GREENPEACE activists are expanding a barrier on the seabed of a fishing ground in the North Sea to prevent “bottom trawling” — the dragging of weighted nets across the seabed to catch fish.

The environmental campaign group vowed to continue dropping granite boulders into Dogger Bank, one of the North Sea’s most productive fishing grounds. Marine Management Organisation has warned against the action, although it poses no harm to the seabed.

Greenpeace has already shielded some 50 square miles of the sea against trawling.

Dogger Bank has marine protected status, which is supposed to conserve fish stocks, allowing them to recover from over-fishing.

But Greenpeace said that bottom trawling is continuing, destroying the seabed and wiping out sea creatures which are a vital part of the marine food-chain.

Dogger Bank is a shallow sandbank home to crabs, starfish, flatfish and sand eels.

Richard Casson of Greenpeace UK said: “Allowing bottom-trawling in a protected area makes about as much sense as allowing bulldozers to plough through protected forests.

“That’s why so many of us have already taken a stand.”