Trans-shipping means fishing boats can stay at sea for an extended period of time, in some cases more than a year, by transferring their stock to another boat and receiving fuel and supplies.
The US-based group, Oceana, said that practice could often involve the laundering of fish, human rights abuses, and labour violations.
Its senior campaign director, Beth Lowell, said trans-shipping was a huge problem around the world and it was also likely to be happening in the Pacific due to its large tuna fisheries.
