Fishing industry leaders have branded an attempt by environmental pressure group Greenpeace to stop a trawler fishing for cod as "suicidal".
Fishermen warned that the protest put lives at risk
Greenpeace members jumped into the North Sea directly in front of the fishing boat Endurance on Saturday.
The protesters claim overfishing could cause stocks of cod to be completely wiped out.
However, a spokesman for the Scottish Fishermen's Federation said the stunt had put lives in danger.
Greenpeace campaigners leapt into the water and swam in front of the fishing boat 40 miles off the coast of Unst, Britain's most northerly island, holding a red sign that read 'Stop - Cod in Crisis'.
The pressure group said its members were swept aside in the trawler's wake after it failed to change its course and had to be picked up by a Greenpeace inflatable boat.
The swimmers involved in the protest were from the Greenpeace ship Arctic Sunrise.
They have called for a halt to cod fishing and for large areas of the North Sea to become protected as 'marine reserves'.
Willie Mackenzie, a campaigner on the ship, said: "The fishing industry and politicians have ignored the scientists and continued to batter cod stocks.
"We're in the North Sea to save the cod from extinction.
"We've had to take action today to stop cod being caught because otherwise it will disappear from the seas and our dinner plates."