Sea Shepherd rescues crew of pirate fishing ship it was chasing

Conservation group Sea Shepherd has come to the rescue of an illegal fishing vessel sinking off the coast of West Africa.

The crew of Melbourne-based vessel Bob Barker had been tailing the Thunder for several months, as it is believed the ship is involved in illegal fishing in Antarctic waters, ABC News reports.

Captain Peter Hammarstedt said he immediately responded to a mayday signal, received around 4pm (AEST) on Monday afternoon, by sending a small boat to offer assistance to Thunder's 35 crew.

A second Sea Shepherd vessel, the San Simon, was also on the scene and preparing to take on the crew.

The Australian Maritime Safety Authority has also been alerted and is relaying information to rescue authorities in Nigeria.

The Thunder is suspected of poaching tens of millions of dollars' worth of toothfish and has been issued with an Interpol notice for suspected illegal fishing activity.

Bob Barker discovered the vessel off the coast of Antarctica over three months ago, where it dumped its illegal gill nets and took off, Captain Hammarstedt said.

"Our suspicion is that the captain of the Thunder, knowing he's run out of fuel, has deliberately sunk his own boat in order to destroy any physical evidence," he said.

He said Bob Barker crew members who boarded the vessel once it had been evacuated found the ship's hatches had been left open, allowing the ship's compartments to flood as it sank.

"I was really hoping this was going to be a case that would be decided in a court of law and that's why we were determined to hand the Thunder over to law enforcement authorities," said Captain Hammarstedt.

"It seems like the captain of the Thunder felt the pressure was too much, that he would certainly see the inside of a jail cell and he'd rather see the evidence sink with his ship and that's the situation we're in."