Sea Shepherd founder Paul Watson has described a US judge's opinion that he is a "pirate" as one-sided and irrelevant.

Chief Judge Alex Kozinski used the term to criticise Mr Watson and the activist group in a court of appeal in San Francisco.

The court was overturning an earlier decision which had dismissed claims from Japanese whalers that Sea Shepherd boats had breached an order to stay 500 yards (457 metres) from them.

"You don't need a peg leg or an eye patch," Chief Judge Kozinski wrote in the unanimous three-judge panel's decision.

Mr Watson said the judge had ignored aggression from the Japanese fleet, which has clashed with Sea Shepherd vessels in recent days.

"I guess that puts me in the same boat as Johnny Depp," Mr Watson told AAP from aboard the Steve Irwin.

"That's an opinion, it's certainly not a judgment.

"He didn't mention anything in there about the fact that the Japanese have destroyed one of our ships (the Ady Gil in 2010), they've thrown concussion grenades at us, hit us with water cannons and laser beams.

"It seems to be a very one-sided opinion."

In the judgment it is Sea Shepherd that is accused of violence in the Southern Ocean.

"When you ram ships; hurl glass containers of acid; drag metal-reinforced ropes in the water to damage propellers and rudders; launch smoke bombs and flares with hooks; and point high-powered lasers at other ships, you are, without a doubt, a pirate, no matter how high-minded you believe your purpose to be," the chief judge writes.

Mr Watson rejected the accusations.

"We don't throw acid. We throw rotten butter," he said.

"I guess technically you can call it acid like you could call milk lactic acid or orange juice citric acid.

"We haven't rammed a single Japanese whaling vessel down here in the entire nine years.

"We've been rammed multiple times.

"The judge obviously has not seen the evidence or the facts; he's just making an opinion based on his own personal prejudices."

Footage released this week by both parties shows Sea Shepherd boats well inside the 500-yard limit but Mr Watson said the restriction applied only to the organisation's US branch.

He said Sea Shepherd USA had withdrawn from the campaign when the injunction imposing the limit was handed down in December last year, and the action was now being managed by the Australian branch.

"The whole thing is irrelevant," Mr Watson said.

"This is beyond the jurisdiction of the Ninth Circuit Court of the United States."

The ruling came as Japan's fisheries minister Yoshimasa Hayashi compared his country's whaling to Australians eating kangaroo meat.

"We don't eat those animals, but we don't stop them from doing that because we understand that's their culture," he said.