Chrrrrp, chrrrp: Our headphones echo with the tinny peeps of a radio-tagged pallid sturgeon (Scaphyrincus albus). Dave Fuller, a Montana Department of Fish, Wildlife and Parks fisheries technician, maneuvers the jet boat up and down the Missouri River on a beautiful October day. The sapphire sky has yet to succumb to winter's haze, and the cottonwoods and willows that line the bank are dressed in fall's finery, their branches sleeved in green and gold.

Soon we zero in: CHRRRP. "It's a female," says Pat Braaten, a U.S. Geological Survey research fish biologist, checking the transmitter. "Code 117." He throws a buoyed net into the water, and we drift downstream until the buoys start dancing: We've got her.

"You can look into (a pallid's) eyes and back into the future -- or something," Fuller says as the two men haul in one of North America's largest and most endangered fish. Braaten smirks at this facetious attempt at eloquence, but Fuller's sentiment is one often evoked by pallid researchers. Aaron Delonay, a USGS ecologist who works with the fish, calls them an "irreplaceable treasure from a time older than the Missouri River itself."

Today's Missouri River pallids are descended from fish that lived alongside dinosaurs more than 70 million years ago. They've weathered ice ages, volcanic explosions and a mass extinction event. Through it all, the fossil record indicates that, in form and function, they've hardly changed.

Lately, though, their remarkable evolutionary tenacity has been tested. Since dam-building and channelization began on the Missouri in the early 1900s, roughly 80 percent of the fish's habitat has been modified or destroyed. Scientists estimate that if they stopped stocking the river with hatchery fish tomorrow, the species could vanish from the Upper Missouri -- the dam-locked stretches of river in Montana and the Dakotas -- by the century's end.

After freeing Code 117 -- a four-foot-long behemoth -- from a dripping tangle of net, Braaten and Fuller slip her into a tank. With the caution of a beast accustomed to life in the murky dimness, she lifts her spade-shaped snout from the water. She has the sickly hue of a corpse but her tiny eyes are the color of warm honey. Her under-slung mouth is fringed with fleshy whiskers, and bony scutes armor her from tip to tail. She is weird and wonderful, beautiful and bizarre.

Fuller pulls on rubber gloves and assembles his tools: scalpel, forceps, suturing thread. He makes a one-inch incision in 117's lower abdomen and peeks inside, looking for signs of recent reproductive activity. If 117 hadn't spawned, her ovaries would be filled with large, gray-black eggs, which she would eventually re-absorb. But her eggs are tiny and white: She's spawned. He stitches her up; then, as if handling a priceless antique, he and Braaten lower 117 back into the river.

In recent years, Fuller and Braaten have biopsied a handful of wild pallids that, like 117, appear to be spawning successfully. What's troubling is what they haven't found: wild offspring, in either the Upper Missouri or the lower reaches of a major tributary, the Yellowstone River. A hatchery and propagation program has bought the species time. But the ultimate goal -- a population that can sustain itself in the wild -- remains elusive.

"We are always hopeful," says Braaten. "But I've been working on this stuff for 10, 11 years, and little sturgeon are just not found. Something is happening between reproduction and the rest of their lifespan."

Figuring out what that something is would help answer a more fundamental question for this and many Western rivers: Can struggling fish be rescued without tearing down dams?