Sea Shepherd activists have rescued a number of crew who abandoned an alleged poaching vessel that has now sunk off the West African coast.

Some Thunder crew members are now onboard a Sea Shepherd ship, the Sam Simon, while others remain in life boats.

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Another of the group's ships, the Bob Barker, had been tailing the Thunder boat for several months and received a distress call from the vessel early on Monday.

Bob Barker captain Peter Hammarstedt said the activists would give evidence of illegal poaching activities to Interpol.

The crew of Sea Shepherd ship Bob Barker went to the aid of Thunder as it went down off West Africa. ( Supplied: Sea Shepherd )

They expect to reach an island port in the Gulf of Guinea today.

Captain Hammerstedt said the Indonesian crew were relieved to be picked up, though officers on board would have preferred a different rescuer.

"The Spanish officers are not happy campers. I don't think we were their preferred rescuer," he said.

"We'll make contact with the port authority there, we're already in contact with the coast guard and we're hoping to be able to get this crew ashore as quickly and as safely as possible."

Sea Shepherd's Adam Burling said the Thunder issued a distress signal about 4:00pm (AEST) on Monday.

The Australian Maritime Safety Authority was informed, and relayed information to rescue authorities in Nigeria.

Captain says vessel deliberately sunk

Sea Shepherd ship Bob Barker had been tailing the Thunder boat for several months and received a distress call from the vessel. ( Supplied: Sea Shepherd )

The Thunder was issued with an Interpol notice for suspected illegal fishing activity in 2013.

Sea Shepherd have been following the ship as part of its Operation Icefish.

"We've been following the internationally wanted poaching vessel Thunder for the past 110 days. They were at the limit of their fuel yesterday," Mr Hammarstedt said on Monday as the Thunder was sinking.

"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.

"I've sent a couple of my crew onboard the Thunder — my chief engineer examined the engine room, which is completely underwater.

"We also notice they've opened several hatches — those that the water, once it's filled one area of the ship, will quickly go into another and it's clear, in my opinion, that this ship was sunk."

He said he hoped Sea Shepherd would still see an end to the case.

"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."

Mr Hammarstedt said his crew had nonetheless managed to collect significant evidence of the Tunder's illegal activity.

"We have a lot of physical evidence on board our vessel that we need to hand over to Interpol," he said.

"They're suspected of a number of fisheries crimes, including human trafficking."

Local coastguard authorities met the Sam Simon and took charge of the Thunder crew.