LONDON/BANGKOK: A three-year investigation into slavery on Thailand's fishing boats has uncovered a well-oiled system of trafficking, abuse and exploitation in the southern port of Kantang, leading to eight arrests this month, a campaign group said on Monday.

The owner of a fishing company, three enforcers and four boat captains were arrested on Nov. 7 after the Environmental Justice Foundation (EJF) handed police evidence against them, including testimonies from fishermen who escaped their boats.

Thai police spokesman Kissana Phathanacharoen said they expected to lay charges early in 2016 with investigations ongoing into the money trail and more arrests likely.

EJF's report comes a week after Swiss food giant Nestle SA admitted that slave labour was used in its Thai seafood supply chain, adding to mounting calls to clean up a $3 billion industry long dogged by allegations of abuse in recent years.

EJF's director Steve Trent said he hoped evidence the organisation had collected would be fully tested in court to protect fishermen, many of whom are migrant workers trafficked from Thailand's poorer neighbours, Cambodia and Myanmar.

"This cannot be an arbitrary kangaroo court just driven to please the international community at this key time for Thailand. It's got to bring people justice," Trent told the Thomson Reuters Foundation in an interview.

EJF said Kantang's fishing industry, which netted 65,000 tonnes of seafood in 2013, is dominated by three companies who help make Thailand the world's third largest seafood exporter.

The Thomson Reuters Foundation tried to contact the three companies - Boonlarp Fishing Co. Ltd, Jor Monchai, and Wor Wattana Sohpon - seeking comment on their operations and crews.

"With respect to the fishermen, we do everything correctly now. We do not violate migrant fishermen's rights. We hire them correctly and treat them right," said a man at Boonlarp's office in Kantang who answered a phone call.

He declined further comment and would not give his name.

Jor Monchai owner Pramote Cholwisit insisted violence and labour violations were a problem of the past.

"On shore, there are no such violations. At sea, there may be some problems - before there were many. When the migrants fight at sea, it can become violent, and we try to solve this problem by putting them on separate boats," Pramote said by telephone from Trang.

"At my pier, I guarantee there are no such problems."

The Thomson Reuters Foundation could not immediately find a working telephone number or email address for Wor Wattana.

ILLEGAL FISHING

EJF's report details how migrants from Cambodia and Myanmar are trafficked to work on Kantang's fishing fleets, becoming trapped in a cycle of debt and abuse that was repeated elsewhere in the country.

Brokers in Kantang, a port in decline about 150 km (90 miles) from the Malaysian border in Trang province, employ a network of enforcers and informants, such as motorbike taxi drivers, to monitor and control crew while they are on shore.

At sea, the fishermen face violence, intimidation, dangerous working conditions and even murder, according to the EJF report.

"They would torture and murder the fishers then throw them into the sea," escaped migrant worker, Tun Thet Soe, told EJF.

The extent of the problem in the Thai fishing industry is hard to gauge.

Thailand's seafood industry employs more than 650,000 people. Activist group Raks Thai Foundation estimates that about 200,000 of them are migrant workers from Myanmar, Cambodia and Laos, many of whom are undocumented.

The European Union threatened earlier this year to ban Thai seafood imports if the country failed to adopt adequate measures against slave labour and illegal fishing.

Trent said overfishing had caused a crisis in Thailand's marine biodiversity, prompting fishing companies to keep boats at sea longer or fish in the waters of Malaysia and Indonesia.

The overall catch per unit effort - a measure of the abundance of fish - in the Gulf of Thailand and the Andaman Sea has plummeted by more than 86 percent since 1966, making Thai waters among the most over-fished in the world, the EJF added.

Faced with diminishing catches, Thai operators routinely use human trafficking networks to provide crew for their boats and cut costs, it said.

Corruption is a major obstacle to ending slave labour in Thailand with some police officers involved in leaking information to fishing companies and brokers, suppressing investigations and, on occasion, taking part in violence against migrant workers, EJF said.

Royal Thai Police deputy spokesman Kissana said police are extending the detention of the eight suspects, who by law can be held for 84 days in total with charges expected soon.

"Right now we are expanding the investigation to find additional suspects involved in the case," he said, adding that police are working with the government's anti-money laundering office to follow the money trail and seize funds.

Trent said more action was needed to clean up the industry.

"Those personal fiefdoms, those small territories that exist in the ports and provinces where the worst of this is happening, has to be pulled under the control of Bangkok," he said.