Scientists are urging swimmers in Britain to avoid gray seals, after a recent revelation that the seals are behind the death and mutilation of hundreds of harbor porpoises.
Scientists discovered harbor porpoises beached on the coast of Netherlands, but they had no idea why the porpoises were dying. Some people suspected that fishermen were harming the porpoises. However, a group of scientists that studied the marine mammals for eight years determined the cause of death: gray seals. The carnivorous seals had learned a deadly new trick, and were killing porpoises.
Furthermore, the scientists said that they had no reason to believe gray seals wouldn't turn their deadly selves on humans, sooner or later. Gray seals are much larger than humans, weighing 551 pounds on average, and also larger than harbor porpoises, which average around 55 pounds.
"People go on excursions to swim or dive with gray seals, unaware that these animals can be quite dangerous for human-sized animals. Most people consider them quite cute," said Lineke Begeman, co-author of the study.
Gray seals can hold down larger prey with their front appendages and use their teeth to break down larger animals into pieces small enough for them to swallow. This allows them to hunt and kill animals larger than they are.
The team of researchers analyzed bite marks on over a thousand of the dead porpoises and found that they matched the teeth of gray seals. The team also found gray seal DNA on some of the porpoises.
The researchers said that not every porpoise attacked by a gray seal ended up dead. Sometimes the seal may have just bitten the porpoise and then let it go, or else the porpoise escaped.
"Some attacked porpoises evidently got away, which is to be expected with a predation attempt. In a few cases, the seal may just have been aggressive, rather than hungry, or may have been disturbed during the encounter," said Mardik Leopold, the lead author of this study, which was published in the latest issue of Proceedings of the Royal Society B.
There are no known cases of gray seals killing humans, but there have been reports of humans being bitten by a gray seal and then let go.
Bitten harbor porpoises have also been observed in the U.K., France, Belgium, and along the Assateague National Seashore in Maryland, all areas where gray seals also are found.
