File photo shows A 125-pound (57kg) beluga whale male calf, in Chicago, in this file image from August 4, 2006. [Agencies]

Conservation groups said they had initially filed a petition to list the population as endangered under the Endangered Species Act in March 1999.

"The science was clear and it has been for a very long time," said marine mammal scientist Craig Matkin of the North Gulf Oceanic Society. "The population is critically endangered."

At the urging of the Palin administration, NOAA delayed a listing decision that had been expected in April this year so that it could conduct one additional summer population survey.

"The State of Alaska has had serious concerns about the low population of belugas in Cook Inlet for many years," Palin said in a statement after the NOAA decision. "However, we believe that this endangered listing is premature."

Denby Lloyd, commissioner of the Alaska Department of Fish and Game, said they had urged NOAA to delay the listing "for a few years to get more population counts".

"Of course, whenever you have a population of marine mammals that is this low, it is a cause for serious concern," Lloyd said. "We just aren't sure that an endangered listing, and all the legal requirements it brings with it, is necessary to assure the health of this population at this time."

Various industry groups have also fought the listing, which they fear will hamper Cook Inlet oil and gas development, cargo shipping, commercial fishing and major construction projects.

The beluga population of Cook Inlet is among five beluga populations in Alaska waters.