International conservation group Sea Shepherd rescued the crew of an alleged pirate ship that sank under mysterious circumstances on Monday, the group said. Sea Shepherd ships "Bob Barker" and "Sam Simon" rescued the crew of the "Thunder," a ship it called a “notorious poaching vessel,” in the exclusive economic zone of Sao Tome off the western coast of Africa.

Peter Hammarstedt, captain of the Bob Barker, said the ship seemed to have been intentionally sunk. “When my Chief Engineer boarded the Thunder in the hours leading up to the sinking, he was able to confirm that there were clear signs that the vessel was intentionally scuttled. Usually when a vessel is sinking, the captain will close all hatches so as to maintain buoyancy. However, on the Thunder, the reverse was done - doors and hatches were tied open and the fishhold was opened. It is an incredibly suspicious situation, to say the least,” he said in a statement.

He added that the Thunder was now "3,800 meters below the surface of the sea,” the Sydney Morning Herald reported. "What was incredibly strange was that the captain of the Thunder, when the vessel did actually sink, started applauding and cheering, which I took to be very unusual behavior and further evidence that he deliberately sank his own vessel."

The crew of the ship was received by the Sam Simon, where they were given food, water and medical attention. Sea Shepherd said it will hand over the Thunder’s crew to local authorities shortly.

The Thunder has used several different names and national flags, and has been accused of reaping over $60 million from illegal fishing, according to Interpol. Sea Shepherd had been pursuing the ship since December. It had named the Thunder part of the “Bandit 6,” a group of ships that engage in illegal, unregulated and unreported (IUU) fishing of whitefish in the Southern Ocean.

The conservation group claims it has recovered illegal fishing equipment, including gillnets that are 25km (16 miles) long, which the Thunder had allegedly illegally abandoned. The Bob Barker also registered several reports to transnational authorities that the Thunder had tried to ram the Sea Shepherd ship and had hurled grappling hooks and other projectiles at the ship during the four-month pursuit, according to the Sydney Morning Herald.

IUU fishing is a major threat to both ocean ecologies and the global fishing industry, with one in five fish caught worldwide being acquired unlawfully, costing the industry over $20 billion a year. The practice has been linked to overfishing, which has affected fish populations that were already over-exploited, including cod, salmon, trout and lobster.