But the agency said it will take other steps to increase federal protection for the "southern resident" pods of killer whales, which summer in Puget Sound.

"We are taking the decline of these killer whales seriously and we will work to sustain and support this population," said Bob Lohn, head of the fisheries service's regional office.

By the end of last year, only 78 killer whales remained in the three pods - down from 98 in 1995, a 20 percent drop in six years. The population is believed to have peaked at around 120 whales in the early 1960s, when dozens were captured for marine aquariums across the country - a practice that stopped in the 1970s.

"They're not quite dead enough yet for the National Marine Fisheries Service to act," said Fred Felleman of the Orca Conservancy and Ocean Advocates, two of 10 groups that petitioned last year for the endangered-species listing.

An 11-member team of biologists reviewed the case and determined that the southern residents, while in danger of extinction, do not qualify as a geographically and genetically distinct subpopulation of orcas - one requirement for protection under the Endangered Species Act.

No one is sure exactly what is causing the decline, but Lohn said stress from pollution, parasites and vessel traffic including whale-watching boats are all possible factors, as well as declining salmon runs. While most killer whales feed on fish and seals, the southern residents - and their northern counterparts in Canada's inland waters - feed on fish alone.

"The bottom line on causes is we don't know the answer," Lohn said.

The agency plans to begin the process of having the orcas designated as "depleted" under the Marine Mammal Protection Act, which would allow allow increased attention to the population, Lohn said.

"It allows more focused attention and certainly means we will write a conservancy plan, which is the equivalent of a recovery plan under an ESA listing," NMFS spokesman Brian Gorman said. "And we will list in pretty clear terms what we think the problems with the population are, and the things that we think would lead to a solution."

He said he did not know whether the agency had funds on hand for more focused killer-whale work. "I think it's a little too early to tell."

The agency will solicit public comment about other ways to help the population and, in conjunction with Canadian authorities, improve whale-watching guidelines, Lohn said.

Brent Plater, attorney for the Berkeley, Calif.-based Center for Biological Diversity and the petition's lead author, contended the southern residents are a distinct population and said the findings are inconsistent with determinations on other populations.

"Today's decision is a new low in the annals of the fisheries service," Plater said. "They've completely abdicated their responsibilities to protect this population."

Lohn said he took the advice of some of the world's top scientists, and "I'll stand by that."

The whales' status will be reviewed again in four years, he said.

"If they wait another four years it is just to suggest they think the status quo is acceptable," Felleman said. "To wait four more years is a recipe for disaster."

Sen. Maria Cantwell, D-Wash., issued a statement saying NMFS "should account for its decision by providing all available information to the public and local experts in order to allow a thoughtful examination of the merits." And she noted that the "depleted" designation "allows us to take advantage of additional research and recovery resources."

"The call to action has been sounded and our regional leaders must act now to preserve this icon of our Northwest waters," she said.

Tuesday's decision came in response to a petition brought by several conservation groups. They argued that the Marine Mammal Protection Act only prevents direct harming of orcas, while an endangered-species listing would also protect their environment.

Other groups pressing for an endangered listing include the Center for Whale Research in Friday Harbor, the Washington Toxics Coalition, the American Cetacean Society, the Sierra Club, Friends of the San Juans, People for Puget Sound, Project Sea Wolf and former Washington Secretary of State Ralph Munro.