Blue sharks: summer visitors

Blue sharks summer visitors

Before the late 1940s the blue shark was only known to live in British seas from occasional specimens caught in the western English Channel. They were found tangled in the drift nets set for pilchards. Apart from the nuisance value – they rolled themselves in the nets and it took a long time to untangle them, and even longer to repair the nets—they were regarded as rather rare fish in British waters.

However, just after the end of World War II Brigadier J.A.L.Caunter began to explore the possibility of catching blue sharks from his boat off Looe, Cornwall. He found that in summer blue sharks were common off the Cornish coast, and from then on shark fishing out of Looe and other coastal ports became a small industry. (Sadly, their num- bers have dwindled over the years because of overfishing.)

True blue

The blue shark is easy to recognize – its long, slimline body, large, upper lobe to the tail and immensely long, sickle-shaped pectoral fins are characteristic. The back is deep blue, the sides a brilliant blue and the belly translucent white. This vivid, distinctive colouring fades to a dull grey soon after death.

Mainly a surface-dwelling shark, it lives to depths of 150m (500ft). Where it occurs is dictated mainly by temperature: it prefers water that is between 7-16°C (45-61°F), but can tolerate temperatures up to as much as 21°C(70°F).

As a result, the blue shark only turns up off British coasts from mid June to the end of September – the water is simply too cold for the rest of the year. It is found off the southern and western coasts of Ireland for more or less the same period, and is occasionally encountered off the coast of western Scotland.

Clockwise migration

Tagging blue sharks has shown they make a circum-Atlantic migration. Leaving British coasts at the end of September, they turn southwards, making a huge clockwise sweep into the tropical Atlantic before crossing southwards towards South America. Here they pick up the warm water of the North Atlantic Drift, a strong current flowing north-eastwards across the northern Atlantic. Passing close to the Azores before hitting south-west Britain, they split into two – one branch continuing up the west coast of Ireland, the other turning south to the Bay of Biscay.

In some years the current is stronger and more persistent than usual and this accounts for the presence of blue sharks in the northern North Sea.

Nocturnal feeders

The blue shark feeds mainly at night on various small fish and squid. In British seas its diet consists mainly of pilchards, herring, mackerel, skippers and garfish. It also feeds on offal and rubbish from boats.

Anglers attract sharks by hanging a bag of mashed bits of oily fish – rubby dubby -over the side, then dropping their bait into the scent trail it creates. Sharks can follow scent trails in the water for long distances and, with their acute sense of smell, can detect wounded fish from a considerable distance.

Single-sex schools

The vast majority of sharks around British coasts are immature females – about one in 5000 is male – and very few are mature. Where the sexes meet and mate is still something of a mystery.

Blue sharks are one of the shark species that bear live young. The number of blue shark pups born at any one time varies with the size of the mother – the greatest number recorded is 135, but in British seas litters of between 20 and 30 are usual (though few have been reported). The young are about 38-46cm (15-18in) long at birth.