The Environmental Justice Foundation said it discovered human rights abuses involving African crews during its investigation into illegal fishing, the British newspaper The Guardian reported.
"We didn't set out to look at human rights but rather to tackle the illegal fishing that's decimating fish stocks, but having been on board we have seen conditions that unquestionably meet the U.N. official definition of forced labor, or modern-day slavery," EJF investigator Duncan Copeland said.
The EJF said conditions on the fishing trawlers included incarceration, violence, withholding of pay, confinement on board for months or years, confiscation of documents and a lack of drinking water.
The group said the hi-tech trawlers all carried European Union identification numbers, indicating they are licensed to import fish to Europe, and passed strict sanitation standards.
Living and working conditions on some of the vessels were deplorable, the report said. It filmed cramped sleeping areas no more than three feet high, and said one crew of 200 had no clean drinking water.
EFT said temperatures in unventilated fish holds on some boats where workers sort, process and pack fish for lucrative markets in Europe and Asia were as high as 45 degrees Celsius (113 degrees Fahrenheit).
The boats were operated in fishing exclusion zones off the coast of Sierra Leone and Guinea over the last four years, the report said.
The group said 13 percent to 31 percent of total catches worldwide are made illegally.