BERLIN (Reuters) — Environmental group Greenpeace started dropping three-tonne granite rocks on the North Sea bed off Germany to try to stop trawling it says is decimating fish, porpoises and other marine life.

Destructive fishing methods, such as deep net trawling near the seabed, are depleting stocks of plaice and sole near the Sylt Outer Reef and destroying the reef itself, a feeding ground for creatures such as common and grey seals, say activists.

The stone reef, an EU-protected area, is similar to a coral reef but made of rock, and is located off one of Germany's North Frisian Islands, near Schleswig Holstein and Denmark.

"The harbour porpoise population is one of the most threatened species of whale and dolphin in Europe and giving protection to this species was one of the primary objectives for the area," said Greenpeace.

The organisation is demanding Germany and the EU enforce a ban on heavy net bottom trawling in the protected area.

The German Office for the Protection of Nature is in talks with fishermen to stop destructive fishing methods.

German fishermen, however, condemned Greenpeace, saying the rocks could damage fishing boats and even endanger human life.

"We believe what they've done is illegal and risks the lives of fishermen," Peter Breckling, general secretary of the German Fishing Association, told Reuters. He denied German fishermen used nets in the area and insisted the reef was not in danger.