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.
Greenpeace denied suggestions it might be damaging marine life itself by dropping the rocks on the seabed.
"We have a very clear knowledge of this and are placing the stones next to the old reef, effectively extending it. There is no damage," Greenpeace oceans campaigner Iris Menn told Reuters.
The German Office for the Protection of Nature agreed the move probably did no ecological damage but expressed concern that it might hinder its talks with fishing groups.
"My worry is that at a time when we had been hoping to reach an agreement with fishermen, the Greenpeace campaign does not help dialogue," Henning von Nordheim, the Office's Scientific Director for Marine Nature Conservation, told Reuters.
He says British, Dutch, Danish and German fishermen are active in the area and that sand and gravel extraction is also a problem for the reef.
The European Commission has long argued for tighter curbs on bottom trawling, which involves dragging a cone-shaped net along the seabed, but getting a deal has proved difficult.