Sea Shepherd activists have left Hobart bound for the Southern Ocean in a bid to stop Patagonian toothfish poaching.

Carrying a crew of 29, the Bob Barker was farewelled by a group of well-wishers at the Elizabeth Street pier.

It has embarked on a three-month Antarctic mission dubbed "Icefish".

There are plans to forcibly stop the illegal harvesting of toothfish and arrest crews.

Captain Peter Hammarstedt said the Southern Ocean's top predator was being targeted by six illegal poaching operations.

"They operate outside the full reach of the law," he said.

"Our mission is to find these bandits, to shut down their illegal fishing activities, to confiscate and destroy any illegal fishing gear that we find."

He said the measure of the mission's success would be the number of fish saved and the number of illegal operations shut down.

Mr Hammarstedt said toothfish were sold as Chilean sea bass and its numbers in the wild are rapidly falling.

"It sells primarily in the United States, Europe, Japan," he said.

"There's an increasing market in China ... the illegal catch is predominantly going to Asia.

"But there's a big concern about trans-shipment of illegal fish and the possible integration of this illegal fish into catches of legal fish."

Upholding the law is dangerous: Bob Brown

Conservationist and former Greens leader Bob Brown said the so-called shadow lands of the Antarctic need to be better policed.

"We need the Australian Government and other government's around the world to have a policing presence at all times," he said.

The former Sea Shepherd chairman praised the crew for its bravery.

"It can be a dangerous business upholding the law," he said.

"They're unarmed but they're prepared to do it and - sterling human beings - doing a job for all of us."

A second Sea Shepherd vessel, the Sam Simon, will set sail for the Antarctic next week from New Zealand.