Ship-borne activists said Tuesday they had targeted fishing boats from South Korea, Taiwan and the United States in high-seas protests against the "plundering" of tuna in the Pacific.

In the latest confrontation, crew from the Greenpeace ship Esperanza boarded a Taiwanese boat, the Nian Sheng 3, to inspect their catch, a spokesman said.

The captain of the tuna boat, which also contained hundreds of frozen shark fins and tails, allowed the activists to board, Greenpeace campaign leader Lagi Toribau told AFP by telephone from the Esperanza.

"Greenpeace are not a violent campaigning organisation," he said, while adding that the activists were prepared to "interfere with their physical fishing activities in order for us to save the last tuna stocks."

On Sunday, Esperanza crew members hopped in a small boat to paint the side of the US vessel Cape Finisterre with the words "Tuna Overkill," Greenpeace said in a statement.

Last Thursday, the group protested alongside the South Korean ship Olympus before activists "confiscated a fish aggregation device" used to attract tuna.

The latest action took place in international waters near the Solomon Islands northeast of Australia where "legal fishers and pirates are both plundering Pacific tuna," Greenpeace said.

Describing tuna as the world's favourite fish, Toribau said, "advances in technology mean large ships are now able to catch as much fish in two days as the fishers of the small Pacific island countries can catch in a year."

The future of the western and central Pacific tuna fishery is crucial for small Pacific states. Tuna is the only major economic resource for many, as well as one of the most important food sources.

Currently, licence fees provide them a small return of around five to six percent of the three billion US dollar annual catch in the region.

Toribau said the fishing carried out by the foreign ships "is technically not illegal but is unregulated," and Greenpeace is campaigning for the pockets of international waters between the island nations to be declared marine reserves.

Delegates from more than 40 countries met last December at a Pacific fisheries conference in Guam amid warning signs that the world's last great tuna fishery -- which supplies more than half the world's tuna -- is heading for crisis.

But the five-day meeting of the Western and Central Pacific Fisheries Commission, which includes delegates from Pacific nations and major fishing countries, failed to reach an agreement.

"Greenpeace took action against this tuna fishing operation because the Western and Central Pacific Fisheries Commission, which is supposed to be managing the fishery and protecting the tuna, are failing to do their job," Toribau said.

"Both time and tuna are running out."

Prized for top quality sashimi in Japan and as a source of cheap canned protein on supermarket shelves all over the world, tuna stocks have been slashed in most of the world's oceans.

"As tuna catches in other oceans have declined because of overfishing, these ships have moved into the Pacific," said Toribau.

"There are now nearly 600 purse seiners and over 3,600 tuna longliners plundering the Western and Central Pacific alone. This is clearly not sustainable."

Some modern purse-seiners, which surround schools of tuna with curtain-like nets, are capable of catching up to 10,000 tonnes annually, while long-line boats tow thousands of baited hooks at a time.

The Esperanza was heading for a stopover in the Solomon Islands before returning to international waters to continue the protests, Toribau said.