Sea Shepherd, the international Conservation Society is chasing a French trawler which it said left Malta last Sunday despite not having a quota for bluefin tuna fishing.

The Sea Shepherd vessel Steve Irwin spotted five French vessels leaving Malta at high speed on Sunday, bound for Libyan waters.

One of the vessels, the Sainte Sophie Francois 2, does not have a bluefin tuna quota for the 2011 season and is not allowed to take part in any such fishing operation, Sea Shepherd said on its website.

The International Commission for the Conservation of Atlantic Tuna (ICCAT) confirmed that Sainte Sophie Francois 2 recently departed from Malta and had no legal right to be carrying nets this year.

Approximately one hour after leaving Malta, the four purse seiner vessels with a quota turned off their Automatic Identification System (AIS) even though this is prohibited, and they disappeared off the radar, Sea Shepherd said.

The Steve Irwin dispatched its helicopter to fly over the area and spotted the Sainte Sophie Francois 2.

"No sooner did the helicopter fly over, the vessel turned off its AIS, changed course and started to head towards Tunisian waters."

The other four vessels were the Jean Marie Christian 6, Jean Marie Christian 7, Sainte Sophie 3 and Cisberlande 5. All have been allocated bluefin tuna quotas this year. Their whereabouts are unknown.

The Brigitte Bardot is deep inside Libyan waters searching for poaching operations.

The Sea Shepherd ships continuing to patrol the waters off the coast of Libya searching for tuna poachers which, it said, are taking advantage of the war in Libya to illegally catch endangered bluefin tuna. All tuna fishing in these waters is prohibited under ICCAT and European fishing regulations.

Sea Shepherd had a number of clashes with Maltese and Malta-based tuna fishermen last year and accused one of the trawlers of ramming its vessle. The society said it had cut the fishermen's nets, saving some 800 tuna.