The suspects were arrested in the remote island village of Benjina in May after the slavery was revealed by The Associated Press in a report two months earlier.
The defendants were being tried separately Monday in Tual, a municipality in southeastern Maluku province, about 2,900 kilometers (1,800 miles) southeast of Jakarta.
Youngyut Nitiwongchaeron, a 53-year-old Thai captain, was the first to go on trial.
The three Indonesian and five Thai boat captains are employees at Pusaka Benjina Resources, one of the largest fishing firms in eastern Indonesia.
If found guilty they could face jail sentences of up to 15 years and fines as high as $46,000.
