Greenpeace claims to have evidence of large-scale illegal fishing by Cambodian, Philippines and Indonesian vessels in the Pacific Ocean.
Esperanza, a Greenpeace ship, has been monitoring the western and central Pacific Fisheries Commission area.
Oceans campaigner Karli Thomas says only registered vessels are allowed to fish in an area known as the Pacific Commons.
She says the vessels identified were not registered and Greenpeace has video and photo evidence of large transfers of skipjack tuna and some yellow fin.
Ms Thomas says the evidence will be presented to a meeting of the region's Fisheries Commission next month.