The trawler FV Margiris, the world’s second-biggest fishing vessel, deposited the mass of fish into the Atlantic Ocean near France, which left a floating carpet of fish carcasses on top of the water.
The French campaign group Sea Shepherd published photos of the spill which showed the ocean’s surface covered in a dense layer of white fish.
The country’s fisheries minister, Annick Girardin, called the images “shocking” and has called on the national fishing surveillance authority to launch an investigation.
The spill, which happened on Thursday, was reportedly caused by a rupture in the trawlers net, said the fishing industry group that represents the vessel’s owner.
The Pelagic Freezer-Trawler Association (PFA) said the fish were blue whiting, a sub-species of cod used to mass-produce fish fingers, fish oil and meal. In a statement, the group called the spill a “very rare occurrence”
Environmental campaigners have disputed this and claimed the fish were illegally dumped.
Campaign group Sea Shepherd – who first published photos of the spill – said it believed the incident was an attempt to release a type of fish it didn’t want to process, which is a banned practice under EU fishing laws.
Trawlers like the FV Margiris use nets over a kilometre long that drag along the sea bed and catch fish – a practice heavily criticised by environmentalists.
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“We are launching an inquiry to national authorities of the fishing area & presumed flag state of the vessel, to get exhaustive information & evidence about the case.”
The PFA’s statement said that the drag net had been ruptured at 5:50 local time (4:50 GMT) on Thursday morning.
It said: “In line with EU law, this has been recorded in the vessel’s log book and reported to the authorities of the vessel’s flag state, Lithuania,” it said. It added that the dead fish would be subtracted from the vessel’s quota.”