On average some 800 harbour porpoises (Phocoena phocoena) are found on Dutch beaches annually. They are one of the smallest marine mammals, up to roughly 6 feet and about 170 lbs, and are also called "puffer" or "puffer pig" by fishermen in New England and eastern Canada.

For several years, though, the cause of heavily mutilated porpoises washing up was a complete mystery. At times as many as 5 carcasses would wash ashore, and over the winter of 2009 record numbers were found. Oddly enough this was only happening in Holland.

After deliberations by marine organizations, it was decided to call in the Dutch Forensic Institute (NFI), to investigate the deaths. NFI is the only institute in the world with forensic knowledge of animal pathology. Suspicions that they had been caught in nets and cut out by fishermen went unconfirmed. On 49 supervised trips, only one was caught in the nets that clearly had been dead a while. Another possible cause, ship’s propellers, was found to be unsubstantiated as well.

Now Belgian scientists have strong evidence that grey seals (Halichoerus grypus) are the culprit. After examining wounds on porpoises, they found dental imprints. The distance between the canine prints in these wounds closely match those of grey seals. Seals had been known to attack and eat large fish such as cod, as well as the occasional bird, but a porpoise is considerably larger than any other known prey. Killer whales have been around a long time, apparently now Holland has killer seals.

Grey seals are up to 11 feet long, and weigh up to 770 lbs, so the porpoises really are no match for them. So far it is a limited number of seals that have learned this new fishing technique, in Zeeland, in the south of The Netherlands, and around Den Helder and the isle of Texel, in the north. Evidence as to whether the porpoises were caught alive as yet has not been found, but it appears the seals have found new prey. Research on the case will continue.

Mardik Leopold, of Imares, Institute for Marine Resources & Ecosystem Studies, said that - unlikely as it may seem now - future human attacks cannot be ruled out. As yet, however, people have little to fear, he concluded.

JB