Transshipment is regarded as such a dirty practice that there have been numerous pushes to put an end to it. After a two-year campaign , Greenpeace finally pressured the world’s largest tuna canning company, Thai Union—parent to the major U.S. brand Chicken of the Sea—to announce a moratorium on transshipment at sea throughout its global supply chain, as well as to implement other changes towards more sustainable fishing. Still, for now, entire fishing industries rely on the practice. As coastal countries have drained the fish stocks close to their shores, it has become increasingly lucrative to send fishing fleets into remote waters of the globe. To keep them fishing there for as long as possible, large carrier ships are sent out to resupply them with fuel and provisions and to collect the catch, which is frozen on board and brought back to port. It is thanks to this “distant-water” fishing model that hardly a corner of our oceans is left untouched. And while economical, this model comes with a host of problems.

For one, keeping fishing boats out at sea for prolonged periods sets up conditions that can lead to human rights abuses. Thailand has become the poster child for this, especially since a high-impact Associated Press exposé in 2015 showed that Thai-caught fish consumed around the world were likely tainted by slave labor. Just last year, Greenpeace published an investigation into a group of rogue Thai fishing vessels in a remote bank in the Western Indian Ocean. The ships were crewed by trafficking victims forced to work in horrendous conditions, some of them so malnourished that they died of an arcane vitamin deficiency disease. The workers were visited only occasionally by the ‘motherships’ that came to collect the fish, which ended up in Thai factories that supply to major pet food brands like Nestlé Purina PetCare. (Nestlé banned transshipment at sea in its supply chains earlier this year.)

Transshipment is also a major entry point for illegally caught fish to slip into the supply chain. The big cargo ships that pick up the catch we eventually eat can rendezvous with many fishing boats along their way, making it possible to launder contraband catch in with legally caught fish.

Yes, those larger ships are supposed to keep logs to make this more difficult. But a lot of documentation on fish trade at sea is made on paper.

“When you only have written logbooks and captains’ statements and this is taking place on the high seas away from any effective enforcement, you can’t be certain that there wasn’t any illegal activity associated with the catch”, explains Graham Forbes, Greenpeace’s global seafood markets project leader, who worked on the Thai Union campaign.