Fifteen Vietnamese citizens were among 26 sailors saved Sunday from being used as slave labour on two fishing boats off San Jose in Costa Rica.
The police had been inspecting the boats for the past four months after nine Vietnamese crewmen escaped from the boats, swam ashore and told their stories.
Among the men rescued were 13 Indonesians, five Filipinos, two Taiwanese and one Chinese.
San Jose officials said the victims were forced to work 20 hours a day, and usually starved and beaten. The men said they were promised US$250 a month in the contracts they signed, but they had received nothing.
"They were in completely unsanitary, inhumane, overcrowded (conditions)," Jorge Rojas Vargas, director of Costa Rica Judicial Investigation Agency (OIJ), was quoted by local newspaper La Nacion as saying.
Mario Zamora, director of Costa Rica migration agency, said: "This is modern slavery."
The identity of the boats' owners has not been released. But authorities have arrested two Taiwanese and a Costa Rican. Managers on the boats who also came from Taiwan said they had sent the men's wages to their families.
Police have impounded the passports of the Taiwanese.
Further investigations have been launched as Costa Rica authorities suspect it is a case was a human trafficking, the third most lucrative black market business after drug and arms trafficking. The trade generates some US$9.5 billion a year globally, according to United Nations figures.