The men [and it is almost always men] who get forced into slavery aboard those ships have often gone willingly, seeking work, says Val Farabee, director of research at Liberty Shared, an organization that fights human trafficking. But once isolated at sea, their wages are withheld and they’re subjected to violent, bleak working conditions for years.

Forced labor and slavery are terms used interchangeably by human trafficking experts to refer to people working against their will. Though well documented in ships that fish illegally, the fishing industries’ dizzying network of enforcement and regulation, as well as the vastness of the oceans, make it difficult for law enforcement to help those trapped on such ships.

It’s unclear how many people are held on fishing boats, but an estimated 21 million people are trapped in enslaved labor around the world, according to the International Labour Organization.