Greenpeace, emboldened by its recent success in harrying the Japanese whaling fleet, is now turning its attention to tuna fishers in the Pacific Ocean. Ships from several countries have been singled out by the environmental activists, who say that they are defending the region's rapidly dwindling tuna stocks from legal and illicit “plundering”.

Protests have involved activists boarding or aggressively drawing up alongside fishing boats from South Korea, Taiwan and the United States. In some cases they have managed to “confiscate” modern machinery capable of hugely increasing the catch.

The Greenpeace protests come amid grave warnings from scientists that the oceans can no longer support surging global appetites for fish. The Western world's recently acquired taste for sushi - and its chief constituent, tuna - is seen as especially unsustainable.

Although Greenpeace insists that its campaign to protect tuna is non-violent, the organisation's success in the Southern Ocean this winter has increased its audacity. In an apparent escalation of its non-physical protests, the organisation has now vowed to “interfere” with the activities of the tuna fishers.

Under the constant harrying and “blocking” tactics of Greenpeace and others, the Japanese whaling fleet returned home in despair earlier this month with less than half its intended haul of minke whales. The drawn-out campaign, in which non-lethal missiles were fired by activists and whalers, was declared a triumph after Japan admitted it had been able to take only 500 whales.

In its attempt to curb the activities of what it called “legal fishers and pirates”, Greenpeace said that crew members of its Esperanza ship had succeeded in boarding a Taiwanese vessel off the coast of the Solomon Islands that turned out to have hundreds of frozen shark fins and tails on board. It then stayed close by the fishing boat until it had sailed out of international waters.

Much of Greenpeace's concern is focused on the lack of regulation of tuna fishing in the Pacific and the great leaps in fishing technology that have vastly improved the success rate of each outing.

Pointing out how vital the $3 billion (£1.5 billion) annual Pacific tuna catch was for the economies of island nations in the region, a Greenpeace spokesman told reporters that “advances in technology mean that 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”.

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