The family of environmental activist Simon Peterffy are worried he faces jail in a foreign country.
News that the leader of Forest Rescue was aboard the Shonan Maru 2 came as a surprise to Mr Peterffy's partner Panda Broad and his mother Maxine Peterffy.
Ms Broad was stunned when told her partner of two years had clambered aboard the security vessel in the dark of night and might be heading to Japan.
"Are they in danger, they must be," she said.
Ms Broad said she was prepared to go to Japan to support him.
"As long as he's going to be OK, and so are the other guys, they will be fine - this is what Simon does."
Mr Peterffy has a long history of colourful public protests, including clashes with native timber loggers, locking himself to machinery in Broome to oppose a gas hub and scaling Bunbury Cathedral
Ms Broad said she was worried he could now face jail.
"Because of previous arrests it's more than likely he's going to go to jail," she said. "God knows what they're doing to the guys on board and how they're being treated. They wouldn't be treated nicely and respectfully.
"He's standing up for the rights of everyone and you have to have people do this, stand up for animals.
"Good on him for doing it."
Mr Peterffy's mother Maxine, from Victoria, said she could say little until more information was available.
Mr Peterffy declared his commitment to the planet in a Facebook message on Saturday.
"I love my life and love you all! Thanks everyone for their commitment to defend the earth. Earth First," he wrote.
The partner of Mr Peterffy said she fears the men will go on a hunger strike if not brought home.
Bridgetown artist Panda Broad is demanding the Australian government act to bring home her partner Mr Peterffy and his colleagues Geoffrey Tuxworth and Glen Pendlebury.
Ms Broad said she had been told by other activists that the men's plan was to go on a hunger strike if detained on the ship.
"I've only heard that that the boys are on a hunger strike, that was the plan if they weren't brought home," she said.
"I think a couple of the boys are on medication and need to be brought home."
Ms Broad said she was desperate for news of her partner and felt Prime Minister Julia Gillard had done little to secure their safety.
"The need to be brought home, if they got on a hunger strike, they need to come home."
Forest Rescue spokesman Michael Montgomery confirmed there was a plan for the men to go on hunger strike if detained.
"This is to show without a shadow of a doubt that they are being held under duress and against their will," he said.
Mr Montgomery said he was unable to make contact with the men to see if they had carried out the plan.
"I'm aware (of the hunger strike plan) but we haven't been able to have contact with them, we believe their radios were taken as soon as they got on board, which is a bit much given that they only wanted to get on to the boat and say please leave Australian waters."
Mr Montgomery said the men represented the view of majority of Australians that whaling should be stopped.