Kicking back in a comfy brown armchair and sipping from a grande soya latte at the back of a Richmond coffee shop, Steve Roest doesn’t look much like a pirate. Most people in the area simply know him as the Green Party’s parliamentary candidate for Twickenham.

The only give away as to his piratical alter-ego is a logo on the upper left hand side of his black t-shirt which features a toothy skull crossed with a trident and a shepherd’s staff.

But unlike Somalian cutthroats who are currently causing chaos in the Gulf of Aden in their search for western hostages and oil tankers, Mr Roest quarry is rather different. He is after Japan’s internationally despised whaling fleet.

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This week the 42-year-old property developer will fly towards Australia to join a controversial band of eco-activists who for the past five years have played an often dangerous game of cat and mouse in the frozen waters of the Antarctic where Japan heads each summer to hunt for whales despite international condemnation.

The battle is always a David versus Goliath affair. On one side is Japan’s vast whaling fleet lead by the Nisshin Maru – an 8,000ton “research vessel” where hundreds of harpooned minke and fin whales will be butchered, packaged and frozen during the three month hunt. On the other side is the Sea Shepherd Conservation Society, an anti-whaling fleet comprised of just a single ship: a converted 885-ton fishing vessel called the MV Steve Irwin.

If previous years are anything to go by things will get dirty. The 32 crewmembers, which include three British citizens this year, will stop at nothing to ensure that the Japanese harpooners miss their targets. In the past activists have thrown rotten butter bombs to contaminate the decks where the whales are cut up, deployed steel cables to foul the ship propellers and have even resorted to ramming vessels using a 7-foot hydraulic steel “can opener”. The Japanese have responded with flash grenades, the odd live round and accusations that the activists of Sea Shepherd are “eco-terrorists”.

“The suggestion that we are eco-terrorists is ridiculous but I have no problem with the term pirates,” says Mr Roest. “We like to call ourselves pirates of compassion. Either that or sea cops, legally operating under the United Nations’ World Charter for Nature.”

The next three months will be a major test for anyone onboard the Steve Irwin, which was renamed in honour of the late Australian television presenter who was intending to join one of Sea Shepherd missions before he died. On 1 December the vessel will leave Brisbane harbour and motor towards the Southern Ocean to try and intercept the Japaense fleet.

“It’s a small boat and there will be a lot of us crammed together in very difficult and icy conditions,” says Mr Roest, seemingly unperturbed by the prospect of three months in the Southern Ocean where the water temperature rarely gets above -5C. “We’ll also be up against boats that are much bigger and more numerous than our own but I can’t wait to get there.”

Founded by Canadian national Paul Watson in 1977, Sea Shepherd have long been regarded as one of the more radical conservation groups working to combat ethically dubious and often illegal fishing practices in the world’s oceans. Watson was one of dozens of activists who helped found Greenpeace in the early 1970s but was later thrown out for breaking the group’s non-violent ethos during a protest against seal hunters and spent much of the past three decades adrift from the mainstream environmental movement.

But his recent operations against Japan’s whaling fleet over the past five years have earned the group renewed and increasing praise. For the past three years the Japanese whalers have been forced to return to harbour with less than half their expected catch. Although commercial whaling was halted in 1986 Japan is permitted to conduct whaling in the name of scientific research but critics say their hunt is just a front for commercial whaling.

“Since 1986 the Japanese have not released a single peer-reviewed piece of research that has come through its lethal whaling operations,” says Mr Roest. “What the Japanese are really up to is thinly disguised commercial whaling.”

This season the Japanese hope to catch 900 minke and 50 fin whales but Sea Shepherd hope to disrupt their operations so badly that they will be forced to return empty handed. This year, however, Sea Shepherd will be on their own. Greenpeace, which still has a frosty relationship with its more radical incarnation, usually sends the Esperanza, a ship that documents whaling activities but refrains from the sort of tactics employed by Watson. But this week the environmental group announced it was instead concentrating on campaigning for two Japanese activists who face jail next year for jail intercepting whale meat stolen by crew from the whaling factory ship Nisshin Maru in a trial which Amnesty International says is politically motivated.

Speaking to the Independent yesterday from the deck of the Steve Irwin, Watson was characteristically critical of the movement he once helped found.

“Obviously the more boats targeting the Japanese the better but Greenpeace have never really been that helpful to us anyway,” he said. “They’ve never shared co-ordinates of the Japanese fleet with us. With or without their help we will do everything we can to stop the whalers killing whales.”

As for Mr Roest, a qualified dive instructor who will be driving one of the fast inflatable boats used to harangue the Japanese fleet when their spotted, he believes direct action is the only way to stop this year’s whale kill.

“The more you get involved in the environmental movement the more you understand just how large the problem is,” he said. “The fact is that in thirty years no-one has been harmed on either side by Sea Shepherd. Everything you do has an element of risk, even crossing the road has its risks, but if you’re ensconced on an 8,000 ton vessel the risk is very small. But nothing compares to the risk we face of losing these beautiful sea creatures that are on the brink of extinction.”