Lang Long’s ordeal began in the back of a truck. After watching his younger siblings go hungry because their family’s rice patch in Cambodia could not provide for everyone, he accepted a trafficker’s offer to travel across the Thai border for a construction job.

It was his chance to start over. But when he arrived, Mr. Long was kept for days by armed men in a room near the port at Samut Prakan, more than a dozen miles southeast of Bangkok. He was then herded with six other migrants up a gangway onto a shoddy wooden ship. It was the start of three brutal years in captivity at sea.

“I cried, said Mr. Long, 30, recounting how he was resold twice between fishing boats. After repeated escape attempts, one captain shackled him by the neck whenever other boats neared.

Mr. Long’s crews trawled primarily for forage fish, which are small and cheaply priced. Much of this catch comes from the waters off Thailand, where Mr. Long was held, and is sold to the United States, typically for canned cat and dog food or feed for poultry, pigs and farm-raised fish that Americans consume.

The misery endured by Mr. Long, who was eventually rescued by an aid group, is not uncommon in the maritime world. Labor abuse at sea can be so severe that the boys and men who are its victims might as well be captives from a bygone era. In interviews, those who fled recounted horrific violence: the sick cast overboard, the defiant beheaded, the insubordinate sealed for days below deck in a dark, fetid fishing hold.

The harsh practices have intensified in recent years, a review of hundreds of accounts from escaped deckhands provided to police, immigration and human rights workers shows. That is because of lax maritime labor laws and an insatiable global demand for seafood even as fishing stocks are depleted.

Shipping records, customs data and dozens of interviews with government and maritime officials point to a greater reliance on long-haul fishing, in which vessels stay at sea, sometimes for years, far from the reach of authorities. With rising fuel prices and fewer fish close to shore, fisheries experts predict that more boats will resort to venturing out farther, exacerbating the potential for mistreatment.

“Life at sea is cheap, said Phil Robertson, deputy director of Human Rights Watch’s Asia division. “And conditions out there keep getting worse.

While forced labor exists throughout the world, nowhere is the problem more pronounced than here in the South China Sea, especially in the Thai fishing fleet, which faces an annual shortage of about 50,000 mariners, based on United Nations estimates. The shortfall is primarily filled by using migrants, mostly from Cambodia and Myanmar.

Many of them, like Mr. Long, are lured across the border by traffickers only to become so-called sea slaves in floating labor camps. Often they are beaten for the smallest transgressions, like stitching a torn net too slowly or mistakenly placing a mackerel into a bucket for herring, according to a United Nations survey of about 50 Cambodian men and boys sold to Thai fishing boats. Of those interviewed in the 2009 survey, 29 said they had witnessed their captain or other officers kill a worker.

The migrants, who are relatively invisible because most are undocumented, disappear beyond the horizon on “ghost ships unregistered vessels that the Thai government does not know exist.

They usually do not speak the language of their Thai captains, do not know how to swim, and have never seen the sea before being whisked from shore, according to interviews in Malaysia, Thailand and Indonesia. These interviews, in port or on fishing boats at sea, were conducted with more than three dozen current deckhands or former crew members.

Government intervention is rare. While United Nations pacts and various human rights protections prohibit forced labor, the Thai military and law enforcement authorities do little to counter misconduct on the high seas. United Nations officials and rights organizations accuse some of them of taking bribes from traffickers to allow safe passage across the border. Migrants often report being rescued by police officers from one smuggler only to be resold to another.

2015 The New York Times Company