SONGKHLA, Thailand — 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.

Mr. Long did not know where the fish he caught ended up. He did learn, however, that most of the forage fish on the final boat where he was held in bondage was destined for a cannery called the Songkla Canning Public Company, which is a subsidiary of Thai Union Frozen Products, the country’s largest seafood company. In the past year, Thai Union has shipped more than 28 million pounds of seafood-based cat and dog food for some of the top brands sold in America including Iams, Meow Mix and Fancy Feast, according to United States Customs documents.

The United States is the biggest customer of Thai fish, and pet food is among the fastest growing exports from Thailand, more than doubling since 2009 and last year totaling more than $190 million. The average pet cat in the United States eats 30 pounds of fish per year, about double that of a typical American.

Though there is growing pressure from Americans and other Western consumers for more accountability in seafood companies’ supply chains to ensure against illegal fishing and contaminated or counterfeit fish, virtually no attention has focused on the labor that supplies the seafood that people eat, much less the fish that is fed to animals.

“How fast do their pets eat what’s put in front of them, and are there whole meat chunks in that meal?” asked Giovanni M. Turchini, an environmental professor at Deakin University in Australia who studies the global fish markets. “These are the factors that pet owners most focus on.”

Little Respite From Danger

It is difficult to overstate the dangers of commercial fishing. Two days spent more than 100 miles from shore on a Thai fishing ship with two dozen Cambodian boys, some as young as 15, showed the brutal rhythm of this work.

Rain or shine, shifts run 18 to 20 hours. Summer temperatures top 100 degrees. The deck is an obstacle course of jagged tackle, whirring winches and tall stacks of 500-pound nets. Ocean spray and fish innards make the floor skating-rink slippery. The ship seesaws, particularly in rough seas and gale winds. Most boys work barefoot; 15-foot swells climb the sides, clipping them below the knees. Much of this occurs in pitch blackness. Purse seiners, like this ship, usually cast their nets at night when the small silver forage fish that they target — mostly jack mackerel and herring — are easier to spot.

When they are not fishing, the Cambodians, most of whom were recruited by traffickers, sort their catch and fix the nets, which are prone to ripping. One 17-year-old boy proudly showed a hand missing two fingers — severed by a nylon line that had coiled around a spinning crank. The migrants’ hands, which are virtually never fully dry, have open wounds, slit from fish scales and torn from the nets’ friction. “Fish is inside us,” one of the boys said. They stitch closed the deeper cuts themselves. Infections are constant.

Thailand’s commercial fishing fleet consists predominantly of bottom trawlers, called the strip-miners of the sea because they use nets weighted to sink to the ocean floor and ensnare almost everything in their path. But purse seine boats, like the one where these Cambodians work, are common too. They use circular nets to target fish closer to the water’s surface. After the nets are hauled upward, they are pinched at the top, like old-style coin purses.

Before arriving on the ship, most of the Cambodians had never seen a body of water larger than a lake. The few who could swim were responsible for diving into the inky sea to ensure that the 50-foot mouth of the nets closed properly. If one of them were to get tangled in the mesh and yanked underwater, it is likely that no one would notice right away. The work is frenzied and loud, as the boys chant in unison while pulling the nets.

Meals on board consist of a once-daily bowl of rice, flecked with boiled squid or other throwaway fish. In the galley, the wheel room and elsewhere, countertops crawl with roaches. The toilet is a removable wooden floorboard on deck. At night, vermin clean the boys’ unwashed plates. The ship’s mangy dog barely lifts her head when rats, which roam all over the ship, eat from her bowl.

Crew members tend to sleep in two-hour snatches, packed into an intensely hot crawl space. Too many bodies share the same air, with fishing-net hammocks hanging from a ceiling that is less than five feet above the floor. Deafening, the engine turbines throb incessantly, shaking the ship’s wooden deck. Every so often, the engine coughs a black cloud of acrid fumes into the sleeping quarters.

These conditions, which are typical on long-haul fishing vessels, are part of the reason that the Thai fishing fleet is chronically short of men. Thailand has one of the lowest unemployment rates in the world — generally less than 1 percent — which means native workers have no trouble finding easier, better paying jobs on land.

“You just have to work hard,” said Pier, 17, one of the migrants on the purse seiner. Pier, who goes by only one name, said he liked life on the ship. “Better than home,” he said, “Nothing to do there.” He flexed his sinewy biceps, showing the results of his labor.

In the dead of night, the captain spotted a school of fish on radar. He roused the crew with an air horn. Pier, in his second year of working on the ship, explained that he still owed the captain some of the $300 he paid a smuggler to get him from the border to port. The rest of his debt, $90, was from a cash advance he sent back to his family, he said. Willing to answer other questions, Pier silently looked down when asked whether he had ever been beaten. Several other boys, questioned about the same, furtively looked to the captain and shook their heads to indicate that they did not want to be interviewed.

Indentured servitude — a “travel now, pay later” labor system where people work to clear a debt typically accrued for getting free passage to another country — is common in the developing world, especially in construction, agriculture, manufacturing and the sex industry. It is more pervasive and abusive at sea, human rights experts say, because those workers are so isolated.

Historically, Thai boat captains paid large advances to deckhands so they could sustain their families during their long absences. But the country’s labor crisis has converted this upfront cash into a price per head (or “kha hua” fee) given to smugglers who ferry workers across the border.

Standing on the boat next to Pier, another Cambodian boy tried to explain how elusive the kha hua debt becomes once they leave land. Pointing to his own shadow and moving around as if he were trying to grab it, he said: “Can’t catch.”

The boat’s Thai crew master, Tang, a man with pockmarked skin and missing front teeth, ordered the boys back to work. He then ticked off a list of the pressures on deep-sea captains. Fuel costs eat up about 60 percent of a vessel’s earnings, double what they did two decades ago. Once fish are caught, storing them in melting ice is a race against the clock. As fish thaw, their protein content falls, dropping their sale price. And, Tang added, because deep-sea fishing boats work on commission, “Crews only get paid if we catch enough.”

Captains fear their crews as intensely as they drive them. Language and cultural barriers create divisions; most boats here have three Thai officers and foreign deckhands. The captain is armed, in part because of the threat of pirates, but Tang also talked of a gruesome mutiny on another ship that left all the officers dead.

Tales of forced labor are not always what they seem, according to the boat’s captain, who insisted on anonymity as a condition of allowing a reporter on board. Some workers sign up willingly, only to change their minds once at sea, while others make up stories of mistreatment in hopes of getting back to their families, he said.

Still, a half-dozen other captains acknowledged that forced labor is common. It is unavoidable, they argue, given the country’s demand for laborers. Every time a boat docks, they said, they fret that their willing workers will bolt to better-paying ships. That is also the moment when captive migrants make a run for it.

Short-handed at the 11th hour, captains sometimes take desperate measures. “They just snatch people,” one captain explained, noting that some migrants are drugged or kidnapped and forced onto boats. “Brokers charge double.”

Litany of Abuses

Traveling the coast of the South China Sea, it can seem that every migrant has his own story of abuse.

Skippers never lacked for amphetamines so laborers could work longer, but rarely stocked antibiotics for infected wounds. Former deckhands described “prison islands” — most often uninhabited atolls, of which there are hundreds in the South China Sea. Fishing captains sometimes maroon their captive crews on those islands, sometimes for weeks, while their vessels are taken to port for dry docking and repair.

Other islands, inhabited but desolate, are also used to hold crew members. Fishing boat workers on an Indonesian island called Benjina were kept in cages to prevent them from fleeing, The Associated Press reported earlier this year. Inaccessible by boat several months a year because of monsoons, Benjina had an airstrip that was rarely used and no phone or Internet service.

Thai government officials said they have stepped up the number of investigations and prosecutions and plan to continue doing so. A registration drive is underway to count undocumented workers and provide them with identity cards, added Vijavat Isarabhakdi, Thailand’s ambassador to the United States until this year. The government has also established several centers around the country for trafficking victims.

San Oo, 35, a soft-spoken Burmese man with weather-beaten skin, predicted that until ship captains are prosecuted, little will improve. He described how on his first day of two and a half years in captivity, his captain warned that he had killed the seaman Mr. Oo was replacing. “If you disobey or run or get sick I will do it again,” he recalled his captain saying.

Pak, a 38-year-old Cambodian who fled a Thai trawler last year, ended up on the Kei Islands, in Indonesia’s eastern Banda Sea. The United Nations estimated that hundreds of migrants there escaped fishing boats over the last decade. “You belong to the captain,” Pak said, recounting watching a man so desperate that he jumped overboard and drowned. “So he can sell you if he wants.”

Critics have faulted Thailand for what they say is a deliberate failure to confront the larger causes of abuse in fishing. Compared to its neighbors, Thailand has less stringent rules on how long boats can remain at sea. Last year, it was the only country to vote against a United Nations treaty on forced labor requiring governments to punish traffickers, before reversing its stance in the face of international pressure.

Thai officials also proposed using prison labor on fishing boats as a way to shift away from migrant workers, a plan dropped after an outcry from human rights groups. On Monday, the State Department renewed Thailand’s ranking on the lowest rung of governments that do not meet minimum standards in countering human trafficking.

The other Thai industry where forced labor is common is sex work, said Mr. Robertson, from Human Rights Watch. The two industries intersect in run-down towns like Ranong, along the Thailand-Myanmar border. Labor brokers operate with impunity in these towns. Karaoke bars double as brothels and debt traps.

A tavern owner named Rui sat down to make his pitch late one night in November, pointing to two prepubescent girls who sat in a corner, wearing caked-on makeup and tight, glittery skirts. He spread a stack of Polaroid pictures of them from a year before. Each clutched a stuffed animal in the photos and looked scared. “Popular,” Rui said of the girls now. “Very popular now.”

A beer at Rui’s tavern cost about $1. Sex with a “popular” girl: $12. For the tattered men, mostly Burmese, who end up here, a couple of evenings at the tavern can add up to kingly sum. Many of them have trekked hundreds of miles by foot, not a cent on them, hoping for work. Meals, drugs and lodging, offered as favors, show up later as fees. To clear these bills, migrants are sometimes sold to the sea.

Checking boats for human rights abuses is difficult. Most fishing vessels are exempt from international rules requiring the onboard tracking systems used by law enforcement. Marine officials in Thailand, Malaysia and Indonesia said that their navies rarely inspect for labor and immigration violations. Authorities in those countries added that they lack boats and fuel needed to reach the ships farthest from shore that are most prone to using captive labor.

Deep-sea fishing generally does not lend itself to timecards or pay stubs. Labor contracts common in the region often include terms that would seem unthinkable in jobs on land.

For instance, a contract from a manpower agency in Singapore, provided to The New York Times, committed deckhands to a three-year tour during which the agency retained the full $200 per month for the first six months and $150 per month thereafter. “Daily working hours will be around 18 hours,” the contract stipulates, adding that there is no overtime pay. Boats may remain at sea for longer than a year per trip. Only seawater may be used for bathing and laundry. Mariners can be traded from boat to boat at the captain’s discretion.

“All biscuits, noodles, soft drinks and cigarettes” are to be purchased by the sailor, the contract says. “Any crew who breaches the contract (own sickness, lazy or rejected by the Captain, etc.) must bear all the expenses incurred in going back home.”

Supply and Demand

The boat that delivered Mr. Long to captivity and subsequently rescued him was known as a “mothership.” Carrying everything from fuel and extra food to spare nets and replacement labor, these lumbering vessels, often over a hundred feet long, function as the roving resupply stores of the marine world. Motherships are the reason that slow-moving trawlers can fish more than 1,500 miles from land. They allow fishermen to stay out at sea for months or years and still get their catch cleaned, canned and shipped to American shelves less than a week after netting.

But once a load of fish is transferred to a mothership, which keeps the cargo below deck in cavernous refrigerators, there is almost no way for port-side authorities to determine its provenance. It becomes virtually impossible to know whether it was caught legally by paid fishermen or poached illegally by shackled migrants.

Bar codes on pet food in some European countries enable far-flung consumers to track Thai-exported seafood to its onshore processing facilities, where it was canned or otherwise packaged. But the supply chain for the 28 million tons of forage fish caught annually around the globe, about a third of all fish caught at sea and much of it used for pet and animal feed, is invisible before that.

Sasinan Allmand, the head of corporate communications for Thai Union Frozen Products, said that her company does routine audits of its canneries and boats in port to ensure against forced and child labor. The audits involve checking crew members’ contracts, passports, proof of payment and working conditions. “We will not tolerate any human trafficking or any human rights violation of any kind,” she said. Asked whether audits are conducted on the fishing boats that stay at sea, like the one where Mr. Long was captive, she declined to respond.

Human rights advocates have called for a variety of measures to provide greater oversight, including requiring all commercial fishing ships to have electronic transponders for onshore monitoring and banning the system of long stays at sea and the supply ships that make them possible. But their efforts have gotten little traction. The profits for seafood businesses still far exceed the risks for those who exploit workers, said Mark P. Lagon, who formerly served as the State Department’s ambassador at large focused on human trafficking.

Lisa K. Gibby, vice president of corporate communications for Nestlé, which makes pet food brands including Fancy Feast and Purina, said that the company is working hard to ensure that forced labor is not used to produce its pet food. “This is neither an easy nor a quick endeavor,” she added, because the fish it purchases comes from multiple ports and fishing vessels operating in international waters.

Some pet food companies are trying to move away from using fish. Mars Inc., for example, which sold more than $16 billion worth of pet food globally in 2012, roughly a quarter of the world’s market, has already replaced fishmeal in some of its pet food and will continue in that direction. By 2020, the company plans to use only non-threatened fish caught legally or raised on farms and certified by third-party auditors as not being linked to forced labor.

Though Mars has been more proactive on these issues than many of its competitors, Allyson Park, a Mars spokeswoman, conceded that the fishing industry has “real traceability issues” and struggles to ensure proper working conditions. This is even more challenging, she said, since Mars does not purchase fish directly from docks but further up the supply chain.

Over the past year, Mars received more than 90,000 cartons of cat and dog food from the cannery supplied by one of the boats where Lang Long was held captive, according to the Customs documents.

Shackled and Afraid

In Songkhla, on Thailand’s southeast coast, Suchat Junthalukkhana thumbed through an inch-thick binder, each page with a photograph of a fleeing mariner whom his organization, the Stella Maris Seafarers Center, had helped.

“We get a new case every week,” he said.

The fate of the men who escape from the fishing boats often relies on chance encounters with altruistic strangers who contact Stella Maris or the other groups that make up an underground railroad that runs through Malaysia, Indonesia, Cambodia and Thailand.

One such inadvertent rescuer was Som Nang, 41, who said his name means “good luck” in Khmer. A squat man, he is quick to show off the retractable metal rod that he keeps with him for protection.

Having worked dockside for several years, Som Nang had heard the tales of fishing-boat brutality. None of it prepared him, however, for what he would witness on his maiden voyage on a mothership late in 2013.

“I wish I had never seen it,” Som Nang said, sitting in his cinder-block home just outside Songkhla. After a four-day trip from shore, Som Nang’s supply boat pulled alongside a dilapidated Thai-flagged trawler with an eight-man crew that had just finished two weeks fishing in Indonesian waters where they were not allowed.

It was difficult not to notice Mr. Long, who crouched near the front of the fishing boat, Som Nang said. Padlocked around his bruised neck was a rusty metal collar attached to a three-foot chain looped to an anchor post. Mr. Long, who was the only Cambodian among the Burmese deckhands and the Thai senior crew, stared, unblinking, at anyone willing to make eye contact.

“Please help me,” Som Nang, who is also Cambodian, recounted Mr. Long whispering in Khmer. That was 30 months after Mr. Long had met a trafficker along the Thai-Cambodian border during a festival. Mr. Long said he never intended to work in Thailand but the job offer was attractive. When he instead arrived at a port near Samut Prakan, the trafficker sold him to a boat captain for about $530, less than a water buffalo typically costs. He was then marched up a gangplank, and sent due west for four days.

A police report later described his account of his arrival in captivity: “Three fishing boats surrounded the supply boat and began fighting for Mr. Long,” the report says. Similar arguments broke out a year later when Mr. Long was sold again in the middle of the night between trawlers.

The longer he spent on the boats, the more his trafficking debt should have lightened, bettering his prospects for release. But the opposite was the case, Mr. Long explained. The more experience he had, the bleaker his fate, the higher the price on his head, the hotter the arguments over him between short-handed trawler captains.

Having never seen the sea before, Mr. Long seemed to tangle his portion of the nets more than others, he said. All the fish looked the same to him — small and silver — making sorting difficult. Slowed at first by intense seasickness, Mr. Long said he sped up after witnessing a captain whipping a man for working too slowly.

Mr. Long suffered similarly. “He was beat with a pole made of wood or metal,” said a case report about him from the Office of the National Human Rights Commission of Thailand. “Some days he had rest of only 1 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.

Mr. Long said he often considered jumping overboard to escape. He did not know how to swim, though, and he never once saw land during his time at sea, Mr. Long told a doctor who later treated him. At night he had access to the ship’s radio. But he had no idea whom or how he could call for help.

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

When Som Nang’s boat showed up, Mr. Long had been wearing the shackle on and off for about nine months. The captain typically put it on him once a week, Mr. Long said, whenever other boats approached.

After offloading fish for about 10 minutes, Som Nang said he asked the captain why Mr. Long was chained. “Because he keeps trying to escape,” the captain replied, according to Som Nang. Based on the looks he got from the crew on his mothership, Som Nang said he figured it best to stop asking questions. But after returning to port, he contacted Stella Maris, which began raising the 25,000 baht, roughly $750, needed to buy Mr. Long’s freedom.

Over the next several months, Som Nang resupplied the fishing boat twice. Each time, Mr. Long was shackled. Som Nang said he discreetly tried to reassure him that he was working to free him.

In April 2014, Mr. Long’s captivity ended in the most undramatic of ways. Som Nang carried a brown paper bag full of Thai currency from Stella Maris to a meeting point in the middle of the South China Sea, roughly a week’s travel from shore. With few words exchanged, the money was handed to Mr. Long’s captain. His debt paid, Mr. Long, rail-thin, stepped onto Som Nang’s boat and began his journey back to solid ground and a hope for home.

Thai immigration officials who have investigated his case say they found it credible. Mr. Long is in the process of being repatriated back to his native village, Koh Sotin, in Cambodia. He hopes to go back to his old job cleaning a local Buddhist temple, he said. Thai and Indonesian marine officials say they are trying to locate his last boat captain but they are not hopeful because there are so many of these illegal vessels.

During his six-day voyage back to shore on the mothership, Mr. Long cried and slept most of the time. Som Nang said the crew hid him to avoid word getting out to other fishing boats about their role in the rescue.

Mr. Long, who has a perpetually vacant gaze, said he never wanted to eat fish again. He added that at first he had tried to keep track of the passing days and months at sea by etching notches in the wooden railing. Eventually he gave that up. “I never thought I would see land again,” he said.

Som Nang, who is now a security guard at a factory, said he stopped working at sea shortly after his rescue trip. His explanation: “I don’t like what is out there.”