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Swimmers could be attacked in British waters by killer grey seals, warns a new report.

Britain's largest carnivorous mammal has been blamed for a spate of mysterious mutilations of harbour porpoises in the North Sea.

Many of the carcasses were washed up on Dutch shores frequented by human bathers who may also be endangered by attacks from the vicious predators. The grey seal's preferred diet is mostly fish, and it had never been previously known to hunt harbour porpoises.

The UK is home to more than 180,000 grey seals, more than a third of the world's population, with about 90 per cent around Scotland.

WARNING: CONTAINS GRAPHIC CONTENT

Seals are being blamed for a spate of mysterious mutilations of harbour porpoises in the North Sea.

Now tell-tale DNA found in fatal, sharp-edged wounds on porpoises found around the southern North Sea has revealed these seals to be more than the cuddly, friendly animals that have enchanted tourists for decades.

Experts say the seals probably first grab their victims with their heavily armed beaks, before tearing off large sections of the calorie rich blubber.

Evidence from 1,081 autopsies indicates grey seals to be involved in 25 per cent of the cases, showing they are regular porpoise predators and may also have swimmers in their sights, reports the journal Proceedings of the Royal Society B.

Biologist Dr Mardik Leopold, of the Institute for Marine Resources and Ecosystem Studies at Wageningen in Holland, said: 'We show a substantial proportion of harbour porpoises that stranded on the Dutch coast were mutilated by grey seals.

'We also conclude most cases involved active killing and only a small proprtion can be attributed to post mortem scavenging.

'This makes predation by grey seals one of the main causes of death in harbour porpoises currently stranding in The Netherlands.'

Photographs of the dead porpoises showed chunks of skin, blubber and muscle ripped out, puncture wounds on the head and broken jaws, revealing the ferocity of the attacks. Acute haemorrhages indicated the wounds were inflicted during life, just before death. Partly digested prey in the stomachs showed they had recently fed.

The mutilated body of a porpoise shows the fatal wounds believed to have been inflicted by a grey seal

The harbour porpoise's wounds (pictured) are thought to be caused by the seals' sharp beaks and claws

The researchers believe their estimate of grey seal attacks on porpoises is 'probably conservative' as mutilated carcasses with an open stomach or throat wound usually sink rapidly, so going unrecorded. And animals that initially escaped an attack may have died later from wounds inflicted.

Dr Leopold said: 'The porpoise population may suffer in ways other than loss of individuals as most of the mutilated animals were healthy and fat prior to the attack suggesting grey seals primarily target juvenile harbour porpoises that are in prime condition and so probably reduce recruitment to breeding age.'

The first grey seal victim was found in 2003, but without accurate information from earlier years it is not known when or why the behaviour first occurred.

Bodies of the porpoises have been washing up on beaches in the Netherlands (pictured) that are popular with swimmers

Bodies of mutilated carcasses have washed up on the popular Dutch island of Texel (pictured), just north of Amsterdam

A grey seal stalks the waters of Duncansby Head, in Caithness, Scotland. The predators are now thought to be responsible for widespread predation of harbour porpoises in the North Sea

Dr Leopold added: 'Many of the mutilated porpoises were found on Dutch shores used frequently by human bathers and surfers and there would appear to be no reason why humans may not be at risk from grey seal attacks.'

The harbour porpoise, one of the smallest marine mammals at just over six feet long, is not under threat of extinction. There are about 350,000 in the North Sea, half the world's population.

Grey seals can reach a length of up to 11 feet and a weight of more than 660lbs (300 kilograms). They can outweigh a harbour porpoise by a factor of four or more. 

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