smaller spots, has small, rounded pectoral fins positioned towards the back of the body. This shark breathes by pumping water over its gills by opening and closing its mouth.
Feeding at night, it relies heavily on its sense of smell, hunting crabs or shellfish. It is during the spring that the females come into the shallow waters around our coastline and lay their distinctive brown capsule eggs (or mermaid's purse), which they attach to seaweeds, rocks or sea grass with the long tendril on the corners. After nine months a miniature dogfish emerges.
Porbeagle, or Mackerel Shark (Lamna nasus)
The Porbeagle can be found in surface waters down to a depth of over 700m but can also occasionally venture into close inshore waters. It can reach up to 3.5m in length and is a dark metallic-blue colour with white underneath.
Porbeagles are part of the mackerel shark family, which include sharks such as makos and great whites, and are a fast-swimming species. Mackerel sharks can become actively warm-blooded by modifying their circulatory system. Warm blood means warm muscles, which enables the shark to react more quickly and swim more powerfully so that it can pursue cold-water fish such as cod, herring or haddock in our cold, temperate waters.
Like much of the biology of the mysterious porbeagle shark, little is known about courtship or mating, although they are known to give birth to live young at a mature age of 13 years. Gestation is thought to last 8-9 months, after which litters of 1-6 shark pups are born.
Porbeagle sharks are critically endangered in the North-east Atlantic due to over-exploitation for their meat and from being accidentally caught in fishing nets. They are listed as endangered on the International Union for Conservation of Nature's Red List.
Blue Shark (Prionace glauca)
The blue shark is large and indigo-coloured and must keep moving for its entire life. It is sleek with long, scythe-like pointed fins and a long upper lobe to the tail fin that prevents it from sinking. Its tapered body, growing up to 3.8m, makes it a graceful and efficient swimmer. A blue shark breathes by allowing the seawater to flow freely into its open mouth, through the pharynx, over their gills and out of the gills' slits.
Found offshore, visiting our Cornish waters in packs of a hundred or more individuals, this shark travels over incredible distances, using a combination of drifting in ocean currents and its own efforts to clock up to 44 miles a day. These long-range movements include crossing the Atlantic from the North American coast to Europe. It has been discovered that it is the females who make this journey, while males stay in the North-west Atlantic. The reason for this is not yet known. The blue shark's diet consists mostly of squid, but it will eat almost anything as it is an opportunistic feeder.
The female can self-fertilise when she reaches sexual maturity at around five years old, giving birth to live pups after a gestation period of almost one year.
Basking Shark (Cetorhinus maximus)
Growing up to 12m long and weighing up to 7 tonnes, this is the second-largest shark in the world and a resident of our British waters. Dark grey or greyish-brown, the basking shark has an unusually long snout.
Although generally elusive, these creatures often cruise the Cornish coast in the spring and summer in search of their food source, zooplankton, tiny, microscopic animals floating in the water column. Like many sharks, the basking shark has excellent senses. It feeds by swimming along with its huge mouth open wide, filtering out seawater through specially adapted gill rakers; a 7-metre shark can filter out up to 25kg of plankton in one hour. These sharks become sexually mature at around 20 years old.
These sharks were once heavily fished for their liver oil, meat and fins, a factor which, when combined with its late maturation age and low number of young born, has led to the species being officially listed as vulnerable.