The tiny Pacific nation of Nauru on Thursday banned transhipping outside its ports in a bid to tackle lucrative fishing "black" trade, after a Taiwanese ship was found allegedly illegally trawling for tuna and carrying shark fins.

Transhipping -- or transferring cargo from one vessel to another -- is believed to be one of the methods facilitating illegal fishing on the high seas, which fall beyond national jurisdictions and are harder to regulate.

Global seas have been fished to dangerously low levels, according to independent panel the Global Ocean Commission, hurting the economies of Pacific islands nations that are dependent on the multi-billion dollar tuna trade.

"As one of the countries that border the high seas, we also regularly observe longliners in the high seas acting suspiciously and intruding on our borders," the Nauru government said in a statement.

"These seas act like a safe haven for pirate boats, and transhipment allows them to stay at sea even longer, and launder fish out of the area."

Nauru's move was sparked by activist group Greenpeace's boarding of Shuen De Ching No.888, reportedly a Taiwanese-flagged tuna longliner, in international waters near neighbouring Papua New Guinea last week.

The group said it discovered 75 kilogrammes (165 pounds) of shark fins on the vessel, taken from at least 42 shark carcasses -- though only three carcasses had been recorded in the ship's log.

Taiwan's Fisheries Agency replied that the boat was unlawfully boarded by Greenpeace but promised to look into the illegal fishing allegations.

Greenpeace welcomed Nauru's ban but said it should be extended to the entire region if overfishing is to be successfully stopped.

"If fishing vessels had to go to land to transfer their catch, it would solve many of the problems out here in the Pacific," Lagi Toribau of Greenpeace Australia Pacific said in a statement.

"Although more than 70 percent of the world's tuna comes from the Pacific, only 20 percent of that is actually caught by Pacific island fleets."