An alleged poaching vessel is sinking off western Africa.

Nigerian-flagged Thunder has been pursued by Sea Shepherd activist vessel 'Bob Barker' for more than 100 days.

Captain of Bob Barker, Peter Hammarstedt, says Thunder suddenly began taking on water today.

"They put out a mayday distress call, that distress call is that they are sinking, and we can clearly see from the deck of the Bob Barker that they are taking on water."

Captain Hammarstedt says all crew of the Thunder are now in Sea Shepherd life rafts.