A joint study of the Faculty of Veterinary Medicine (University of Utrecht), IMARES Wageningen UR and the NIOZ Royal Netherlands Institute for Sea Research found that grey seals were killing dozens of porpoises per year. The finding implies that it won't be long enough before humans become victims too.

Dozens of heavily mutilated porpoises are often found dead ashore in Dutch coastline and scientists have wondered of the animals' cause of death. There were also dead porpoises found in the U.K., France, Belgium and along the Assateague Island National Seashore in Maryland. Initial theory suggested that grey seals might be eating the porpoises, but no clear evidence was made to back up the claim.

As part of the investigation, the researchers collected DNA samples on the wounds of three dead porpoises found in August, October and December 2013. These samples were used to determine the real cause of death of the porpoises, whether they were attacked by another species or were victims of illegal fishing, according to the university release.

Based on the autopsies performed on more than 1,000 porpoises between 2012 and 2013, about 25 percent were bitten or purposely attacked by the grey seals on the same side of the body.

Grey seals feed on fish, but since their population is continuously rising, there is not enough fish to sustain them all. As a result, these species had found a new food: porpoise blubber.

Researchers are worried now because grey seals might start attacking humans as well, mistaking them as porpoises. Some porpoises and humans have the same size.

"People go on excursions to swim or dive with grey seals, unaware that these animals can be quite dangerous for human-sized animals. Most people consider them quite cute," study co-author Lineke Begeman, of Utrecht University's Faculty of Veterinary Medicine, told Discovery News.

The findings of the study serve as a warning to humans to be cautious when swimming near the grey seals. On a positive note, fishermen from the Netherlands were relieved as they were often accused of causing the deaths of the porpoises.

"These earlier assumptions caused emotional responses of those who were held responsible, and rightly so as these accusations were false," Begeman said.

This study was published in the Nov 26 issue of The Royal Society Proceedings B.