Swimmers are at risk of being attacked in British waters by killer grey seals, a new report has warned. The large carnivorous mammals have 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. WARNING: CONTAINS GRAPHIC CONTENT . Seals are being blamed for a spate of mysterious mutilations of harbour porpoises in the North Sea. 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. Now tell-tale DNA found in fatal, sharp-edged wounds on porpoises around the southern North Sea has revealed a dark side to the creatures. Experts say the seals probably first grab their victims with their heavily armed beaks, before tearing off large sections of the calorie rich blubber. The mutilated body of a porpoise shows the fatal wounds believed to have been inflicted by a grey seal . 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. The attacks may have been learned behaviour causing by fishing nets. The study suggests that grey seals originally scavenged the bodies of porpoises which had become entangled in fishing nets and drowned. Once getting a taste for them, they then moved on to hunting the animals, and may soon be hunting humans, said the researchers. ‘Most of the porpoises are juvenile, so they were very small,’ Ken Collins, a senior research fellow, Ocean and Earth Science at University of Southampton told MailOnline . ‘Seals are very nippy underwater. Here in the south we have evidence of them catching salmon in the sea. They are quite fast and the thought is the seals moved from scavenging to attacking young porpoises.’ Many of the porpoises targeted  had thin blubber and lots of parasites. Although they can move faster than seals, they often rest at the sea surface. They may have been ambushed by the seals, and been too weak to react fast enough. Grey seals are also very territorial, and despite their cuddly reputation can be vicious creatures. Their usual diet consists of fish, crabs, squid, and at times sea birds. They grab onto their prey using claws on their front flippers while ripping into skin and flesh with their teeth. This same feeding style would have caused the injuries seen on the porpoises. Mr Collins said it is ‘extremely unlikely’ grey seals would attack humans given how they interact with divers. He also doesn’t believe this type of behaviour would extend to the common seal, who is smaller. 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 attacks may have been learned behaviour caused by fishing nets. The study suggests that grey seals originally scavenged the bodies of porpoises which had become entangled in fishing nets and drowned. Once getting a taste for them, seals then moved onto hunting the animals. ‘Most of the porpoises are juvenile, so they were very small,’ Ken Collins, a senior research fellow, Ocean and Earth Science at University of Southampton told MailOnline. ‘Seals are very nippy underwater. Here in the south we have evidence of them catching salmon in the sea. They are quite fast and the thought is the seals moved from scavenging to attacking young porpoises.’ Many of the porpoises targeted had thin blubber and lots of parasites. Although they can move faster than seals, they often rest at the sea surface. They may have been ambushed by the seals, and been too weak to react fast enough. Grey seals are very territorial. Their diet consists of fish, crabs, squid, and at times, sea birds. They usually grab onto their prey using claws on their front flippers while ripping into skin and flesh with their teeth. This same feeding style would have caused the injuries seen on the porpoises. 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'. They say mutilated carcasses with an open stomach or throat wound usually sink rapidly, so can go unrecorded. Animals that initially escaped an attack may also 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.
Experts discover grey seals are mutilating and killing harbour porpoises .
They use their sharp beaks to catch the porpoises and tear them apart .
Ravaged carcasses are being found on the shores of Dutch beaches .
Pictures show porpoise bodies with chunks of skin and blubber  ripped out .
Seals may originally scavenged the bodies of porpoises caught in nets .
Once getting a taste, they may have moved on to hunting the creatures .
Researchers believe the seals could also be a danger to swimmers .
Britain is home to  180,000 grey seals - a third of the world's population .
Swimmers are at risk of being attacked in British waters by killer grey seals, a new report has warned. The large carnivorous mammals have 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. WARNING: CONTAINS GRAPHIC CONTENT . Seals are being blamed for a spate of mysterious mutilations of harbour porpoises in the North Sea. 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. Now tell-tale DNA found in fatal, sharp-edged wounds on porpoises around the southern North Sea has revealed a dark side to the creatures. Experts say the seals probably first grab their victims with their heavily armed beaks, before tearing off large sections of the calorie rich blubber. The mutilated body of a porpoise shows the fatal wounds believed to have been inflicted by a grey seal . 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. The attacks may have been learned behaviour causing by fishing nets. The study suggests that grey seals originally scavenged the bodies of porpoises which had become entangled in fishing nets and drowned. Once getting a taste for them, they then moved on to hunting the animals, and may soon be hunting humans, said the researchers. ‘Most of the porpoises are juvenile, so they were very small,’ Ken Collins, a senior research fellow, Ocean and Earth Science at University of Southampton told MailOnline . ‘Seals are very nippy underwater. Here in the south we have evidence of them catching salmon in the sea. They are quite fast and the thought is the seals moved from scavenging to attacking young porpoises.’ Many of the porpoises targeted  had thin blubber and lots of parasites. Although they can move faster than seals, they often rest at the sea surface. They may have been ambushed by the seals, and been too weak to react fast enough. Grey seals are also very territorial, and despite their cuddly reputation can be vicious creatures. Their usual diet consists of fish, crabs, squid, and at times sea birds. They grab onto their prey using claws on their front flippers while ripping into skin and flesh with their teeth. This same feeding style would have caused the injuries seen on the porpoises. Mr Collins said it is ‘extremely unlikely’ grey seals would attack humans given how they interact with divers. He also doesn’t believe this type of behaviour would extend to the common seal, who is smaller. 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 attacks may have been learned behaviour caused by fishing nets. The study suggests that grey seals originally scavenged the bodies of porpoises which had become entangled in fishing nets and drowned. Once getting a taste for them, seals then moved onto hunting the animals. ‘Most of the porpoises are juvenile, so they were very small,’ Ken Collins, a senior research fellow, Ocean and Earth Science at University of Southampton told MailOnline. ‘Seals are very nippy underwater. Here in the south we have evidence of them catching salmon in the sea. They are quite fast and the thought is the seals moved from scavenging to attacking young porpoises.’ Many of the porpoises targeted had thin blubber and lots of parasites. Although they can move faster than seals, they often rest at the sea surface. They may have been ambushed by the seals, and been too weak to react fast enough. Grey seals are very territorial. Their diet consists of fish, crabs, squid, and at times, sea birds. They usually grab onto their prey using claws on their front flippers while ripping into skin and flesh with their teeth. This same feeding style would have caused the injuries seen on the porpoises. 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'. They say mutilated carcasses with an open stomach or throat wound usually sink rapidly, so can go unrecorded. Animals that initially escaped an attack may also 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.
hunt harbour porpoises. The UK is home to more than 180,000 grey seals, more than a third of the the creatures. Experts say the seals probably they are regular porpoise predators and may also have swimmers in their sights. The attacks may originally scavenged the bodies of porpoises moved on to hunting the animals, and may soon be porpoises showed chunks of skin, blubber and muscle ripped out, puncture wounds on the head Once getting a taste for them, seals then moved to be caused by the seals' sharp beaks and claws conservative'. They say mutilated carcasses with mutilated porpoises were found on Dutch shores the North Sea, half the world's population. Grey