WASHINGTON -- Government biologists on Thursday affirmed the need for a spring rise and more shallow summer waters in the Missouri River to ensure the survival of the endangered pallid sturgeon.

But conservationists said the remedy proposed by the U.S. Fish and Wildlife Service isn't enough to save the fish from extinction. And they castigated the service for asserting that two endangered and threatened bird species can survive without changing river operations.

The attempt to revise operations of the nation's longest river has dragged on for nearly 15 years, caught up in a mighty tug-of-war between economic and political interests.

All sides of the issue were disappointed in the biological opinion released by the service, which is part of the Interior Department. Downriver farmers, shippers and upriver recreation industry advocates all said the recommendations were unacceptable and inadequate.

The agency called for flows to be reduced beginning next July to 25,000 cubic feet per second or less from the Gavins Point Dam on the South Dakota-Nebraska border.

The number is higher than the agency's 2000 recommendation, which said releases should be decreased even more, to 21,000 cubic feet per second or less.

''The service today basically said we will forgo immediately creating hundreds of acres of shallow water habitat that juvenile sturgeon need to survive to adulthood," said Scott Faber, a spokesman for the conservation group Environmental Defense.

''That would be more shallow water habitat than the corps has created through artificial means in the last two decades. It's a stunning reversal," Faber said.

If the corps moves quickly to create more shallow water habitat in the most critical area, between the Gavins Point Dam and the mouth of the Platte River south of Omaha, Neb., then the service would allow higher releases. The service said the fish has lost 98 percent of its habitat in those reaches since the river was dammed and channelized from the 1940s to the 1960s.

In 2000, the agency said the sturgeon as well as two bird species, the endangered interior least tern and the threatened piping plover, could survive only if the river's flow was changed to mimic conditions before the Missouri was dammed and channeled.

Those conditions included a spring rise in the river to trigger spawning and a lower level in the summer establishing a suitable habitat for young sturgeon.

The new biological opinion said the spring rise should actually be two ''pulses," one in March to mimic snow melt from the plains and one later to mimic spring snowmelt in the Rocky Mountains.

''What we're trying to do is look at the Missouri's own signature heartbeat and trying to mimic it where we can," said Mike Olson, an official of the U.S. Fish and Wildlife Service in Bismarck, N.D.

Thursday's biological opinion represents the agency's final word on what must be done under the Endangered Species Act. The Army Corps of Engineers intends to use the findings to update river operations that have gone virtually unchanged for more than four decades. The service gave the corps until 2006 to come up with a permanent plan for following Thursday's recommendations.

The Bush administration has resisted the flow changes and last month yanked the service biologists who originally ordered the changes off the project to bring in a new team. The new team issued Thursday's recommendations.

Communities in Montana and the Dakotas want the more seasonal ebb-and-flow advocated by environmentalists because it would benefit the multimillion dollar boating and fishing industry in those states. They said the new recommendations would not keep enough water upstream.

North Dakota Democratic Sen. Byron Dorgan denounced the findings as ''a triumph of politics over science."

Downstream, below the Gavins Point dam, farmers and barge shippers oppose the changes because they rely on barges, which need consistent depths to carry multimillion-dollar cargo toward the Mississippi River at St. Louis. The barges generally need about 28,500 cfs, far more than the service called for, to operate along the river in Nebraska, Iowa, Kansas and Missouri.

''Once again, this places the navigation system in the category of unreliability," said Chris Brescia, who heads the MARC 2000 shipping coalition.

The specter of sudden flooding during a spring rise also frightens those who live along the lower Missouri.

On the Net: U.S. Fish and Wildlife Service: http://www.fws.gov