The use of migrant workers in the US seafood supply chain has led to the creation of exploitative conditions that are equivalent to forced labor, according to a report released on Wednesday.

The report was released by the National Guestworker Alliance (NGA) and is based on previous findings as well as interviews with 126 seafood processing workers in New Bedford, Massachusetts, and a range of in-depth case studies.

In order to succeed in a highly competitive global market, US seafood processors have increasingly come to rely on temporary labor. Such workers include H-2B visa laborers as well as undocumented workers. Their immigration status makes these workers vulnerable to exploitation. The NGA found that these workers were unlikely to report abuse on the job due to threats from employers not to hire them in subsequent seasons and sponsor them for H-2B visa. Undocumented workers have been threatened with immigration enforcement.

“This new research exposes a reality that workers know all too well,” said Daniel Castellanos, former H-2B guest worker and a co-founder of the NGA. “Seafood processing workers are routinely subjected to severe forms of exploitation by companies producing cheap seafood for major retailers and food distributors like Walmart. And when they speak out or try to resist abuse, they are punished severely for it.”

A Walmart spokeswoman told the Guardian that the company cares about the men and women in its supply chain.

“Walmart’s standards for suppliers lists our social and environmental expectations for our suppliers, specifically addressing working conditions, voluntary labor, pay, the cultivation of a safe and healthy work environment, and freedom of association,” she said. “Walmart also is committed to participating in collaborative efforts with external stakeholders to advance the goal of identifying risk and building a more transparent supply chain, including participation in industry groups that serve as a vehicle to share experiences and gain new information that can help improve operations and the global supply chains as a whole.”

Among the indicators of forced labor detected by NGA in the US supply chain were verbal abuse, curfews, threats of denunciation to immigration authorities and poor treatment by supervisors. Those workers surveyed also reported having their wages stolen, not being paid overtime, and being paid less than originally promised.

According to the report, a majority of workers processing scallops and groundfish in the large plants in New Bedford were hired on a temporary basis. The NGA also referenced a 2009 report that found nearly 75% of seafood processing workers in the city “were undocumented migrants, generally hired through temp agencies”. In Louisiana, seafood employers rely heavily on H-2B visa workers. According to the NGA, utilization of H-2B visas among seafood employers tripled between 1990 and 2000.

“In the 1970s, when I started my business, I was lucky enough to have the refugees from Vietnam come in, so we brought in roughly 40 of them to help us through that time,” Frank Randol, president of Louisiana-based crawfish processor Randol Inc told Congress last year. “Over the course of the years, we came to the 1990s when that started to wane just a little bit, we discovered the H-2B program and started bringing in the guestworkers from Mexico.”

Randol described the jobs at his processing plant as “one of America’s least desired jobs” and said that union activity has made it difficult for him to hire workers.

The NGA report was released at the end of the International Labour Conference, which is currently being held in Geneva. The NGA has called on the International Labour Organization (ILO) to “move towards a binding legal convention regulating global value chains”. The alliance also asked the ILO to organize a conference to explore the adverse impact of contracting and purchasing practices on migrant workers’ rights, and to assemble research on how purchasing practices of multinational corporations affect core labor standards and the wages and benefits of workers.

“Even on the end of seafood supply chains in the US, competition from low-cost producers in Asia and elsewhere has created a powerful incentive for US processors to cut labor costs through wage theft, unsafe working conditions and at times forced labor conditions,” said JJ Rosenbaum, the NGA’s legal adviser. “As the report shows, the ILO’s role ensuring decent work including living wages and the right to organize for all supply chain workers is fundamental to reverse the race to the bottom and support workers and their organizations in raising standards.”