What exactly is slavery? does it have to last a lifetime, or is a child who is sold for a set period of time also enslaved? If parents are promised money for the child's labor, is that a salary or a purchase price? Lured by an agent with promises of money, 14-year-old Andy Irawan's parents forced him to join a group of eight other boys living on a jermal, a tennis-court-sized platform of rotting wood and leaky, rusted roofs 10 km off the north coast of Sumatra in the Malacca Strait. The boys are promised pay—around $30 at the end of a three-month stint. But after deductions are made for food, the agent's cut and other fees and expenses, the boys are left with little or nothing. They are captives on the jerry-built island. Syahman Purba, who runs a school for former jermal workers, has no doubt the employment is modern slavery: "These kids aren't treated like human beings. They're given just enough food so they can work and won't die."

There are an estimated 250 million child laborers in the world. No one knows how many are in forced labor like Andy, sold by their parents for weeks or years to agents who promise salaries that turn out to be inflated, are whittled away by fictitious expenses or are nonexistent. But for mind-numbing work like netting fish on a jermal, children are the ideal employees—cheap, docile and easily cowed. "They said I could go home after three months," Andy recalls, clutching his right hand still swollen from a sea snake bite. "But there was no replacement so they said I had to stay."

A working day on a jermal lasts 18 hours and the boys are isolated; their only contact with the outside world is when operators pick up the catch and drop off water, rice and instant noodles. Flattened cardboard boxes serve as mattresses. Mangy dogs defecate on the platform surface where fish are sorted from the sea snakes and jellyfish. In the past five years, six boys have died at sea, the victims of accidents and failed escapes. Andy was rescued last July, one of scores of boys who have been removed from the jermals since the International Labor Organization began an anti-child-labor program in Indonesia a year ago. Despite the increased monitoring, employers continue to lie about children's ages, and working conditions are worsening. Overfishing is causing stocks of squid and fish to dwindle, which means, says one foreman, a jermal veteran of 18 years: "We have to work the kids twice as hard as we used to."