A London-based environmental group charged Thursday that Japan's coastal whaling programme was on track to wipe out the marine mammals from local waters.
The number of whales being caught off the coast is on a steady decline, the Environmental Investigation Agency said, with fishermen having to travel further afield to find their targets.
"A comprehensive analysis of the available scientific data demonstrates unequivocally that there are grave concerns regarding the sustainability of these hunts," said Sarah Baulch, the group's cetaceans campaigner.
The campaigners looked at coastal whaling, which is distinct from Japan's annual whale hunt in the Antarctic that draws international opprobrium and has seen Australia lodge a case with the International Court of Justice.
Small-time coastal whaling is allowed under the rules of the International Whaling Committee, which regards it as similar to that of communities engaged in aboriginal subsistence whaling elsewhere in the world.
The practice was brought to worldwide attention by the Oscar-winning anti-whaling documentary "The Cove", which graphically depicted the slaughter of the animals in the small town of Taiji in Japan's southwest.
The Japanese government has maintained that coastal whaling is the socio-economic foundation of fishing communities. But the argument does not wash in many Western countries, whose publics want it banned.
The outrage abroad, particularly the more extreme actions of militant campaigners in the Southern Ocean, has had the effect of making whaling a rallying cry for nationalists, who insist the desire to ban it is cultural imperialism.
The Environmental Investigation Agency, citing figures from the whaling industry, said the falling catch was indicative of a diminishing whale population, while charging the Japanese government is not carrying out proper surveys.
The group also charged that cruel methods employed in killing dolphins, whales and porpoises, in which they are chased a long way before being butchered, "likely" causes stress to the wider cetacean population.
The government should phase out the practice to allow the populations to recover while helping fishermen to find different jobs, it said.