While forced labour exists throughout the world, nowhere is the problem more rampant than in the South China Sea, and especially in the Thai fishing fleet. Tens of thousands of migrants from Cambodia and Myanmar are whispered into Thailand each year to make up a chronic shortfall of mariners. Then, unscrupulous captains buy and sell the men and boys.

With rising fuel prices and fewer fish close to shore, maritime labour researchers predict that more boats will resort to venturing farther out to sea, making the mistreatment of migrants more likely. The work is brutal. Captains require crew members to simply do what they were told, when they were told. No complaints are tolerated, no matter how long the hours, how little the food, or how paltry the pay. In short, these captains rely on sea slaves.

“Life at sea is cheap,” said Phil Robertson, deputy director of Human Rights Watch’s Asia division. Conditions are worsening, he says, because of lax maritime labour laws and an insatiable global demand for seafood, even as overfishing depletes fish stocks.

Chained up

“I wish I had never seen it,” said the security guard Som Nang, describing what he had witnessed hundreds of miles from shore. In late 2013, Som Nang embarked on his maiden voyage on a boat that resupplied fishing vessels in the South China Sea. After four days on the water, Som Nang’s ship pulled up alongside a dilapidated Thai-flagged trawler.

At the front of the trawler, a shirtless, emaciated man huddled with a rusty metal shackle around his bruised neck and a 3ft chain anchoring the collar to a post on the deck. The man had tried to escape the boat, the captain of the fishing vessel later explained, so he locked the metal collar on the man and chained him up every time another ship drew near.

The name of the shackled man was Lang Long, and like thousands of other men and boys in the Thai fishing fleet he was trafficked across the border from Cambodia into Thailand. Long had never intended to go to sea. Near his village outside Phnom Penh, Cambodia’s capital city, he met a man at a Buddhist festival who offered him a construction job in Thailand and help getting into the country.

Long, who was 30, saw it as his chance to start over; he was tired of watching his younger siblings go hungry because their family’s rice paddy back home could not provide for everyone. So Long travelled by night to a port city on the gulf coast of Thailand, traversing over bumpy dirt roads in the back of a flatbed truck.

When he arrived, he waited for days in a room guarded by armed men near the port at Samut Prakan, more than a dozen miles southeast of Bangkok. The trafficker sold Long to a boat captain for about $530, less than the going price for a water buffalo.

He was then herded with six other migrants up a gangway onto a shoddy wooden ship. It was the start of three brutal years of captivity at sea, during which Long was resold twice between fishing boats.

I met Long in Songkhla, on Thailand’s southeast coast, in September 2014 while I was reporting on forced labour. Long had been rescued seven months earlier by a Catholic charity called the Stella Maris International Seafarers’ Centre, which paid a captain for his release.

As I waited to meet Long, I spent hours at the Stella Maris office poring through a binder full of case files. It was a horrifying catalogue of cruel abuses, torture, and murder at sea. In page after page, in photographs and scribbled notes, the documents described the sick being cast overboard, the defiant beheaded, and the insubordinate sealed for days below deck in a dark, fetid fishing hold.

“We get a new case every week,” said Suchat Junthaluhana, the centre’s director.

Surviving these ordeals often depended upon chance encounters with altruistic strangers who contacted Stella Maris or other groups involved in the clandestine rescue of sea slaves, part of a mariners’ underground railroad stretching through Malaysia, Indonesia, Cambodia, and Thailand.

Som Nang had worked on a type of boat known as a mother ship. Carrying everything from fuel and extra food to spare nets and replacement labour, these lumbering vessels, often more than a 100ft long, functioned as the Walmarts of the ocean – floating, all-purpose resupply stores. The same kind of boat delivered Long to captivity and subsequently rescued him as well. Mother ships were the reason slow-moving trawlers could fish more than 1,500 miles from land. They allowed fishermen to stay out at sea for months or years.

After the four-day trip from shore, Som Nang’s mother ship pulled alongside Long’s battered Thai-flagged trawler, whose eight-man crew had just finished two weeks of illegal fishing in Indonesian waters.

The only Cambodian among the Burmese deckhands and Thai senior crew, the shackled Long stared unblinking at anyone willing to make eye contact. “Please help me,” Som Nang recounted Long whispering in Khmer.

Vacant gaze

Thin and tall, Long had pockmarked skin the colour of creamed coffee. He sat unnaturally still, like an upright cadaver, and breathed only through his nose as though he was afraid to open his mouth. A shell of a man, he wore a perpetually vacant gaze.

After sitting for nearly half an hour, Long said that at first he tried to keep track of the passing days and months at sea by using a rusty fishing hook to etch notches in the wooden railing. Eventually, he stopped. “I never thought I would see land again,” he said, his voice trailing off.

The longer he worked on the boats, the lighter Long’s debt should have become for the money he owed to the captain who had paid the cost for his illegal travel across the border. Instead, time only tightened Long’s bondage. His captivity began looking like a life sentence. The more experience he gained at sea, the higher the price other shorthanded captains were willing to pay to buy him for their boats.

At first, he made rookie mistakes. Having no fishing experience and having never seen the sea before his captivity, Long seemed to tangle his portion of the nets more than other deckhands did, he said. All the fish looked the same to him – small and silver – making sorting difficult. Slowed at first by intense seasickness, Long said he sped up after witnessing a captain whip a man for working too slowly.

Despite his efforts, Long faced severe punishments. “He was beat with a pole made of wood or metal,” his case report from the Thai government’s Office of the National Human Rights Commission said. “Some days he had rest of only one hour.”

When drinking water ran low, deckhands stole foul-tasting ice from the barrels of fish. If one of the seamen put gear away incorrectly, the crew master docked the day’s meal for the offender.

Long said he often considered jumping overboard to escape. He told a doctor who later treated him that he never once saw land during his three years at sea. At night, there were times when no one was guarding the ship’s radio, but Long said that he had no idea whom to call for help, or how.

As much as he feared the captains, Long said, the ocean scared him more. Waves, some of them several stories high, battered the deck in rough seas.

When Som Nang’s boat showed up, Long had been wearing the shackle on and off for about nine months.

Buying freedom

The only thing more shocking than seeing the man shackled, Som Nang said, was the fact that no one else around him on the mother ship seemed surprised by it. After returning to port, Som Nang contacted Stella Maris, which began raising the 25,000 baht, roughly $750, needed to buy Long’s freedom. I remember being sickened when I heard this number: the price of Long’s life was less than my plane fare from Washington, DC, to Bangkok.

It was not the last time Som Nang would see Long. Over the next several months, Som Nang resupplied the fishing boat twice. Each time the mother ship drew alongside, Long was shackled. “I’m trying to get you free,” Som Nang whispered to him on one of the supply runs.

In April 2014, Long’s captivity ended in the most anticlimactic of ways. On his supply ship’s next rendezvous with Long’s captors, Som Nang carried a brown paper bag full of Thai currency from Stella Maris to the meeting point in the middle of the South China Sea, roughly a week’s travel from shore.

What the rescuers saw as a ransom, the captain viewed as a “debt payment”– an amount still owed in work from Long. With few words, Som Nang handed the bag of money to Long’s captain. Long then stepped onto Som Nang’s boat and began his journey back to solid ground.

During his six-day voyage back to shore on the mother ship, Long cried and slept most of the time. The crew hid him to avoid word getting out to other fishing boats about their role in the rescue, because they feared that other ship captains might resent that a supply company was playing a role in what they viewed as a labour dispute.

Som Nang stopped working at sea shortly after his rescue trip. He took a new job as a factory security guard. I visited with him in his cinderblock home just outside Songkhla, and Som Nang said he still had nightmares about what he saw offshore. “I don’t like what is out there,” he said.

Extreme case

Long’s case was extreme. Most fishing captains don’t resort to shackling their crew. Typically, debt and distance from shore are enough to keep workers captive. The ships in the Thai fleet that were known to have the worst conditions were the long-haul boats that stayed away from shore for months or years on end and relied most on migrant workers. It felt journalistically important to try to see this deprivation and exploitation in person.

Joined by a British photographer, Adam Dean, and a young female Thai translator, I hoped to see for myself the conditions on these long-haul ships. I soon discovered this was an ambitious goal. Captains seemed baffled as to why we would want to spend time on ships that everyone knew to be dangerous and dirty places. I said we simply wanted to witness the work and chronicle the lives of these men.

We finally found a Thai purse seiner with a crew of 40 Cambodians, some of whom were boys who appeared no older than 15. Worse for wear, the ship was crowded and rusty, looking as if it had been at sea for years.

Thailand’s fishing fleet consists mostly of bottom trawlers, which drag a wall of mesh behind them. Purse seiners use more rudimentary circular nets that are dropped to target fish closer to the water’s surface, hauled upward, then constrained at the top like a drawstring coin purse. To ensure the 50ft mouth of the nets closed properly, the boys dove into the inky-black sea. If one of them was to get tangled in the mesh and yanked into the fathoms below, it was likely that no one would notice right away over the frenzy, darkness, and noise.

For the crews, injury was a constant danger. Throughout my reporting, deckhands on these ships routinely turned to me as though I had medical expertise. Because they saw me take vitamins in the mornings, they figured that meant I knew how to administer medicine.

On one boat in the Philippines, a man showed me his scalp wound that he said was crawling with worms (I couldn’t see them). Off the coast of Somalia, I met a deckhand who coughed up blood and spat it in the water as though it were normal. It had been that way for months, he told me through a translator.

Rashes were the most common ailment. In Indonesia, a deckhand worked without trousers or underwear, just a towel around his waist, because, he told me, the itchy sores on his crotch were otherwise too uncomfortable. In many of these cases the men asked me for help and I gave them what medicines or ointments I thought might at least ease their symptoms.

Particularly pervasive

Common in the developing world, especially in construction, agriculture, manufacturing, and the sex industries, debt bondage is particularly pervasive and abusive at sea because these workers are so isolated.

In Thailand, boat captains historically paid large upfront sums in advance to deckhands so that these workers could sustain their families during their long absences. However, because more of the deckhands were migrant workers, captains no longer paid the upfront money to the men themselves. Instead, these captains paid this money to the smugglers who snuck the deckhands into the country.

Thailand’s sea slavery problem is connected to the emergence of the country’s middle class. Among Asia’s “tiger economies”, Thailand’s gross domestic product grew in the late 1980s by an average rate of 9 per cent annually, peaking at 13 per cent in 1988. Its exports also expanded by an average of 14 per cent each year. Wages on land rose, making Thai nationals even less inclined to take jobs offshore.

As of 2016, Thailand had one of the lowest unemployment rates in the world – generally less than 1 per cent. The fishing industry became dependent on cheap foreign labour, particularly from Myanmar, Cambodia, and Laos.

Still, the Thai fishing fleet was chronically shorthanded. The shortage was worse because the industry resisted investing in labour-saving technologies, relying instead on gear like purse seines that require large crews.

Thailand’s labour and human rights abuses are also connected to its environmental problems. As the number of Thai boats grew, so did the size of their hauls, causing fish stocks to plummet. In fisheries and conservation biology, the catch per unit effort, or CPUE, is an indirect measure of the abundance or scarcity of a target species.

In both the Gulf of Thailand and the Andaman Sea, on Thailand’s western side, the CPUE on fishing boats fell by more than 86 per cent between the mid-1960s and the early years of the 21st century, making Thai waters among the most overfished on the planet. Even though there were fewer fish to catch, Thai boats were catching more, partly by travelling to more distant waters.

All of these grander economic and environmental forces conspired to make debt bondage that much more tightly woven into the fabric of fishing on the South China Sea.

This is an edited extract from The Outlaw Ocean – Crime and Survival in the Last Untamed Frontier, by Ian Urbina , published by Bodley Head