Amsterdam - Dutch police on Thursday said the sudden increase of mutilated porpoises washing onto the Dutch coast remains a mystery.

Between December and early March, more than 113 of the mammals had washed ashore on the northern Dutch coast. Almost half of them were severely mutilated, the cadavres showing multiple knife cuts or being cut in entirely half.

Dutch police said a microanalysis invasive trauma team of the veterinary faculty of Utrecht University and the national forensic institute were to investigate and study the remains of the mammals.

Justus van der Broerk, spokesman of Ecomare, an institute for the preservation of the Wadden sea nature reserve in the northern Netherlands, said "no one saw anything ... However, we suspect the porpoises got caught in the fishing nets of some fishing boats, and subsequently drowned."

He added fishermen often prefer to cut a porpoise to death after it gets caught in their nets, rather than damage their relatively expensive fishing nets or risk a fine for having caught one, a protected mammal.

Ecomare said Dutch authorities should prohibit the type of fishing nets currently still used by many Dutch fishermen, and require them to use a type of net which makes a noise that deters porpoises and so prevents the animals from becoming caught. (dpa)