Police arrested environmental activist Captain Paul Watson on his ship Sunday in the town of Nuuk, Greenland. Watson is a prominent campaigner against commercial whaling.

When he was arrested, Watson was apparently engaged in a mission to traverse the Northwest Passage, a series of waterways through the arctic archipelago of Canada that connects the Atlantic Ocean to the Pacific Ocean. The goal was to intercept the newly-built Japanese vessel, “Kangei Maru,” seen by many as “the most dangerous whaling ship in the world.” According to the Captain Paul Watson Foundation, the crew had stopped to refuel in Nuuk when they were boarded by Danish federal police.

While the foundation said the arrest may be connected to a prior Interpol Red Notice—or international arrest warrant—for Watson’s prior anti-whaling activities in the Antarctic, the foundation later added that the arrest was in relation to Watson’s anti-whaling operations in the Faroe Islands. The arrest came as a surprise to Watson and his organization, as they were under the impression that the Red Notice had been withdrawn.

Watson and his crew of mainly volunteers fight against commercial whaling by using techniques to disrupt whaling methods. While Watson says he and his team are merely enforcing Japan’s obligations to the International Convention for the Regulation of Whaling, Japan claims that their actions constitute dangerous piracy on the high seas.

In 2013, the US Court of Appeals for the Ninth Circuit ruled that Watson’s previous US-based anti-whaling group Sea Shepherd Conservation Society (SSCS) were modern-day pirates, which cleared the path for Japanese whalers to pursue legal action against them. The whaling group, Institute of Cetacean Research (ICR), claimed that SSCS was preventing them from engaging in what they said was lawful activity. The whalers claimed their activities were permitted under Article 8 of the International Convention for the Regulation of Whaling, which allows for the taking of whales if a signatory nation has issued them a license. ICR possessed a license issued by Japan. Writing the opinion for the appeals court, Chief Judge Alex Kozinski said of SSCS at the time:

You don’t need a peg leg or an eye patch. When you ram ships; hurl glass containers of acid; drag metal-reinforced ropes in the water to damage propellers and rudders; launch smoke bombs and flares with hooks; and point high-powered lasers at other ships, you are, without a doubt, a pirate, no matter how high-minded you believe your purpose to be.

After Watson’s arrest on Sunday, the Captain Paul Watson Foundation reported that he was transported to Denmark.