BOISE, Idaho -- Each year, endangered white sturgeon lay millions of fertilized eggs on the silty bed of the Kootenai River in northern Idaho -- in vain.

The white sturgeon, the largest freshwater fish in North America, has not successfully reproduced in the Kootenai since Libby Dam was completed in Montana in 1974, reducing the river velocity and trapping critical nutrients upstream.

Downstream, however, pools at a hatchery run by the Kootenai Indian Tribe teem with thousands of year-old sturgeon, inch-long miniatures of the two armor-plated wild adults in another tank that measure 6 to 7 feet long. Another tank holds the medium-size fish raised at the hatchery since birth.

Bred from captured wild sturgeon, the young sturgeon may be the last hope of saving the species from extinction.

''Old sturgeon don't die, they just fade away,'' said Sue Ireland, the fish and wildlife program manager for the Kootenai Tribe. ''Entire populations just fade away because people don't realize there is no recruitment (additions to the number of spawning fish) going on.''

Over the past decade, the hatchery has released 80,000 juvenile sturgeon into the river. But since the fish don't reach sexual maturity until about age 30, the oldest of those hatchery raised sturgeon are not expected to begin spawning until 2025.

And the number of fish that will survive until that time will be significantly less than 80,000, said Bob Hallock, Kootenai white sturgeon recovery team leader for the U.S. Fish and Wildlife Service.

''The first year the fish are released, about 60 percent survive,'' he said. ''After that, the rate is about 90 percent annually. So you play that out over 30 years and that's not a lot of fish.''

Ireland said researchers are unsure whether the hatchery fish will return to the place they were put into the river or spawn elsewhere along the only place they are found, a 167-mile stretch of river between Kootenai Falls in Montana across Idaho to Corra Linn Dam at the end of British Columbia's Kootenay Lake.

''We don't really know if they will come back,'' she said. ''We'd like to start incubating and rearing fish upstream, because if there is a specific homing instinct, maybe we should be rearing them in the location we want them to return to.''

That location is an 18-mile section of the river that was designated critical habitat for the white sturgeon by the federal wildlife agency. The designation means federal and state agencies must consult with the agency before undertaking projects in that section of the river that might affect the sturgeon, which was declared an endangered species by the federal government in 1994.

The habitat designation also means that Libby Dam operations must be modified to better replicate the higher, faster flows of the Kootenai before the dam. It's estimated that will cost between $360 million and $780 million between now and 2025, for modifications to the dam and in lost hydropower revenue.

Biologists say the river is moving more slowly than it did prior to the dam, allowing silt to fill the river bottom and cover rock crevices where young sturgeon can hide from predators. They believe increasing flows would clear out sand and silt from spawning beds, providing a gravelly place for fertilized eggs to incubate and giving cover to young sturgeon.

The agency is proposing that the Army Corps of Engineers put rocks and gravel in the river downstream from Bonners Ferry to improve spawning beds, along with possibly dredging and narrowing the channel to increase water velocity.

Meanwhile, planners with the Bonneville Power Administration and the corps have until April 14 to devise a strategy to stop the steady demise of the estimated 500 remaining wild sturgeon, a number expected to dwindle to 50 by 2030 barring successful spawning by the hatchery sturgeon.

''We still have 500 adults left and we know a lot more about the biology of this species and the physical makeup of the river system than when we started,'' Hallock said. ''So we feel we can save this fish from extinction.''

On the Net:

http://www.fws.gov/endangered/features/sturgeon/