John Vidal takes a tour of the Steve Irwin, part of the Sea Shepherd organisation's fleet of activist vessels that pursue marine lawbreakers with 'aggressive non-violence' guardian.co.uk

As the Steve Irwin approached the equator last week, word that Japan would be sending a strengthened whaling fleet to Antarctica next month reached the bridge of the old Aberdeen-built customs vessel. The crew of activists on board cheered, as their veteran leader, Captain Paul Watson, resigned himself to his eighth "whale war" among the icebergs and 100mph winds of the Southern ocean.

Watson, on what is nearly his 350th voyage in nearly 40 years defending whales and other marine wildlife at the helm of Sea Shepherd Conservation Society, is sending three ships to intercept, chase and harass the Japanese. He promises "aggressive non-violence", while the Japanese, still smarting from last year's humiliation when their fleet took only a fifth of its planned whale catch, say they will heighten security and take an armed government fisheries patrol vessel.

"We intend to carry out the [whale] research after enhancing measures to assure that the fleet is not obstructed," said fisheries minister, Michihiko Kano.

The two fleets expect to meet in the Antarctic whale sanctuary before Christmas and will shadow and confront each other for at least 12 weeks. Both have helicopters and water cannon. In addition, the Steve Irwin has iron spikes to prevent the Japanese from boarding, and Watson's crew has a store of vile-smelling butyric acid stink bombs to fling aboard any vessel that comes close. Both fleets are expected to wage a media and diplomatic battle, as well as engage in a dangerous physical tussle on the high seas.

But it was Australia, which fired the first diplomatic shots, this week condemning Japan and urging it not to send its fleet. "There is widespread concern in the international community at Japan's whaling programme and widespread calls for it to cease", said foreign minister, Kevin Rudd, this week. Australia last year took Japan to the international court of justice seeking an end to the harpooning which it conducts under a "scientific" loophole.

Few people realise, said Watson in London before setting off for the Antarctic, how dirty this old-fashioned sea war can get, with hand-to-hand combat, collisions, bombardments and sinkings. "Some of the scenes look like out of world war two. There are a lot of ships at sea, seven or eight at a time, water cannons going … We get help finding them [the whaling vessels]. Tourist ships and fishing boats, research stations give us their co-ordinates."

Although he is on Interpol's wanted list and is classed as an eco-terrorist in Japan, Watson says he has been on the side of the law since he was first mate on the first Greenpeace voyages of the early 1970s. "We don't protest, we intervene. We are not there to witness but to stop crimes being committed," he says. "They call me a pirate but what is a pirate? Drake and Raleigh were pirates. John Paul Jones, who started the US and the Russian navies, was a pirate. Pirates challenge the status quo."

Watson, for years little known in Europe, has recently become a star of Discovery Channel reality TV programme Whale Wars. Although the show has been criticised for being more showbusiness than documentary, the TV exposure has tripled the group's membership and income.

But Watson has his critics. He was savagely satirised in the South Park animation Whale Whores for being media-hungry, and a long-standing row with Greenpeace has resulted in the two organisations not talking to each other.

"[Greenpeace's international executive director Kumi Naidoo] should be running the Red Cross. He's not an environmentalist. He's an anti-apartheid organiser who has stated that the only way to save the planet is through alleviating world poverty. It can't be done. There are just not enough resources. Why does he want to do the job of Oxfam or the Red Cross? Greenpeace seems to have lost their direction," says Watson.

Watson is a confirmed "biocentrist" who believes worms and cockroaches are more important than humans. "I say look at earth as a spaceship travelling at 500km a second. Our life support system is the biosphere. It provides the air and temperature, and it's run by a crew, not us. We are just passengers, busy entertaining ourselves, but the crew are the bacteria, the worms and the fish we are killing off. There's only so many crew we can kill before things fall apart. They are more important than we are. If the fish die the ocean dies and if the ocean dies we die. We cannot live without them. I measure intelligence by the ability to live in harmony with the natural world and by that criteria cockroaches are more intelligent than we are."

Too many humans, he says, is by far the greatest problem facing earth. "Earth can probably only carry one billion humans. As long as human populations continue growing, the battle [to save the planet] will be lost.

"One of two things will happen. Some incredibly imaginative, intelligent person will come along or planet Earth will take care of it for us. The reason we had great age of affluence is we had four continents to exploit. But we have now far exceeded earth's carrying capacity which is why we're in the middle of this major extinction. There will inevitably be a resource crash, but we are in denial about it."

There's a waiting list of thousands from dozens of countries wanting to sail with Watson, who prides himself on never having caused or sustained an injury in his 34 years of taking amateurs to sea in often dangerous situations.

"Because our crew are amateurs and not professional there's much more precaution taken in everything we do. Sometimes professionals get themselves into trouble because they take things for granted.

"Passion is the most important thing."