Victims say they get two or three hours sleep each day, are beaten and drugged to keep them working. Human rights advisor Hornung has heard reports that men who fell ill were thrown overboard.

Declining fish stocks close to land mean many boats spend months at sea, docking only with mother-ships to unload their catch. That makes escape impossible.

"In a current case, we have one person who stayed consecutively for three years on a boat without seeing land," said Hornung. "He was basically sold on the high seas from boat to boat over a three-year period. And these cases are not infrequent."

Hornung estimates the range of time people are enslaved on boats runs anywhere from three months to several years. And he adds none of the 60 men the group Licadho interviewed received any pay for their work.

Experts say Cambodia's poverty drives most of the trafficking in the country.

Louise Rose, a victim protection officer for The Asia Foundation, says a survey of 258 Cambodian men - most of whom worked on foreign fishing boats - found that debt had driven half to seek work abroad. But two other factors were even more significant.

"Lack of food was a huge one," said Rose. "Three-quarters of the men reported not enough food being a motivator for migrating. And the other one that was even higher again - no source of income. That was about 78 percent."

One in five of the men in the Asia Foundation survey said they had worked in slave-like conditions on Thai and Malaysian fishing boats.