They spent up to 10 years at sea, toiling in some of the harshest conditions distant-water fishing crews ​can face. Many never set foot on land because their Chinese captains did not want them to be seen by the port authorities. Most of their salaries went directly to their government, and some of their catch has likely ended up on dining tables in Europe and Asia.

These were North Koreans assigned by their government to work on board Chinese tuna longliners operating in the Indian Ocean, according to a report ​published on Monday by the London-based Environmental Justice Foundation. Through them, North Korea’s leader, Kim Jong-un, established new source of revenue ​for his cash-strapped regime.

The United Nations bans member states from hiring North Korean workers because its Security Council says Mr. Kim’s government uses them to raise funds for its nuclear weapons program. But Mr. Kim sends tens of thousands of his people abroad to earn cash and other benefits for his regime.

They have worked in factories and restaurants in China, logging camps and construction sites in Russia and farms and shipyards in Eastern Europe​. They have sweated on construction sites in the Middle East and built monuments for dictators in Africa. An estimated 11,000 North Koreans were sent to fight for Russia in its war against Ukraine.

Until now, the fate of North Korean fishermen has drawn little global attention because they were in vast oceans, cut off from the rest of the world and even their own families for years at a time.

The conditions they faced “would constitute forced labor of a magnitude that surpasses much of that witnessed in a global fishing industry already replete with abuse,” the foundation said in its report, shared with The New York Times ahead of its publication.