Japanese officials said they were considering legal action against militant protesters that disrupted the five-month Antarctic voyage.

But they declared the mission a success despite the lower-than-expected haul, vowing to continue the annual hunt despite diplomatic pressure by anti-whaling countries led by Australia.

Escorted by patrol boats, the 8,044-tonne Nisshin Maru mother ship with 143 crew on board docked at a heavily guarded Tokyo port early Tuesday.

The fleet of six vessels killed a total of 551 whales, less than its original target of about 950 whales including 50 humpbacks.

Nisshin Maru captain Tomoyuki Ogawa said he felt a "real danger of collision" when the fleet was earlier pursued by the protesters and a patrol boat sent by Australia to monitor the Japanese mission.

"I want (protesters) to stop any acts which threaten people's lives and the safety of ships," he told a news conference after his return.

Demonstrators on a Sea Shepherd Conservation Society ship threw what they described as stink bombs filled with rancid butter onto the decks of the whaling ships.

Japan, which says the bombs contained acid that stings the eyes, fired warning flash grenades at the activists. Two protesters also boarded a whaling factory ship, resulting in a two-day standoff.

"The disturbances were an unforgivable act," said Shigeki Takaya, assistant director of the Fisheries Agency's whaling bureau.

"We will take preventive measures, resorting to legal procedures and gaining international cooperation," he told AFP.

"But we can call the mission a success as we were able to pursue our research," Takaya said. "We want to firmly continue whaling, which is based on an international treaty and scientific grounds."

No major protests were staged as the mother ship docked but four Greenpeace members sailed past the vessel with a banner that read: "Failed."

Dock workers unloaded leftover harpooning ammunition and frozen whale meat already processed on the ship to be sold at fish markets.

Coastguard officials were seen boarding the main ship, whose hull was scratched and slightly dented, although it was unclear whether the damage was related to the high-seas clashes.

An inspection of the fleet and questioning of the crew over the incidents are likely to begin as early as Wednesday, an official said.

Meanwhile Tuesday Australia said it remained determined to end Japanese whaling.

"It remains Australia's firm view that there is no scientific justification for Japan's whale hunt in the Southern Ocean," Foreign Affairs Minster Stephen Smith and Environment Minister Peter Garrett said in a joint statement.

"The government's objective continues to be the cessation of whaling by Japan in the Southern Ocean."

Japan had aimed to kill 850 minke whales and 50 fin whales on the mission. It dropped plans also to kill up to 50 humpbacks, beloved by whale-watchers.

Japan, which kills whales using a loophole in a 1986 whaling moratorium that allows "lethal research" on the giant mammals, says that it is monitoring whale numbers but makes no secret that the meat ends up on dinner tables.

"If it's aimed purely at research, why does Japan have to kill whales to sell?" said Junichi Sato, an anti-whaling campaigner at Greenpeace Japan.

"If Japan really wants to secure leadership in the international community, ending whaling is a prerequisite," Sato said.

Whale meat holds sentimental value for some Japanese who ate it after the devastation of World War II.

Most Japanese have seldom eaten whale since the international moratorium was imposed although Japan has been trying to give young people a taste for whale, which has been sold in curry and burgers.

"I didn't know the whaling vessel returned today," said Yuko Mitsui, a 23-year-old beautician who has never eaten whale meat. "I don't understand why Japan has to keep catching whale as we have no problems with poverty any more."

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