Greenpeace activists have dropped 18 large boulders on the seabed off the coast of south-west England to disrupt destructive industrial fishing.
The rocks were dropped on Thursday from Greenpeace's Arctic Sunrise research vessel in an area of the South West Deeps (East) Conservation Zone, about 120 kilometres off the west coast of Cornwall.
Campaigners say the limestone boulders, which weigh between 500 and 1,400 kilograms, will make bottom-trawling off limits in the area, which is one of the most heavily fished in Britain.
Anna Diski, UK oceans campaigner at Greenpeace UK, said the action would make it "impossible for them to drag the heavy fishing gear along the seabed, destroying the habitat and disturbing the carbon".
Artists created a giant ammonite sculpture, inspired by the fossil often found in Portland limestone, using one of the boulders, which was also placed on the seabed.
The names of celebrity backers such as Stephen Fry and chef Hugh Fearnley-Whittingstone, were inscribed on the rocks.
Greenpeace UK's head of oceans, Will McCallum, said the boulders were dropped to protect the ocean as a "last resort" because of government inaction on the problem.
Mr McCallum said it was outrageous that bottom-trawlers are allowed to operate on the seabed in protected areas.
"They destroy huge swaths of the marine ecosystem and make a mockery of our so-called 'protection'," he said.
The action comes after the latest round of UN talks to try to secure protection for marine life in international waters broke up without agreement.
Greenpeace said the 4,600-square-kilometre South West Deeps is "one of the most heavily fished so-called Marine Protected Areas in the UK".
It cited figures from the Global Fishing Watch monitoring agency that said 110 vessels fished for 18,928 hours in area in the 18 months to July.
Of that, industrial vessels with bottom-towed fishing gear spent 3,376 hours fishing in the zone.
Bottom-trawling is banned in only four of the UK's 76 offshore Marine Protected Areas, and the government is consulting over the possible bans in a further 13.
"The problem is that the majority of the UK's MPAs don't have any actual protection at all," said Jasmine Watkiss, one of those on board the Arctic Sunrise.
"The government needs to get serious about ocean protection before it's too late.
She said that the next UK prime minister should ban industrial fishing in protected areas by making changes to commercial fishing licences.
Neil Whitney, a fisherman from East Sussex in southern England, said bottom-trawling was "like ploughing a combine harvester through a national park".
"They're able to take out entire ecosystems, and if they cause a fishery to collapse, they just move on to the next one," he said.
He said it was absurd that bottom-trawling was legal in MPAs. "MPAs are supposed to be the areas where fish stocks can recover, so that we can fish for generations to come.
"It's a case of common sense."