Thailand's multibillion-dollar fishing industry is facing allegations of using slave labor, following the publication of an investigation into the exploitation of migrant workers on shrimping ships.

The report, “Sold to the Sea: human trafficking in Thailand's fishing industry,” was published Wednesday by the British-based NGO the Environmental Justice Foundation (EJF). It documents the case of 15 Burmese workers, beaten and abused at the hands of a Thai fishing crew, who forced them to work more 20 hours a day for little or no money. The men, who are now in custody after being rescued, reported seeing other Burmese workers murdered by crew members on the ships in Kantang in southern Thailand.

EJF and other anti-trafficking groups are calling for Thailand to be downgraded in the US state department’s annual Trafficking in Persons (TIP) report, which grades the scale and severity of trafficking globally on a three-tiered scale.

However, some analysts doubt the US would actually take the step to relegate Thailand to a lower status, given the two countries’ close trade and tourist ties. America is the No. 1 purchaser of Thai fish products, including tuna and shrimp. In 2011, Thai frozen and fresh exports to the US were valued at $1.8 billion.

The TIP report, due out next month, has put Thailand on the Tier 2 Watch List for three consecutive years and the US has warned that the country could face lowered status if promises by the Thai authorities to clear up the industry are not followed up on. Earlier this month, Thailand’s deputy prime minister, Surapong Tovichak Chaikul, met with Secretary of State John Kerry on a visit to the US in which he promised reforms and urged his counterpart not to downgrade Thailand.

Relegation to Tier 3 would rank Thailand among the worst countries for trafficking in the world and could lead to restrictions on US foreign aid and access to global financial institutions including the World Bank.

Thai authorities are yet to comment on the report but analysts said its findings were unsurprising.

“We have known about these abuses for a long time,” says Phil Robertson, deputy Asia director at Human Rights Watch. “The Thai authorities have consistently avoided investigating this issue despite a growing body of evidence.”

The investigation by EJF is based on interviews with six of the Burmese workers who were taken into custody by the Thai authorities in March after five months at sea.

The men said they were deceived by labor brokers, who promised work and then sold them to the ships’ senior crew. On board they said they were detained against their will and regularly beaten.

Two of the men reported seeing at least five fellow migrant workers tortured, killed, and thrown over board for trying to escape. Another man said he saw multiple bodies tossed out to sea by crew members, who forced workers to watch as they beat them to death.

"We were shocked by the extreme levels of violence inflicted on and witnessed by migrant men held as captive workers on these boats,” said Steve Trent, executive director of EJF in a statement. “This is not an isolated case, but indicative of the widespread acceptance and use of modern slavery in an industry that feeds a global appetite for seafood."

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The report also documents collusion between the traffickers and the Thai authorities, who are accused of failing to properly investigate the case and breaking procedures by allowing the traffickers access to the victims once they were in custody. Statements recorded by the Burmese also allege police forced them to work on a rubber plantation owned by a senior member of the force and paint a police cell.

"We have been genuinely surprised by the levels of collusion by agents of the state, who instead of stopping these awful human rights abuses, are ignoring and even benefiting from it," Mr. Trent said.