File photo: For the better part of a decade, hundreds of harbour porpoises washed up along the southeastern coastline of the North Sea coastline.

Paris - It seemed like an open-and-shut case - a beach mystery that a 10-year-old detective with an ice cream and some time on his hands could figure out.

For the better part of a decade, hundreds of harbour porpoises washed up along the southeast coastline of the North Sea coastline.

Their lifeless bodies were gashed with horrible injuries. The slash marks pointed to a pretty obvious perpetrator - some apex predator of the sea or maybe a ship's propellor.

Instead, many of the slayings should be laid at the flippered feet of the innocent-eyed grey seal, says a team of detective biologists in the Netherlands.

DNA found in bite wounds implicate grey seals in the massacre... suggesting the mammals may also pose a risk to humans, the researchers said.

“Tell-tale grey seal DNA found in the bite wounds on porpoise carcasses reveals these seals to be more than cuddly, friendly animals,” said a summary of the study published in the British journal, Proceedings of the Royal Society B.

“They probably first grab their victims with their heavily-armed beaks (jaws) before tearing off large sections of the calorie-rich blubber.”

The sleuths began to suspect seals after finding telltale DNA in bitemarks on three mutilated porpoise carcasses.

They compared the wounds with those in photographs from post-mortem examinations of another 1 081 animals stranded between 2003 and 2013.

They looked at whether the wounds were consistent with seal bitemarks, whether they matched a hunting or scavenging bite pattern and whether the porpoise had flesh marks associated with having been caught in a net when attacked.

Grey seals were the cause of death in at least 17 percent of the cases, the team found.

“This makes predation by grey seals one of the main causes of death in harbour porpoises currently stranding in the Netherlands,” they wrote.

The phenomenon had also been observed in northeast England - prompting shark warnings for bathers.

But the Dutch team said their findings suggested swimmers should be wary of seals as well.

“Many of the mutilated porpoises were found on Dutch shores used frequently by human bathers and surfers, and there would appear to be no a priori reason why humans may not be at risk from grey seal attacks.” - Sapa-AFP