The Nauru government has banned transhipments, a practice of shifting fish catch to a mother vessel employed by many foreign fishing fleets.
The Guardian reports that Nauru's move comes after an alleged illegal operation by a Taiwanese ship caught near its waters by Greenpeace last week.
Transhipments enable fleets to remain in, and plunder, ocean fishing grounds for years at a time while dodging mechanisms which monitor their catch
The Nauru Fisheries and Marine Resources Authority says the crackdown on vessels unloading their catch to "motherships" at sea would help "end the laundering of fish" by high seas "pirates".
The Greenpeace ship Rainbow Warrior II last week claimed to have uncovered a huge pirate tuna fishing operation in waters around Nauru, Papua New Guinea and Micronesia.
Greenpeace said the Taiwanese longliner Shuen De Ching No.888 appeared to have been fishing in those waters without permission for two months.
The activists allegedly found 75kg of shark fins from some 42 sharks, where the ship's log recorded only three shark carcasses - an alleged violation of Taiwanese and Pacific regulations dictating that fins make up 5% or less of the weight of the total catch.
The Shuen De Ching had also logged an improbably low catch of five tonnes of fish, the activists said, suggesting it had been unloading to a mothership.
The NFMRA, which credited Greenpeace's exposure of an "illegal operation" for prompting the Nauru government ban, said it regularly observed "longliners in the high seas acting suspiciously and intruding on our borders".
Nauru has become the third Pacific nation to issue a blanket ban on transhipments in its exclusive economic zone, after Marshall Islands and Tuvalu.
American Samoan call for more longliners.
American Samoa's former governor, Togiola Tulafono, is supporting a call for large longliners to be allowed to fish in his territory's coastal waters.
The federal government has proposed opening up the Large Vessel Protected Area, which is currently reserved for small fishing boats.
Togiola says it must be remembered that fisheries is the territory's only major industry, and that this will be the case for many years to come.
He says fisheries is the only thing that has proven to endure and be sustainable.