These unlicensed, rotting, illegal fishing vessels are crewed with slave labor
A lot of complicated diplomatic negotiations go into the laws that dictate which countries can take which fish from which international waters. But the business of high seas fishing is as lucrative as it is difficult to police. And who can say which fish really came from which waters? Many officials are inclined (with bribes) to look the other way. Others may not even realize there is a problem with a load of fish that are actually from mixed sources.
These are the market forces that drive the "Chinese zombie ships." These ships are former fishing vessels which were abandoned by their original owners, sold, or simply stolen. They chug slowly through the planet's oceans, fishing illegally, and selling their catch to unscrupulous "real" fishing vessels.
These pirate ships ply international waters, or anchor just off the coast of countries like Guinea with no effective navy or Coast Guard equivalent to police ships anchored in her waters. The ships are rotting away - maintenance costs money - and all safety measures have long since been abandoned. Leaking fuel and oil, fishing illegally, these ships rarely dock in port. Docking in port raises too many questions, and exposes the ships to the authorities.
