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.