The first dolphins of the season were slaughtered on Tuesday in the small Japanese town of Taiji, campaigners and a local fishermen's union said, commencing an annual cull repeatedly condemned by animal rights groups.

Activists from the environmentalist group Sea Shepherd have been monitoring a bay in Taiji, southwestern Japan, since the six-month dolphin hunting season began earlier this month.

"First pod of 2014-2015 being driven into cove now," the activists from Sea Shepherd, who call themselves "Cove Guardians", tweeted at 10:33 am (0133 GMT).

About an hour later, @CoveGuardians said: "First dolphin murder of the drive hunt season is complete as dead bodies are dragged to Taiji butcherhouse."

An official from the local fishermen's union also confirmed they had made the first catch of the season.

"We caught 12 Risso's dolphins," he said, adding they had already been killed for their meat.

There were no dolphins left in the bay, he said, and fishermen would continue with their hunt over the coming days.

The campaigners are streaming live footage of a secluded bay, into which local fishermen corral hundreds of dolphins for slaughter, a practice that thrust the small town into the global spotlight in 2010 when it became the subject of the Oscar-winning documentary "The Cove".

Defenders say it is a tradition and point out that the animals it targets are not endangered, a position echoed by the Japanese government.

They say Western objections are hypocritical and ignore the vastly larger number of cows, pigs and sheep butchered to satisfy demand elsewhere.

But critics of the practice say there is insufficient demand for the animals' meat, which in any case contains dangerous levels of mercury.

The annual dolphin hunting season opened on September 1, and is expected to continue until the end of February.

Japan has set self-imposed restrictions on dolphin hunting, allowing the catching of seven species with quotas for each prefecture in which they are hunted.

In the case of Wakayama prefecture, where Taiji is located, quotas for local fishermen are set at a total of just under 2,000 short-finned pilot whales and false killer whales, and Pacific white-sided, striped, bottlenose, Pantropical spotted, and Risso's dolphins, according to the local government.

"These restrictions are imposed so that the marine resources can be used sustainably," said a Fisheries Agency official.

Last season, the slaughter sparked renewed global criticism after US Ambassador to Japan Caroline Kennedy tweeted her concern at the "inhumaneness" of the hunt.

The first dolphin killing of the season came as the sharply split International Whaling Commission meeting opened in Slovenia on Monday, where Japan is expected to announce its controversial plans to resume Antarctic whaling.

It is the first global meeting on whaling since the UN's highest court handed down a verdict in March that Japan had abused a loophole in the international whaling moratorium that allows for lethal scientific research.