Japan and other pro-whaling countries failed on Friday to stop the group from discussing the fate of dolphins, porpoises and small whales, which are not covered by the 1986 moratorium on commercial whaling but which conservationists say are as threatened as the great whales.
The 32-30 vote against a proposal to remove so-called small cetaceans from the IWC's agenda at a gathering on the Caribbean island state of St Kitts and Nevis was seen as a bellwether of the balance of power at the agency.
Environmental groups and anti-whaling nations had feared that Tokyo might finally have been in a position to start challenging the ban, which is credited by all sides with saving great whales from extinction.
Patrick Ramage of the International Fund for Animal Welfare said it was a "big vote for small cetaceans"
"It's a big vote for small cetaceans. It's not the big vote of the day in terms of signalling where this forum is headed"
Patrick Ramage, International Fund for Animal Welfare
"It's not the big vote of the day in terms of signalling where this forum is headed," Ramage said, adding that a pending vote on adopting secret ballots within the IWC would be the decisive indicator of which side held the upper hand.
Japan and other whaling nations such as Norway and Iceland have fought since 1974 to convince the IWC that it should limit its conservationist efforts to large whales, and stop discussing the fate of dolphins and porpoises, which it hunts in large numbers in its coastal waters primarily for food and other products.
Japan has abided by the moratorium on commercial whaling but uses a loophole that allows for scientific whaling.
Its fleets brought back 850 minke whales from Antarctic waters last season and 10 fin whales, and it plans to start hunting humpbacks.
Iceland also conducts scientific whaling while Norway, the only nation to defy the international ban, has set its hunters a quota this year of 1,052 minke whales, a small species whose meat is eaten as steaks.