Activists from the militant environmentalist group Sea Shepherd said the hunters were herding the animals into a screened-off area because they wanted to hide what they were doing.
"They continue to use tarps to cover the slaughter, and physically drive the pod under the tarps... to avoid cameras," Melissa Sehgal told AFP by telephone from Taiji.
"You can hear the dolphins splashing below," she said, as the fishermen stab a metal spike into their spinal cords.
"It was approximately 30 dolphins - striped dolphins - that were all slaughtered this morning."
Boats search the open ocean off Japan's Pacific coast for pods of dolphins. When a group is located the fishermen drive them towards the cove by banging on submerged metal poles attached to their boat.
This creates a sonar wall from which they flee. By positioning several boats in an arc, the hunters can funnel the creatures into a small bay. Once there, nets are strung across the mouth of the cove to prevent the dolphins' escape.
Many of the rest are killed for their meat, which features in the diets of a small number of coastal communities in rural Japan. It is not widely consumed and the Japanese government recommends limiting intake because of the high levels of mercury it contains.
"Over 1,200 dolphins have been driven into the cove since September 1, when the season began," Sehgal said. "Of those 1,200, over 600 dolphins have been slaughtered, not including today, and 149 have been taken captive."
The hunting season in Taiji is to go on until the end of February, according to an official of the local fishermen's association.
Local defenders of the hunt say it is a tradition and point out that the animals it targets are not endangered. They say Western objections are hypocritical and ignore the vastly larger number of cows, pigs and sheep butchered to satisfy demand elsewhere.
They also insist the methods they use are humane and say they screen off the killing area for the same reasons that abattoirs do not advertise the slaughter of mammals.
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