Images of tens of thousands of dead fish floating on the Atlantic Ocean off the coast of France angered environmental groups and authorities in an incident blamed on a huge fishing and factory ship.

Dutch-owned trawler FV Margiris, the world's second-biggest fishing vessel, shed over 100,000 dead fish into the Atlantic Ocean off France, forming a floating carpet of carcasses that environmental campaigners spotted.

The spill, which happened on early Thursday, was caused by a rupture in the trawler's net, said fishing industry group PFA, which represents the vessel's owner. In a statement, the group called the spill a "very rare occurrence."

An environmental group disputed that account, alleging it was an illegal discharge of over 100,000 unwanted fish that covered an area of approximately 3,000 square meters (32,291 square feet).

The French arm of campaign group Sea Shepherd first published images of the spill, showing the ocean's surface covered by a dense layer of blue whiting, a sub-species of cod, used to mass-produce fish fingers, fish oil and meal.

Sea Shepherd France said it did not believe the incident was accidental, but rather an attempt by the trawler to discharge a type of fish that it did not want to process, a practice known as discharging bycatch which is banned under European Union fishing rules.

Lamya Essemlali, head of the campaign group in France, told Reuters she believed the fish were deliberately discharged. Sea Shepherd France said the spill affected over 100,000 fish.

The group said that at least four trawlers, including Margiris, have been operating in the Bay of Biscay off the French port city of La Rochelle.

France's Maritime Minister Annick Girardin called the images of the dead fish "shocking" and said she had asked the country's national fishing surveillance authority to launch an investigation into the accident.

Virginijus Sinkevičius from the EU's Ocean and Fishing Commission also stated that the body is initiating an investigation.

Trawlers like the Margiris use drag nets measuring over a kilometer in length and process the fish in onboard factories, a practice heavily criticized by environmentalists.

Following protests by activists, the Margiris was forced to leave Australian waters in 2012.

Traffic data by marinetraffic.com on Friday showed the vessel, owned by the Dutch company Parleviliet & Van der Plas and sails under the flag of Lithuania, was still engaged in fishing activities off France's coast.